Law of Constant Proportions – Long Answer Questions
Medium Level (Application & Explanation)
Q1. State the Law of Constant Proportions. Explain it with water and ammonia as examples.
Answer:
The Law of Constant Proportions says a compound always has the same elements in a fixed ratio by mass.
This ratio does not change with the source, method of preparation, or amount.
In water (H₂O), hydrogen and oxygen are always in the 1:8 mass ratio.
So, if hydrogen is 2 g, oxygen will be 16 g, and the ratio stays 1:8.
In ammonia (NH₃), nitrogen and hydrogen are in the 14:3 mass ratio.
Thus, compounds have a definite composition, which does not vary.
Q2. Use the law to calculate oxygen needed for given hydrogen to make water. Explain your steps.
Answer:
In water, hydrogen:oxygen = 1:8 by mass.
If hydrogen is 3 g, oxygen needed = 3 × 8 = 24 g.
If hydrogen is 5 g, oxygen needed = 5 × 8 = 40 g.
If total water is 18 g, hydrogen = 2 g and oxygen = 16 g (still 1:8).
These examples show the fixed proportion holds in every case.
The numbers change, but the ratio by mass stays the same.
Q3. Who proposed this law? Describe his observation and its importance.
Answer:
The law was proposed by Joseph Proust.
He observed that compounds from different places or methods had the same composition.
This means their elements were in a constant mass ratio.
For example, water from a river and water made in a lab both show 1:8 for H:O.
His work proved that compounds are not random; they are definite in composition.
This idea became a foundation for modern chemistry.
Q4. Which parts of Dalton’s Atomic Theory support the law? Explain clearly.
Answer:
Dalton said matter is made of atoms which are tiny and indivisible in chemical reactions.
Atoms of the same element have the same mass and properties.
Atoms of different elements have different masses.
Atoms combine in small whole number ratios to form compounds.
So, the number of each atom in a compound is fixed, giving a fixed mass ratio.
Hence, Dalton’s 5th and 6th postulates explain the Law of Constant Proportions.
Q5. Two samples of ammonia are tested. Sample A has N = 14 g and H = 3 g. Sample B has N = 28 g and H = 6 g. Do they follow the law? Explain.
Answer:
For Sample A, the mass ratio N:H = 14:3.
For Sample B, the mass ratio N:H = 28:6.
Simplify 28:6 by dividing by 2 to get 14:3.
Both samples show the same 14:3 mass ratio.
This proves ammonia has a constant composition.
So yes, both follow the Law of Constant Proportions.
High Complexity (Analysis & Scenario-Based)
Q6. River water and laboratory water give these results: Sample 1 has H = 2 g and O = 16 g; Sample 2 has H = 4 g and O = 32 g. Analyze whether the law holds.
Answer:
In Sample 1, H:O = 2:16 which simplifies to 1:8.
In Sample 2, H:O = 4:32 which also simplifies to 1:8.
Both water samples show the same mass ratio.
The source (river or lab) does not change the composition.
This supports the Law of Constant Proportions.
Water always has hydrogen and oxygen in 1:8 by mass.
Q7. A new compound X is tested from two sources. Sample A: element A = 12 g, element B = 32 g. Sample B: element A = 9 g, element B = 24 g. Are both samples of the same compound? Justify.
Answer:
For Sample A, A:B = 12:32. Divide by 4 to get 3:8.
For Sample B, A:B = 9:24. Divide by 3 to get 3:8.
Both give the same 3:8 mass ratio.
This means the composition is the same in both samples.
So, both are likely the same compound X.
If results were different, we would suspect impurities or a different compound.
Q8. A reaction gives: 5.3 g sodium carbonate + 6 g acetic acid → 8.2 g sodium acetate + 2.2 g CO₂ + 0.9 g H₂O. Explain how this supports atomic theory and related laws.
Answer:
Total reactants = 5.3 + 6 = 11.3 g.
Total products = 8.2 + 2.2 + 0.9 = 11.3 g.
So, mass is conserved in the reaction.
This agrees with Dalton’s 2nd postulate: atoms are not created or destroyed.
Each compound formed also has a definite composition.
Thus, the data supports both the Law of Conservation of Mass and Constant Proportions.
Q9. You discover a new compound. Describe a plan to test if it follows the Law of Constant Proportions.
Answer:
Collect samples from different sources and methods of preparation.
Accurately measure masses of each element in the compound.
Calculate the mass ratio of the elements for each sample.
Simplify the ratios and compare them across all samples.
If ratios match, the compound shows a fixed composition.
If ratios differ, check for experimental error or impurities.
Q10. A student says, “Sea water and rain water give different water composition.” Use the law and reasoning to respond.
Answer:
The compound water (H₂O) always has H:O = 1:8 by mass.
Sea water and rain water may have dissolved salts or gases.
But the water molecules themselves stay H₂O with the same ratio.
If measured properly, pure water from any source keeps 1:8.
Differences come from impurities, not from water’s composition.
Thus, the Law of Constant Proportions still holds true.
