Questions on Law of Definite Proportions
Multiple-Choice Questions: Law of Definite Proportions
1. What does the Law of Definite Proportions state?
A) Elements react in pairs of two
B) A compound is made of elements in a fixed mass ratio
C) Mass is neither created nor destroyed
D) Gases occupy definite volume
E) Reactions must produce energy
2. Who is credited with discovering the Law of Definite Proportions?
A) John Dalton
B) Antoine Lavoisier
C) Joseph Proust
D) Robert Boyle
E) J.J. Thomson
3. The Law of Definite Proportions is also known as:
A) Law of Equal Masses
B) Proust's Law
C) Dalton's Principle
D) Law of Compound Masses
E) Mendeleev’s Law
4. According to the law, water always contains:
A) 2 parts hydrogen and 3 parts oxygen
B) 2 grams of hydrogen and 1 gram of oxygen
C) 1 gram of hydrogen and 8 grams of oxygen
D) Equal parts hydrogen and oxygen
E) 100% oxygen
5. The Law of Definite Proportions applies to:
A) All mixtures
B) All elements
C) Pure compounds
D) Noble gases
E) All solutions
6. If a compound is always made of 40g calcium and 60g chlorine, what does this illustrate?
A) Law of Conservation of Mass
B) Law of Definite Proportions
C) Law of Gases
D) Law of Multiple Proportions
E) Law of Radioactivity
7. A compound contains 80g of oxygen and 20g of hydrogen. What is the mass ratio?
A) 1:4
B) 4:1
C) 2:1
D) 8:1
E) 1:1
8. What must remain constant in all samples of a pure compound?
A) Volume
B) Density
C) Elemental composition by mass
D) Color
E) Taste
9. Which of the following does not follow the Law of Definite Proportions?
A) CO₂
B) H₂O
C) NaCl
D) Air
E) H₂O₂
10. The Law of Definite Proportions is not applicable to:
A) Water
B) Oxygen gas
C) Iron(III) oxide
D) Table salt
E) Baking soda
11. A sample of MgO contains 24g of Mg and 16g of O. Another sample has 12g of Mg. How much O should it have?
A) 4g
B) 6g
C) 8g
D) 10g
E) 12g
12. If one sample of CO₂ has 12g carbon and 32g oxygen, and another has 24g carbon, how much oxygen should it have?
A) 24g
B) 48g
C) 64g
D) 12g
E) 36g
13. Which statement best supports the Law of Definite Proportions?
A) All elements are made of atoms
B) The same compound always contains the same elements in the same proportion by mass
C) Atoms are indivisible
D) Energy is conserved
E) Gases expand to fill containers
14. If a sample of water is decomposed and gives 2g of hydrogen, how much oxygen is produced?
A) 4g
B) 6g
C) 8g
D) 10g
E) 12g
15. What happens to the ratio of elements in a compound if it obeys the Law of Definite Proportions?
A) The ratio changes with mass
B) The ratio depends on sample size
C) The ratio stays the same
D) The ratio increases with temperature
E) The ratio varies with pressure
16. Why is the Law of Definite Proportions important?
A) It helps identify unknown gases
B) It proves that air is a compound
C) It distinguishes compounds from mixtures
D) It shows atoms are indestructible
E) It helps measure atomic number
17. A compound has 3g of H and 24g of O. What's the H:O mass ratio?
A) 1:8
B) 1:6
C) 1:3
D) 1:2
E) 3:1
18. A new sample of the same compound has 6g H. How much oxygen should it contain to follow the law?
A) 36g
B) 42g
C) 48g
D) 24g
E) 18g
19. Which concept is confirmed by the Law of Definite Proportions?
A) Mixtures are homogeneous
B) Chemical reactions produce light
C) Compounds have fixed ratios
D) Atoms can be split
E) Energy is always absorbed
20. Which of the following statements is true regarding this law?
A) All compounds contain hydrogen
B) All elements form compounds
C) Every compound has a variable composition
D) A pure compound always has the same mass ratio of elements
E) Elements cannot combine
Answer Key with Explanations
1. B – The law states compounds always have the same elements in fixed ratios by mass.
2. C – Joseph Proust proposed this in the early 1800s.
3. B – It’s also called Proust’s Law.
4. C – Water has a mass ratio of 1g H to 8g O.
5. C – It applies only to pure compounds.
6. B – Repeated ratios of 40:60 show the law in action.
7. B – 80:20 simplifies to 4:1.
8. C – The mass ratio remains fixed in a compound.
9. D – Air is a mixture, not a compound.
10. B – Elements like O₂ don’t follow the law (not a compound).
11. C – 24:16 = 12:8, so 8g of O matches the ratio.
12. B – If 12g C needs 32g O, 24g C needs 64g O.
13. B – The essence of the law: same compound = same ratio.
14. C – In water, the ratio is 1:8, so 2g H → 16g O ÷ 2 = 8g.
15. C – The ratio stays constant regardless of the sample size.
16. C – The law differentiates compounds (fixed ratio) from mixtures (variable ratio).
17. A – 3g:24g = 1:8.
18. C – Doubling H to 6g means doubling O to 48g (to keep 1:8).
19. C – The law confirms compounds consist of elements in fixed mass ratios.
20. D – True: a pure compound has a constant composition.
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