+20 Questions on Law of Constant Proportions
Here are 20 multiple choice questions on the Law of Constant Proportions (also known as Proust’s Law), ideal for middle and high school students studying basic chemistry. The answers are listed at the end.
Multiple Choice Questions
1. What does the Law of Constant Proportions state?
A) Compounds are made of different proportions of elements
B) A compound always contains the same elements in the same ratio by mass
C) Elements combine in random ratios
D) Elements are destroyed in chemical reactions
2. Who formulated the Law of Constant Proportions?
A) John Dalton
B) Antoine Lavoisier
C) Joseph Proust
D) Dmitri Mendeleev
3. Water (H₂O) always contains hydrogen and oxygen in a mass ratio of:
A) 2:16
B) 1:8
C) 8:1
D) 2:1
4. Which of the following is an example of the Law of Constant Proportions?
A) A compound of hydrogen and oxygen varies in composition
B) Every sample of pure CO₂ has carbon and oxygen in the same mass ratio
C) The ratio of sodium and chlorine in salt changes with temperature
D) Water can contain any amount of oxygen
5. Which of these statements violates the Law of Constant Proportions?
A) H₂O always has 2 hydrogen atoms and 1 oxygen atom
B) CO₂ from different sources has a 1:2 carbon to oxygen atom ratio
C) A compound's composition changes with time
D) Pure NaCl has the same ratio of sodium to chlorine
6. Why does the Law of Constant Proportions apply only to pure compounds?
A) Mixtures have no atoms
B) Mixtures are not chemically combined
C) Compounds don't follow chemical laws
D) Pure compounds can be separated physically
7. In CO₂, the mass of carbon is 12 g and oxygen is 32 g. What is the mass ratio of carbon to oxygen?
A) 1:1
B) 3:8
C) 12:32
D) 1:2.67
8. According to the Law of Constant Proportions, how will 36 g of water decompose?
A) 4 g hydrogen, 32 g oxygen
B) 2 g hydrogen, 34 g oxygen
C) 36 g hydrogen
D) 36 g oxygen
9. The Law of Constant Proportions supports the idea that:
A) All substances are mixtures
B) Compounds have fixed chemical formulas
C) Elements can form any compound randomly
D) Mass is never conserved
10. Which of these best demonstrates constant proportion in a compound?
A) Varying amounts of sugar in tea
B) Equal parts iron and sulfur in iron sulfide
C) Identical hydrogen-to-oxygen ratio in all water samples
D) Different samples of salt water have different salt amounts
11. Which pair of substances violates the Law of Constant Proportions?
A) H₂O and H₂O₂
B) CO and CO₂
C) NaCl and NaCl
D) NH₃ and NH₃
12. If 40g of a compound contains 10g of element A, what percent is element A?
A) 25%
B) 50%
C) 20%
D) 10%
13. Two samples of carbon dioxide are analyzed. One has 3g of carbon and 8g of oxygen. The other has 6g of carbon. How much oxygen will it contain?
A) 12g
B) 16g
C) 6g
D) 8g
14. How does the Law of Constant Proportions support chemical formulas?
A) It allows changes in formula
B) It ensures variable compositions
C) It proves elements combine in fixed ratios
D) It makes formulas unnecessary
15. Which is a correct interpretation of Proust’s Law?
A) A compound's components vary by source
B) Element ratios in compounds are always fixed
C) Elements cannot form compounds
D) Atoms are randomly arranged
16. Why is water always H₂O and not H₂O₃ or H₂O₂ in natural conditions?
A) Atoms cannot bond
B) H₂O is the most stable form
C) Water changes shape
D) Mass changes with temperature
17. Which statement is false regarding the Law of Constant Proportions?
A) It applies to chemical compounds
B) It does not apply to mixtures
C) It proves atoms rearrange in reactions
D) It allows different proportions in one compound
18. Which compound obeys the Law of Constant Proportions?
A) Milk
B) Salt (NaCl)
C) Air
D) Sugar solution
19. What is the mass ratio of hydrogen to oxygen in water (H₂O)?
A) 1:16
B) 2:1
C) 1:8
D) 8:1
20. Which statement best summarizes the Law of Constant Proportions?
A) Elements in compounds combine randomly
B) Every pure compound has a fixed composition
C) Elements disappear during reactions
D) Matter is destroyed in chemical changes
Answer Key
1. B) A compound always contains the same elements in the same ratio by mass
2. C) Joseph Proust
3. B) 1:8
4. B) Every sample of pure CO₂ has carbon and oxygen in the same mass ratio
5. C) A compound's composition changes with time
6. B) Mixtures are not chemically combined
7. D) 1:2.67
8. A) 4 g hydrogen, 32 g oxygen
9. B) Compounds have fixed chemical formulas
10. C) Identical hydrogen-to-oxygen ratio in all water samples
11. A) H₂O and H₂O₂
12. A) 25%
13. B) 16g
14. C) It proves elements combine in fixed ratios
15. B) Element ratios in compounds are always fixed
16. B) H₂O is the most stable form
17. D) It allows different proportions in one compound
18. B) Salt (NaCl)
19. C) 1:8
20. B) Every pure compound has a fixed composition
Dive into the groundbreaking science of neurotransmitters—your brain’s invisible architects—in Chemical Harmony: How Neurotransmitters Shape Our Lives (2025). This meticulously researched book reveals how serotonin, dopamine, GABA, and other brain chemicals silently orchestrate every aspect of your existence, from decision-making and relationships to mental health and emotional resilience.Click here to buy
👉 +3000 Biology Questions with Answers Key.
Share Online!
