Always the same proportion
A mass ratio compares two masses. Pure water from any source can be split into hydrogen and oxygen. Their masses are in the same ratio each time. For example, 1 g of hydrogen to 8 g of oxygen.
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The rule
A compound's elements are always in the same mass ratio. This holds however much you make. Extra of one element is left over, unused.
This is the Law of Constant Proportions. It holds for a compound. A mixture has no fixed ratio.
- Hydrogen and oxygen particles, unjoined. Key: circle is hydrogen, square is oxygen.
- Two hydrogen join one oxygen. Four water groups form. This counts atoms, not mass.
- One oxygen has no partner left. It stays unjoined.
Use the ratio
| Water made | Hydrogen | Oxygen |
|---|---|---|
| 9 g | 1 g | 8 g |
| 90 g | 10 g | 80 g |
Each compound has its own fixed ratio. Magnesium oxide has magnesium to oxygen at 3 : 2. Sodium chloride has sodium to chlorine at 23 : 35.5.
How much chlorine joins 69 g of sodium? The ratio is 23 g of sodium to 35.5 g of chlorine. Sodium goes up 3 times, as 69 ÷ 23 = 3. The ratio is fixed. So chlorine also goes up 3 times: 35.5 × 3 = 106.5 g.
Two students make magnesium oxide. One uses 6 g of magnesium and 4 g of oxygen. The other uses 12 g and 8 g. Divide 6 : 4 by 2 to get 3 : 2. Divide 12 : 8 by 4 to get 3 : 2. Both ratios are the same, so the law holds.