Chemistry / Unit 2 Chem 0189 worked example, 60 to 90 seconds
Applying the Law of Multiple Proportions
The idea
Applying the law of multiple proportions means fixing the mass of one element in two compounds, then reducing the other element's two masses to a small whole-number ratio.
In practice
Compound A contains 4.27 g carbon and 5.69 g oxygen; compound B contains 5.19 g carbon and 13.84 g oxygen. Dividing oxygen by carbon in each gives an oxygen-to-carbon ratio of about 1.332 for A and about 2.666 for B. Dividing the larger ratio by the smaller gives almost exactly 2, a small whole number, confirming the law and showing A and B are distinct compounds — B carries twice the oxygen per unit of carbon that A does. A plausible pair matching this pattern is carbon monoxide, CO, for A and carbon dioxide, CO2, for B.
Where people slip
Skipping the "ratio of ratios" step and stopping at the two separate oxygen-to-carbon numbers looks like progress but proves nothing; the law is confirmed only once those two ratios are compared to each other and reduced to small whole numbers.
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