Class 11 Chemistry

Chapter 1: Some Basic Concepts of Chemistry

Question Bank – Part 1

Multiple Choice Questions (MCQs)

Section A: Basic Concepts of Chemistry MCQs

1. Chemistry is the study of:

A. Only living organisms
B. Matter and its transformations
C. Forces and motion
D. Energy only

Answer: B

2. Which of the following is NOT matter?

A. Water
B. Oxygen
C. Light
D. Carbon dioxide

Answer: C

3. Matter is defined as anything that:

A. Has colour only
B. Occupies space and has mass
C. Produces energy
D. Can be seen only

Answer: B

4. The branch of science dealing with composition and properties of substances is:

A. Physics
B. Chemistry
C. Biology
D. Mathematics

Answer: B

5. Ancient Indian philosopher who proposed the idea of tiny particles called Paramanu was:

A. Aryabhata
B. Kanada
C. Charaka
D. Sushruta

Answer: B

Section B: Matter and Its Classification

6. Which of the following is a pure substance?

A. Air
B. Milk
C. Sodium chloride
D. Soil

Answer: C

7. Which of the following is an element?

A. Water
B. Carbon dioxide
C. Oxygen
D. Sodium chloride

Answer: C

8. A compound differs from a mixture because:

A. It has variable composition
B. Its components are chemically combined
C. Its components retain properties
D. It can be separated physically

Answer: B

9. Air is an example of:

A. Element
B. Compound
C. Homogeneous mixture
D. Heterogeneous mixture

Answer: C

10. Which one is a heterogeneous mixture?

A. Salt solution
B. Air
C. Sugar solution
D. Sand and water

Answer: D

11. Brass is an example of:

A. Compound
B. Homogeneous mixture
C. Element
D. Heterogeneous mixture

Answer: B

12. The composition of a compound is:

A. Always variable
B. Fixed
C. Random
D. Dependent on source

Answer: B

13. Which method can separate components of a mixture?

A. Chemical reaction
B. Physical method
C. Nuclear reaction
D. Atomic conversion

Answer: B

Section C: States of Matter

14. The state of matter having maximum intermolecular force is:

A. Gas
B. Liquid
C. Solid
D. Plasma

Answer: C

15. Gases are highly compressible because:

A. They have high density
B. Particles are far apart
C. Particles are fixed
D. They have strong forces

Answer: B

16. Liquids have:

A. Fixed shape and volume
B. Fixed volume but no fixed shape
C. No volume
D. Fixed shape only

Answer: B

17. Conversion of liquid into gas is called:

A. Condensation
B. Freezing
C. Vaporisation
D. Sublimation

Answer: C

18. Gas changes into liquid by:

A. Melting
B. Condensation
C. Sublimation
D. Fusion

Answer: B

19. The particles in a solid:

A. Move freely
B. Are completely stationary
C. Vibrate around fixed positions
D. Have no attraction

Answer: C

Section D: SI Units and Measurement

20. The SI unit of mass is:

A. Gram
B. Kilogram
C. Pound
D. Milligram

Answer: B

21. The SI unit of temperature is:

A. Celsius
B. Fahrenheit
C. Kelvin
D. Joule

Answer: C

22. The SI unit of amount of substance is:

A. Mole
B. Gram
C. Atom
D. Kilogram

Answer: A

23. The SI unit of density is:

A. g/cm³
B. kg/m³
C. kg/L
D. g/L

Answer: B

24. One litre is equal to:

A. 10 mL
B. 100 mL
C. 1000 mL
D. 10000 mL

Answer: C

25. The instrument used for accurate transfer of a fixed volume of liquid is:

A. Measuring cylinder
B. Pipette
C. Beaker
D. Test tube

Answer: B

Section E: Scientific Notation & Significant Figures

26. The number 0.00450 has how many significant figures?

A. 2
B. 3
C. 4
D. 5

Answer: B

27. The number 1005 contains:

A. 2 significant figures
B. 3 significant figures
C. 4 significant figures
D. 5 significant figures

Answer: C

28. Scientific notation of 0.0000012 is:

A. 1.2 × 10⁶
B. 1.2 × 10⁻⁶
C. 12 × 10⁻⁶
D. 0.12 × 10⁻⁶

Answer: B

29. Significant figures in 5.00 are:

A. 1
B. 2
C. 3
D. 4

Answer: C

30. Accuracy refers to:

A. Closeness among repeated values
B. Closeness to true value
C. Number of digits
D. Unit conversion

Answer: B

Section F: Chemical Laws

31. Law of conservation of mass was proposed by:

A. Dalton
B. Lavoisier
C. Proust
D. Avogadro

Answer: B

32. According to law of conservation of mass:

A. Mass increases during reaction
B. Mass decreases during reaction
C. Mass remains constant
D. Mass disappears

Answer: C

33. Law of definite proportions was given by:

A. Dalton
B. Proust
C. Gay-Lussac
D. Avogadro

Answer: B

34. Water always contains hydrogen and oxygen in:

A. Variable ratio
B. Fixed ratio
C. Random ratio
D. Equal ratio

Answer: B

35. Law of multiple proportions was proposed by:

A. Dalton
B. Lavoisier
C. Bohr
D. Rutherford

Answer: A

36. Avogadro’s law is related to:

A. Mass
B. Gas volumes
C. Atomic structure
D. Density

Answer: B

Section G: Atomic Mass and Mole Concept

37. One atomic mass unit is based on:

A. Hydrogen atom
B. Oxygen atom
C. Carbon-12 atom
D. Nitrogen atom

Answer: C

38. One mole contains:

A. 6.022 × 10²³ particles
B. 6.022 × 10²² particles
C. 3.011 × 10²³ particles
D. 1 × 10²³ particles

Answer: A

39. Avogadro number is represented by:

A. M
B. NA
C. K
D. R

Answer: B

40. Molecular mass of H₂O is:

A. 16 u
B. 17 u
C. 18 u
D. 20 u

Answer: C

41. Formula mass is generally used for:

A. Molecular compounds
B. Ionic compounds
C. Gases only
D. Elements only

Answer: B

42. Molar mass is expressed in:

A. u
B. g mol⁻¹
C. kg only
D. mol only

Answer: B

Section H: Stoichiometry

43. A balanced chemical equation follows:

A. Law of multiple proportions
B. Conservation of mass
C. Avogadro’s law only
D. Gas law only

Answer: B

44. The reactant consumed first in a reaction is:

A. Excess reagent
B. Catalyst
C. Limiting reagent
D. Product

Answer: C

45. Percentage yield is calculated using:

A. Actual yield and theoretical yield
B. Mass only
C. Volume only
D. Temperature only

Answer: A

46. Empirical formula represents:

A. Actual molecular structure
B. Simplest ratio of atoms
C. Number of molecules
D. Total mass

Answer: B

47. Molecular formula is related to empirical formula by:

A. Addition
B. Multiplication by whole number
C. Division
D. Subtraction

Answer: B

Question Bank – Part 2

Advanced MCQs + Multiple Correct Questions + Assertion–Reason

(All questions are original and copyright-free)

Section A: Advanced MCQs

48. Which of the following is a chemical change?

A. Melting of ice
B. Dissolving sugar in water
C. Rusting of iron
D. Cutting of paper

Answer: C

49. The property that can be measured without changing the identity of a substance is:

A. Chemical property
B. Physical property
C. Nuclear property
D. Biological property

Answer: B

50. Which of the following is an example of a chemical property?

A. Density
B. Melting point
C. Colour
D. Flammability

Answer: D

51. The density of a substance depends on:

A. Mass only
B. Volume only
C. Ratio of mass to volume
D. Temperature only

Answer: C

52. If the mass of a substance is doubled and volume remains constant, density becomes:

A. Half
B. Double
C. Zero
D. Unchanged

Answer: B

53. The SI unit of volume is:

A. L
B. mL
C. m³
D. cm³

Answer: C

54. Which instrument is used to measure mass accurately in a laboratory?

A. Thermometer
B. Balance
C. Burette
D. Pipette

Answer: B

55. The number of significant figures in 0.005060 is:

A. 3
B. 4
C. 5
D. 6

Answer: C

Explanation:

Leading zeros are not significant, but zeros between and after decimal are significant.

56. The scientific notation of 450000 is:

A. 45 × 10⁴
B. 4.5 × 10⁵
C. 4.50 × 10⁶
D. 0.45 × 10⁵

Answer: B

57. The value of 1 nanometre is:

A. 10⁻³ m
B. 10⁻⁶ m
C. 10⁻⁹ m
D. 10⁹ m

Answer: C

58. Which of the following has the highest density?

A. Gas
B. Liquid
C. Solid
D. Plasma

Answer: C

59. The conversion of solid directly into gas is:

A. Fusion
B. Sublimation
C. Condensation
D. Evaporation

Answer: B

60. Which of the following is an ionic compound?

A. CO₂
B. CH₄
C. NaCl
D. H₂O

Answer: C

Atomic Theory and Laws

61. According to Dalton, atoms of the same element:

A. Have different masses
B. Have identical properties
C. Can be converted into other atoms
D. Are made of molecules

Answer: B

62. The discovery of isotopes proved that:

A. Atoms cannot combine
B. All atoms are identical
C. Atoms of same element may have different masses
D. Compounds have variable composition

Answer: C

63. The law of definite proportions supports that:

A. Compounds have fixed composition
B. Mixtures have fixed composition
C. Atoms disappear during reactions
D. Gases occupy no volume

Answer: A

64. Which law explains why CO and CO₂ have different oxygen ratios?

A. Conservation of mass
B. Multiple proportions
C. Definite proportions
D. Avogadro law

Answer: B

65. Equal volumes of gases under identical conditions contain equal numbers of:

A. Atoms
B. Molecules
C. Masses
D. Densities

Answer: B

Mole Concept Questions

66. Number of molecules in one mole of oxygen gas is:

A. 6.022 × 10²³
B. 3.011 × 10²³
C. 16
D. 32

Answer: A

67. One mole of carbon atoms has a mass of:

A. 6 g
B. 12 g
C. 18 g
D. 24 g

Answer: B

68. Number of moles present in 18 g water is:

(Molar mass of water = 18 g/mol)

A. 0.5
B. 1
C. 2
D. 18

Answer: B

69. The number of atoms in 1 mole of helium is:

A. 4
B. 6.022 × 10²³
C. 2
D. 1

Answer: B

70. Molecular mass of CO₂ is:

(C = 12, O = 16)

A. 28 u
B. 32 u
C. 44 u
D. 48 u

Answer: C

71. Formula mass of CaCl₂ is:

(Ca = 40, Cl = 35.5)

A. 71
B. 111
C. 75
D. 95

Answer: B

Empirical Formula and Stoichiometry

72. The empirical formula of benzene (C₆H₆) is:

A. CH
B. C₂H₂
C. C₆H₆
D. CH₂

Answer: A

73. The empirical formula of hydrogen peroxide (H₂O₂) is:

A. H₂O₂
B. HO
C. H₂O
D. OH₂

Answer: B

74. A balanced chemical equation obeys:

A. Conservation of mass
B. Conservation of volume only
C. Boyle’s law
D. Charles law

Answer: A

75. In the reaction:

$2H_2+O_2\rightarrow2H_2O$ 2H2+O2→2H2O

The mole ratio of H₂ : O₂ is:

A. 1:1
B. 2:1
C. 1:2
D. 2:2

Answer: B

76. The reactant present in excess after completion of reaction is called:

A. Catalyst
B. Limiting reagent
C. Excess reagent
D. Product

Answer: C

77. The maximum product predicted by calculations is called:

A. Actual yield
B. Theoretical yield
C. Percentage yield
D. Experimental error

Answer: B

Section B: Multiple Correct Answer Questions

(More than one option may be correct)

78. Which of the following are SI base quantities?

A. Mass
B. Length
C. Volume
D. Temperature

Answers: A, B, D

79. Which are examples of mixtures?

A. Air
B. Water
C. Brass
D. Sodium chloride

Answers: A, C

80. Which statements are true about compounds?

A. They have fixed composition.
B. They can be separated physically.
C. They contain chemically combined elements.
D. They have properties different from their elements.

Answers: A, C, D

81. Significant figures include:

A. Non-zero digits
B. Leading zeros
C. Captive zeros
D. Trailing zeros after decimal

Answers: A, C, D

82. Mole concept connects:

A. Mass
B. Number of particles
C. Amount of substance
D. Temperature only

Answers: A, B, C

83. Which are examples of physical properties?

A. Density
B. Colour
C. Burning ability
D. Melting point

Answers: A, B, D

Section C: Assertion–Reason Questions

Choose:

A. Both Assertion and Reason are true, and Reason correctly explains Assertion.
B. Both are true, but Reason does not explain Assertion.
C. Assertion true, Reason false.
D. Assertion false, Reason true.

84.

Assertion: Mass remains unchanged during a chemical reaction.
Reason: Atoms are rearranged but not destroyed.

Answer: A

85.

Assertion: Water has a fixed composition.
Reason: Hydrogen and oxygen combine in a definite ratio.

Answer: A

86.

Assertion: Gases are highly compressible.
Reason: Gas particles have large spaces between them.

Answer: A

87.

Assertion: Molecular mass and molar mass have the same numerical value.
Reason: Their units are identical.

Answer: C

88.

Assertion: NaCl has formula mass instead of molecular mass.
Reason: NaCl exists as an ionic lattice.

Answer: A

89.

Assertion: Empirical formula always represents the actual molecule.
Reason: It gives the simplest ratio of atoms.

Answer: D

90.

Assertion: Precision and accuracy are the same.
Reason: Both describe measurement quality.

Answer: D

Question Bank – Part 3

Fill in the Blanks + True/False + Match the Following + One Word Answers

(All questions are original and copyright-free)

Section A: Fill in the Blanks

1. Chemistry is the study of ______, its composition, properties and transformations.

Answer: Matter

2. Anything that has mass and occupies space is called ______.

Answer: Matter

3. The smallest unit of an element that participates in chemical reactions is called an ______.

Answer: Atom

4. The SI unit of mass is ______.

Answer: Kilogram

5. The SI unit of temperature is ______.

Answer: Kelvin

6. The SI unit of amount of substance is ______.

Answer: Mole

7. The SI unit of density is ______.

Answer: kg m⁻³

8. The conversion of solid into liquid is called ______.

Answer: Melting

9. The conversion of gas into liquid is called ______.

Answer: Condensation

10. The direct conversion of solid into gas is called ______.

Answer: Sublimation

Classification of Matter

11. A substance containing only one type of particle is called a ______ substance.

Answer: Pure

12. A pure substance that cannot be broken down into simpler substances is called an ______.

Answer: Element

13. Sodium chloride is an example of a ______.

Answer: Compound

14. Air is a ______ mixture.

Answer: Homogeneous

15. Sand and water form a ______ mixture.

Answer: Heterogeneous

16. The composition of a mixture is generally ______.

Answer: Variable

17. The composition of a compound is always ______.

Answer: Fixed

Measurement and Units

18. A measurement consists of a numerical value and a ______.

Answer: Unit

19. The International System of Units is abbreviated as ______.

Answer: SI

20. The SI unit of length is ______.

Answer: Metre

21. The SI unit of time is ______.

Answer: Second

22. One litre is equal to ______ millilitres.

Answer: 1000

23. Density is calculated by dividing mass by ______.

Answer: Volume

24. The instrument used for accurate transfer of liquid is ______.

Answer: Pipette

25. The instrument used for delivering variable volume of liquid is ______.

Answer: Burette

Significant Figures

26. Significant figures indicate the ______ of a measurement.

Answer: Precision

27. Leading zeros are ______ significant figures.

Answer: Not

28. Zeros between non-zero digits are ______.

Answer: Significant

29. The number 5.00 has ______ significant figures.

Answer: Three

30. Scientific notation is written in the form ______ × 10ⁿ.

Answer: N

Chemical Laws

31. The law of conservation of mass was proposed by ______.

Answer: Lavoisier

32. The law of definite proportions was proposed by ______.

Answer: Proust

33. The law of multiple proportions was given by ______.

Answer: Dalton

34. Equal volumes of gases contain equal numbers of molecules according to ______ law.

Answer: Avogadro’s

35. The law stating that mass remains constant during reaction is the law of ______.

Answer: Conservation of mass

Atomic Mass and Mole Concept

36. One atomic mass unit is represented by ______.

Answer: u

37. Atomic mass unit is based on the ______ atom.

Answer: Carbon-12

38. One mole contains ______ particles.

Answer: 6.022 × 10²³

39. Avogadro constant is represented by ______.

Answer: Nₐ

40. The mass of one mole of a substance is called ______.

Answer: Molar mass

41. Molecular mass is expressed in ______.

Answer: u

42. Molar mass is expressed in ______.

Answer: g mol⁻¹

43. The formula mass is generally used for ______ compounds.

Answer: Ionic

Formula and Stoichiometry

44. The simplest whole-number ratio of atoms in a compound is called the ______ formula.

Answer: Empirical

45. The actual number of atoms in a molecule is shown by the ______ formula.

Answer: Molecular

46. A chemical equation must be ______ before calculation.

Answer: Balanced

47. The reactant consumed completely is called the ______ reagent.

Answer: Limiting

48. The reactant left after reaction is called the ______ reagent.

Answer: Excess

49. The maximum amount of product calculated theoretically is called ______ yield.

Answer: Theoretical

50. Actual yield divided by theoretical yield gives ______ yield.

Answer: Percentage

Section B: True or False

1. Matter has mass and occupies space.

Answer: True

2. Light is a form of matter.

Answer: False

3. Solids have fixed shape and volume.

Answer: True

4. Gases have strong intermolecular forces.

Answer: False

5. Compounds can be separated by physical methods.

Answer: False

6. Mixtures have fixed composition.

Answer: False

7. SI unit of temperature is Kelvin.

Answer: True

8. Mass and weight are the same physical quantities.

Answer: False

9. Density is mass divided by volume.

Answer: True

10. Leading zeros are significant.

Answer: False

11. Carbon-12 is used as standard for atomic mass.

Answer: True

12. One mole contains 6.022 × 10²³ particles.

Answer: True

13. Molecular mass is expressed in grams per mole.

Answer: False

14. Molar mass and molecular mass have the same numerical value.

Answer: True

15. Empirical formula always shows the actual number of atoms.

Answer: False

16. Stoichiometry deals with quantitative relationships in reactions.

Answer: True

17. Limiting reagent determines the amount of product formed.

Answer: True

18. Dalton considered atoms indivisible.

Answer: True

19. Isotopes have different atomic numbers.

Answer: False

20. A balanced equation follows conservation of mass.

Answer: True

Section C: Match the Following

Set 1

Column A	Column B
1. Lavoisier	a. Mole concept
2. Proust	b. Conservation of mass
3. Avogadro	c. Definite proportions
4. Dalton	d. Atomic theory

Answer:

1 → b
2 → c
3 → a
4 → d

Set 2

Column A	Column B
1. Pipette	a. Mass measurement
2. Balance	b. Fixed volume transfer
3. Burette	c. Variable liquid delivery
4. Thermometer	d. Temperature

Answer:

1 → b
2 → a
3 → c
4 → d

Set 3

Column A	Column B
1. H₂O	a. Formula mass
2. NaCl	b. Molecular mass
3. CO₂	c. Compound
4. Air	d. Mixture

Answer:

1 → b
2 → a
3 → b
4 → d

Section D: One Word / One Term Answers

1. Scientist who proposed atomic theory.

Answer: Dalton

2. Scientist who discovered law of conservation of mass.

Answer: Lavoisier

3. Unit used for atomic mass.

Answer: u

4. Number of particles in one mole.

Answer: Avogadro number

5. Simplest formula of a compound.

Answer: Empirical formula

6. Substance that limits product formation.

Answer: Limiting reagent

7. SI unit of amount of substance.

Answer: Mole

8. Instrument used to measure temperature.

Answer: Thermometer

9. Gas law scientist Avogadro.

Answer: Avogadro

10. Mass per unit volume.

Answer: Density

Question Bank – Part 4

Short Answer Questions + Long Answer Questions + Conceptual Questions

(All questions are original and copyright-free)

Section A: Very Short Answer Questions (1 Mark)

1. What is chemistry?

Answer:
Chemistry is the branch of science that studies the composition, structure, properties and transformations of matter.

2. Define matter.

Answer:
Matter is anything that has mass and occupies space.

3. Name the three common states of matter.

Answer:
Solid, liquid and gas.

4. What is an element?

Answer:
An element is a pure substance made up of only one type of atom.

5. Give one example of a compound.

Answer:
Water (H₂O).

6. What is a mixture?

Answer:
A mixture is a physical combination of two or more substances in any proportion.

7. Define density.

Answer: $Density=\frac{Mass}{Volume}$ Density=VolumeMass

It is the mass present per unit volume of a substance.

8. What is the SI unit of density?

Answer: $kg\,m^{-3}$ kgm−3

9. Define atomic mass unit.

Answer:
One atomic mass unit is defined as one-twelfth of the mass of one carbon-12 atom.

10. What is Avogadro number?

Answer: $6.022\times10^{23}$ 6.022×1023

It represents the number of particles present in one mole.

11. Define mole.

Answer:
A mole is the amount of substance containing $6.022\times10^{23}$ 6.022×1023 particles.

12. What is molar mass?

Answer:
The mass of one mole of a substance is called molar mass.

13. Write the unit of molar mass.

Answer: $g\,mol^{-1}$ gmol−1

14. What is empirical formula?

Answer:
The simplest whole-number ratio of atoms present in a compound is called empirical formula.

15. What is limiting reagent?

Answer:
The reactant that gets completely consumed first during a reaction is called limiting reagent.

Section B: Short Answer Questions (2–3 Marks)

1. Differentiate between element and compound.

Answer:

Element	Compound
Contains one type of atom	Contains two or more elements
Cannot be broken chemically	Can be decomposed chemically
Example: Oxygen	Example: Water

2. Differentiate between mixture and compound.

Answer:

Mixture	Compound
Physical combination	Chemical combination
Variable composition	Fixed composition
Components retain properties	New properties are formed
Can be separated physically	Requires chemical methods

3. Explain law of conservation of mass.

Answer:

The law states that mass can neither be created nor destroyed during a chemical reaction.

Therefore: $Mass\ of\ reactants=Mass\ of\ products$ Mass of reactants=Mass of products

Example: $12gC+32gO_2=44gCO_2$ 12gC+32gO2=44gCO2

4. Explain law of definite proportions.

Answer:

This law states that a pure compound always contains the same elements combined in the same proportion by mass.

Example:

Water always contains hydrogen and oxygen in: $1:8$ 1:8

mass ratio.

5. Explain law of multiple proportions with example.

Answer:

When two elements form more than one compound, the masses of one element combining with a fixed mass of another are in simple whole-number ratios.

Example:

CO and CO₂:

Oxygen ratio: $16:32=1:2$ 16:32=1:2

6. Write the limitations of Dalton’s atomic theory.

Answer:

Atoms are not indivisible.
Same elements can have atoms with different masses (isotopes).
Atoms of different elements may have similar masses.

7. What are significant figures?

Answer:

Significant figures are meaningful digits in a measured quantity that indicate its precision.

Example:

5.00 has three significant figures.

8. Explain accuracy and precision.

Answer:

Accuracy: Closeness of a measurement to the true value.

Precision: Closeness among repeated measurements.

9. Calculate molecular mass of CO₂.

Given:

C = 12 u
O = 16 u

Solution:

$CO_2=12+2(16)$ CO2=12+2(16) $=44u$ =44u

10. Calculate formula mass of NaCl.

Given:

Na = 23 u
Cl = 35.5 u

Solution:

$NaCl=23+35.5$ NaCl=23+35.5 $=58.5u$ =58.5u

Section C: Conceptual Questions

1. Why is the atomic mass unit needed?

Answer:

Atoms have extremely small masses, which cannot be conveniently expressed in grams. Therefore, atomic mass unit (u) is used for comparing atomic masses.

2. Why does a compound have fixed composition?

Answer:

Elements in a compound combine in a definite number ratio of atoms, giving a fixed composition.

3. Why can mixtures have variable composition?

Answer:

Components of mixtures are physically combined and can exist in any proportion.

4. Why are gases highly compressible?

Answer:

Gas particles have large spaces between them, allowing compression.

5. Why does NaCl have formula mass instead of molecular mass?

Answer:

NaCl forms an ionic crystal lattice and does not exist as separate molecules. Therefore, formula mass is used.

6. Why is carbon-12 chosen as the standard for atomic masses?

Answer:

Because it is stable, accurately measurable and provides a convenient reference.

7. Why must chemical equations be balanced?

Answer:

A balanced equation follows the law of conservation of mass and ensures equal numbers of atoms on both sides.

Section D: Long Answer Questions (5 Marks)

1. Explain Dalton’s Atomic Theory.

Answer:

Dalton proposed atomic theory in 1808.

Main postulates:

Matter is made of tiny particles called atoms.
Atoms of the same element were considered identical.
Atoms of different elements have different properties.
Atoms combine in simple whole-number ratios to form compounds.
Chemical reactions involve rearrangement of atoms.

Limitations:

Atoms contain subatomic particles.
Isotopes exist.
Atoms of different elements may have similar masses.

2. Explain the mole concept.

Answer:

The mole is a counting unit used to express very large numbers of particles.

One mole contains: $6.022\times10^{23}$ 6.022×1023

particles.

It connects:

Atomic mass
Molecular mass
Laboratory measurements

Important formulas: $n=\frac{Mass}{Molar\ Mass}$ n=Molar MassMass $Number\ of\ particles=n\times6.022\times10^{23}$ Number of particles=n×6.022×1023

3. Explain steps for calculating empirical formula.

Answer:

Steps:

Convert percentage composition into grams.
Convert grams into moles.
Divide each mole value by the smallest value.
Obtain simplest whole-number ratio.
Write empirical formula.

4. Explain stoichiometry.

Answer:

Stoichiometry deals with quantitative relationships between reactants and products.

Steps:

Write balanced chemical equation.
Convert given quantities into moles.
Apply mole ratio.
Convert answer into required unit.

Applications:

Finding product amount
Finding required reactants
Determining limiting reagent

5. Explain limiting reagent with example.

Answer:

The reactant consumed completely first is the limiting reagent.

Example: $2H_2+O_2\rightarrow2H_2O$ 2H2+O2→2H2O

Two moles of hydrogen require one mole oxygen.

If hydrogen is insufficient, hydrogen becomes limiting reagent and controls product formation.

Section E: Reasoning-Based Questions

1. Why is the mass of products equal to mass of reactants?

Answer:
Because atoms are neither created nor destroyed during chemical reactions.

2. Why does 1 mole of different substances have different masses?

Answer:
Because different substances have different molar masses.

3. Why is molecular mass expressed in u while molar mass is expressed in g/mol?

Answer:
Molecular mass represents relative mass of one molecule, while molar mass represents mass of one mole.

4. Why does actual yield differ from theoretical yield?

Answer:
Due to incomplete reactions, side reactions and experimental losses.

Question Bank – Part 5

Numerical Problems with Step-by-Step Solutions

(All problems are original and copyright-free)

Section A: Measurement and Density Numericals

1. Calculate the density of a substance having mass 50 g and volume 25 cm³.

Solution:

Formula: $Density=\frac{Mass}{Volume}$ Density=VolumeMass

Given:

Mass = 50 g
Volume = 25 cm³ $Density=\frac{50}{25}$ Density=2550 $\boxed{Density=2g\,cm^{-3}}$ Density=2gcm−3

2. A metal piece has a mass of 78 g and volume of 10 cm³. Find its density.

Solution:

$Density=\frac{78}{10}$ Density=1078 $\boxed{Density=7.8g\,cm^{-3}}$ Density=7.8gcm−3

3. A liquid has density 1.5 g/cm³. Calculate its mass if volume is 200 cm³.

Solution:

Formula: $Mass=Density\times Volume$ Mass=Density×Volume $=1.5\times200$ =1.5×200 $\boxed{Mass=300g}$ Mass=300g

Section B: Significant Figures

4. Count the significant figures in the following:

(a) 0.00450

Solution:

Leading zeros are not significant.

Significant digits:

4, 5, 0 $\boxed{3\ significant\ figures}$ 3 significant figures

(b) 2050

The zero between non-zero digits is significant. $\boxed{4\ significant\ figures}$ 4 significant figures

(c) 7.080

Digits:

7,0,8,0 $\boxed{4\ significant\ figures}$ 4 significant figures

Section C: Mole Concept Numericals

5. Calculate the number of moles present in 36 g of water.

Given:

Molar mass of H₂O = 18 g/mol

Formula: $n=\frac{Mass}{Molar\,mass}$ n=MolarmassMass $n=\frac{36}{18}$ n=1836 $\boxed{n=2mol}$ n=2mol

6. Calculate the number of molecules present in 2 moles of oxygen gas.

Given: $N_A=6.022\times10^{23}$ NA=6.022×1023

Formula: $Number\ of\ molecules=n\times N_A$ Number of molecules=n×NA $=2\times6.022\times10^{23}$ =2×6.022×1023 $\boxed{1.2044\times10^{24}\ molecules}$ 1.2044×1024 molecules

7. Find the mass of 0.5 mole of carbon dioxide.

Given:

Molar mass of CO₂: $=12+2(16)$ =12+2(16) $=44g/mol$ =44g/mol

Formula: $Mass=n\times Molar\,mass$ Mass=n×Molarmass $=0.5\times44$ =0.5×44 $\boxed{22g}$ 22g

8. How many atoms are present in 1 mole of helium?

Solution:

1 mole contains: $6.022\times10^{23}$ 6.022×1023

atoms.

Answer: $\boxed{6.022\times10^{23}\ atoms}$ 6.022×1023 atoms

Section D: Molecular Mass and Formula Mass

9. Calculate molecular mass of sulphuric acid (H₂SO₄).

Atomic masses:

H = 1
S = 32
O = 16

Solution: $H_2SO_4$ H2SO4 $=2(1)+32+4(16)$ =2(1)+32+4(16) $=2+32+64$ =2+32+64 $\boxed{98u}$ 98u

10. Calculate molecular mass of ammonia (NH₃).

Atomic masses:

N = 14
H = 1 $NH_3=14+3(1)$ NH3=14+3(1) $\boxed{17u}$ 17u

11. Calculate formula mass of calcium carbonate (CaCO₃).

Atomic masses:

Ca = 40
C = 12
O = 16 $CaCO_3=40+12+3(16)$ CaCO3=40+12+3(16) $=40+12+48$ =40+12+48 $\boxed{100u}$ 100u

Section E: Percentage Composition

12. Calculate percentage of oxygen in water.

Molecular mass of water: $H_2O=18$ H2O=18

Mass of oxygen: $=16$ =16

Formula: $\%O= \frac{16}{18}\times100$ %O=1816×100 $\boxed{88.89\%}$ 88.89%

13. Calculate percentage of carbon in CO₂.

Molecular mass: $CO_2=44$ CO2=44

Carbon mass: $=12$ =12 $\%C= \frac{12}{44}\times100$ %C=4412×100 $\boxed{27.27\%}$ 27.27%

14. Calculate percentage of hydrogen in methane (CH₄).

Molar mass: $=12+4(1)$ =12+4(1) $=16$ =16

Hydrogen mass: $=4$ =4 $\%H= \frac4{16}\times100$ %H=164×100 $\boxed{25\%}$ 25%

Section F: Empirical Formula Problems

15. Find empirical formula of a compound containing:

Carbon = 40%
Hydrogen = 6.67%
Oxygen = 53.33%

Step 1: Assume 100 g compound

C = 40 g
H = 6.67 g
O = 53.33 g

Step 2: Convert into moles

Carbon: $\frac{40}{12}=3.33$ 1240=3.33

Hydrogen: $\frac{6.67}{1}=6.67$ 16.67=6.67

Oxygen: $\frac{53.33}{16}=3.33$ 1653.33=3.33

Step 3: Divide by smallest value

C: $3.33/3.33=1$ 3.33/3.33=1

H: $6.67/3.33=2$ 6.67/3.33=2

O: $3.33/3.33=1$ 3.33/3.33=1

Ratio: $1:2:1$ 1:2:1

Empirical formula: $\boxed{CH_2O}$ CH2O

Section G: Molecular Formula Problems

16. Find molecular formula if:

Empirical formula = CH₂O

Molar mass = 180 g/mol

Empirical formula mass: $12+2+16=30$ 12+2+16=30

Calculate n: $n=\frac{180}{30}$ n=30180 $n=6$ n=6

Molecular formula: $(CH_2O)_6$ (CH2O)6 $\boxed{C_6H_{12}O_6}$ C6H12O6

Section H: Stoichiometry Problems

17. Calculate mass of water formed when 4 g hydrogen reacts completely with oxygen.

Reaction: $2H_2+O_2\rightarrow2H_2O$ 2H2+O2→2H2O

Molar masses:

H₂ = 2 g/mol

H₂O = 18 g/mol

From equation:

2 moles H₂ produce 2 moles H₂O

So:

2 g H₂ produces 18 g H₂O

Therefore:

4 g H₂ produces: $\frac{18}{2}\times4$ 218×4 $\boxed{36g\ H_2O}$ 36g H2O

18. Calculate CO₂ produced when 12 g carbon burns completely.

Reaction: $C+O_2\rightarrow CO_2$ C+O2→CO2

12 g carbon produces 44 g CO₂.

Therefore: $\boxed{44g\ CO_2}$ 44g CO2

Section I: Limiting Reagent Problems

19. In the reaction:

$2H_2+O_2\rightarrow2H_2O$ 2H2+O2→2H2O

5 moles H₂ react with 2 moles O₂.

Find limiting reagent.

Requirement:

2 mol H₂ needs 1 mol O₂.

For 5 mol H₂:

Required O₂: $=2.5mol$ =2.5mol

Available O₂: $=2mol$ =2mol

O₂ is insufficient.

Therefore: $\boxed{O_2\ is\ the\ limiting\ reagent}$ O2 is the limiting reagent

Section J: Percentage Yield

20. The theoretical yield of a product is 50 g and actual yield is 40 g. Calculate percentage yield.

Formula: $\%Yield= \frac{Actual}{Theoretical}\times100$ %Yield=TheoreticalActual×100 $=\frac{40}{50}\times100$ =5040×100 $\boxed{80\%}$ 80%

Numerical Practice Summary

Students should master:

✅ Density calculations
✅ Significant figures
✅ Mole conversions
✅ Particle calculations
✅ Molecular mass
✅ Formula mass
✅ Percentage composition
✅ Empirical formula
✅ Molecular formula
✅ Stoichiometry
✅ Limiting reagent
✅ Percentage yield

Question Bank – Part 6 (Final)

NEET/JEE Foundation Level Questions + Case-Based Questions + Mock Test

(All questions are original and copyright-free)

Section A: Higher Order Thinking MCQs

1. Two samples of carbon dioxide obtained from different sources contain carbon and oxygen in the same ratio. This supports:

A. Law of multiple proportions
B. Law of definite proportions
C. Law of conservation of energy
D. Avogadro’s law

Answer: B

2. A compound contains 40% carbon, 6.67% hydrogen and 53.33% oxygen. Its empirical formula is:

A. CHO
B. CH₂O
C. C₂H₄O₂
D. C₆H₁₂O₆

Answer: B

3. The number of atoms present in 4 g of helium is:

(Atomic mass of He = 4 g/mol)

A. $6.022\times10^{23}$ 6.022×1023
B. $3.011\times10^{23}$ 3.011×1023
C. $12.044\times10^{23}$ 12.044×1023
D. 4

Answer: A

4. Which contains the maximum number of molecules?

A. 18 g H₂O
B. 44 g CO₂
C. 32 g O₂
D. All contain equal number of molecules

Answer: D

Explanation:
Each represents 1 mole.

5. The mass of $3.011\times10^{23}$ 3.011×1023 molecules of oxygen gas is:

(O₂ = 32 g/mol)

A. 8 g
B. 16 g
C. 32 g
D. 64 g

Answer: B

6. Which has the greatest number of atoms?

A. 1 mole He
B. 1 mole H₂
C. 1 mole O₂
D. 1 mole CO₂

Answer: D

Explanation:
CO₂ contains 3 atoms per molecule.

7. The ratio of atoms in H₂SO₄ is:

A. 2:1:4
B. 1:2:4
C. 2:4:1
D. 4:2:1

Answer: A

8. The molecular formula of a compound with empirical formula CH and molecular mass 78 is:

(C = 12, H = 1)

A. CH
B. C₂H₂
C. C₆H₆
D. C₇H₇

Answer: C

9. A reaction produces less product than calculated theoretically because:

A. Atoms disappear
B. Reaction may not be complete
C. Mass is destroyed
D. Law of conservation fails

Answer: B

10. The limiting reagent is important because it:

A. Increases reaction speed
B. Determines maximum product formed
C. Acts as catalyst
D. Remains unchanged

Answer: B

Section B: Multi-Step Numerical Problems

11. Calculate the number of oxygen atoms in 18 g water.

Given:

Molar mass of water = 18 g/mol

Step 1:

18 g water = 1 mole water molecules

Number of molecules: $6.022\times10^{23}$ 6.022×1023

Each molecule contains one oxygen atom.

Therefore: $\boxed{6.022\times10^{23}\ oxygen\ atoms}$ 6.022×1023 oxygen atoms

12. Calculate the number of hydrogen atoms in 18 g water.

One molecule of water contains 2 hydrogen atoms.

Number of hydrogen atoms: $2\times6.022\times10^{23}$ 2×6.022×1023 $\boxed{1.2044\times10^{24}\ atoms}$ 1.2044×1024 atoms

13. How many moles are present in 98 g sulphuric acid?

Molar mass: $H_2SO_4=98g/mol$ H2SO4=98g/mol

Formula: $n=\frac{98}{98}$ n=9898 $\boxed{1mol}$ 1mol

14. Find mass of 3 moles of sodium hydroxide.

Molar mass:

NaOH: $23+16+1=40g/mol$ 23+16+1=40g/mol

Mass: $=3\times40$ =3×40 $\boxed{120g}$ 120g

15. Calculate percentage of nitrogen in NH₃.

Molar mass: $NH_3=14+3=17$ NH3=14+3=17

Percentage nitrogen: $=\frac{14}{17}\times100$ =1714×100 $\boxed{82.35\%}$ 82.35%

Section C: Case-Based Questions

Case Study 1: Water Analysis

A student analyses pure water and finds that hydrogen and oxygen are always present in a fixed ratio.

Questions:

16. Which law explains this observation?

A. Multiple proportions
B. Definite proportions
C. Avogadro law
D. Gas volume law

Answer: B

17. The mass ratio of hydrogen and oxygen in water is:

A. 1:1
B. 1:4
C. 1:8
D. 2:1

Answer: C

18. The formula of water represents:

A. Variable composition
B. Fixed atomic ratio
C. Mixture composition
D. Random combination

Answer: B

Case Study 2: Chemical Reaction

A reaction is represented as: $2H_2+O_2\rightarrow2H_2O$ 2H2+O2→2H2O

19. The mole ratio of hydrogen to oxygen is:

A. 1:1
B. 2:1
C. 1:2
D. 2:2

Answer: B

20. If 4 moles hydrogen react completely, oxygen required is:

A. 1 mole
B. 2 moles
C. 3 moles
D. 4 moles

Answer: B

21. This equation follows:

A. Law of conservation of mass
B. Boyle’s law
C. Charles law
D. Graham’s law

Answer: A

Section D: Competitive Concept Questions

22. Which statement is correct about one mole of any gas at the same temperature and pressure?

A. It has same mass
B. It has same number of molecules
C. It has same density
D. It has same volume always

Answer: B

23. 1 mole of NaCl contains:

A. $6.022\times10^{23}$ 6.022×1023 atoms
B. $6.022\times10^{23}$ 6.022×1023 formula units
C. 1 molecule
D. 1 atom

Answer: B

24. Which quantity remains unchanged when temperature changes?

A. Volume
B. Density
C. Mass
D. Pressure

Answer: C

25. If empirical formula mass is 30 and molecular mass is 90, value of n is:

A. 1
B. 2
C. 3
D. 4

Solution: $n=\frac{90}{30}=3$ n=3090=3

Answer: C

Section E: Chapter Mock Test

Choose the correct option.

26. The number of significant figures in 0.02030 is:

A. 2
B. 3
C. 4
D. 5

Answer: C

27. Formula mass of MgCl₂ is:

(Mg = 24, Cl = 35.5)

A. 59.5
B. 95
C. 71
D. 106

Answer: B

28. The simplest formula of glucose (C₆H₁₂O₆) is:

A. CHO
B. CH₂O
C. C₂H₄O₂
D. C₆H₁₂O₆

Answer: B

29. The scientist associated with atomic theory is:

A. Dalton
B. Newton
C. Einstein
D. Bohr

Answer: A

30. The mass of one mole of oxygen molecules is:

A. 16 g
B. 18 g
C. 32 g
D. 64 g

Answer: C