One water, one ratio
The Law of Constant Proportions.
Water from three places
Water is collected from a river, a borewell and the ocean. Each sample is purified and then decomposed into hydrogen and oxygen.
How do you think the mass ratio of hydrogen to oxygen compares in the three samples?
What this lesson covers
The idea
In any compound the elements are always combined in a fixed ratio by mass, whatever its source or method of preparation (Law of Constant or Definite Proportions, Proust's Law).
Water from three places
Water is collected from a river, a borewell and the ocean. Each sample is purified and then decomposed into hydrogen and oxygen.
How do you think the mass ratio of hydrogen to oxygen compares in the three samples?
- The same in all three
- Different in each, because the sources differ
- Ocean water gives the most oxygen
Analyse the water
Pick the source and the mass of purified water that is decomposed.
A fixed ratio by mass
If 9 g of purified water from any source is decomposed, 1 g of hydrogen and 8 g of oxygen are obtained. The ratio 1:8 does not depend on where the water came from.
In any compound the elements are combined in a fixed ratio by mass, whatever its source or method of preparation. This is the Law of Constant Proportions (also called the Law of Definite Proportions, or Proust's Law).
Notes
A compound has its elements in a fixed ratio by mass, whatever the source. Water: hydrogen to oxygen = 1:8. This is Proust's Law.
Check yourself
Water from a borewell is purified and 36 g of it is decomposed. How many g of hydrogen are obtained? (Hydrogen and oxygen are in the mass ratio 1:8.)
Answer: 4 g
36 g is 4 lots of 9 g. Each 9 g gives 1 g of hydrogen, so 4 g of hydrogen (and 32 g of oxygen).
Water from the ocean and water from a river are each purified and analysed. What is the mass ratio of hydrogen to oxygen?
Sodium chloride contains sodium and chlorine in the mass ratio 23:35.5. How many g of chlorine are needed to react completely with 46 g of sodium?
Answer: 71 g
46 g is 2 times 23 g, so chlorine needed is 2 times 35.5 g = 71 g.
Who proposed that the elements in a compound combine in a fixed ratio by mass?
- More oxygen in ocean water. The ratio does not depend on the source. Purified water from any source has hydrogen and oxygen in the mass ratio 1:8.
- More hydrogen in river water. The ratio does not depend on the source. It is 1:8 for river water and for ocean water.
- The same, 1:8, in both — correct. Yes!
- Joseph Proust — correct. Yes!
- Antoine Lavoisier. Lavoisier proposed the Law of Conservation of Mass, about the total mass before and after a reaction.
- John Dalton. Dalton's atomic theory was built on the two laws, after Proust.