Concept preview

What you will be able to do

1. 01Explain how relative atomic masses were first assigned
2. 02State Avogadro's hypothesis
3. 03Use combining volumes to deduce a simple molecular formula

Dalton compared combining masses to infer relative atomic masses, but the inference depends on a compound's formula. For the lesson data, $2.74\ \text{g}$ of hydrogen combines with $97.26\ \text{g}$ of chlorine. Assuming one hydrogen atom for each chlorine atom, the mass ratio $97.26/2.74$ gives chlorine a relative mass of about $35.5$ on a hydrogen reference of one. A different assumed atom ratio would give a different inferred mass from the same measurements. Independent evidence for the formula is therefore essential.