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Water from a river, a well or the ocean — purify any of it and you always find hydrogen and oxygen in the mass ratio 1:8. A compound always sticks to its recipe.
- In any compound, the elements combine in a fixed ratio by mass .
- This holds whatever the source of the compound.
- Water is always H : O = 1 : 8 by mass — 9 g water gives 1 g hydrogen and 8 g oxygen.
- Also called the Law of Definite Proportions or Proust’s Law .
- A prominent French chemist known for careful experimental work.
- Showed that compounds always contain elements in fixed ratios by mass .
- Studied copper carbonate and found the same proportion however it was prepared.
In many ancient civilisations a red pigment from rocks — known in India as hingula and elsewhere as cinnabar — was used in painting. Heating cinnabar yields two elements, mercury and sulfur, in a mass percentage of about 86.22% and 13.78%. Grinding mercury and sulfur in this same ratio can reform cinnabar, though the toxicity of both kept the process from becoming widespread.
Sodium chloride (NaCl) contains sodium and chlorine in the mass ratio 23 : 35.5. If 46 g of sodium reacts fully, how much chlorine is needed?
Chlorine required \(= \dfrac{35.5}{23}\times 46 = 71\ \text{g}\).
- 3. Compound is 40% sulfur, 60% oxygen by mass, i.e. S : O = 40 : 60 = 2 : 3. For 20 g sulfur, oxygen \(= \dfrac{3}{2}\times 20 = \mathbf{30}\ \text{g}\).
- 4. CO has C : O = 3 : 4. For 9 g carbon, oxygen \(= \dfrac{4}{3}\times 9 = \mathbf{12}\ \text{g}\).
- 5. A mixture can be combined in any ratio, but a compound has a fixed composition — so the law holds only for compounds.
- 6. Ratios 4:1 and 8:2 are the same (both 4:1), so yes, X and Y’s results justify the Law of Constant Proportions.
What if atoms could combine in any ratio instead of a fixed one? Then the same “compound” could have different properties each time — substances around us would be unpredictable and unreliable.
- Law of Constant Proportions — in a compound the elements are present in a fixed ratio by mass, whatever the source.
- Law of Definite Proportions — another name for the same law, also called Proust’s Law.
