SAT Subject Test Chemistry
REVIEW OF MAJOR TOPICS
LAWS OF DEFINITE COMPOSITION AND MULTIPLE PROPORTIONS
In the problems involving percent composition, we have depended on two things: each unit of an element has the same atomic mass, and every time the particular compound forms, it forms in the same percent composition. That this latter statement is true no matter the source of the compound is the Law of Definite Composition. There are some compounds formed by the same two elements in which the mass of one element is constant, but the mass of the other varies. In every case, however, the mass of the other element is present in a small-whole-number ratio to the weight of the first element. This is called the Law of Multiple Proportions. An example is H2O and H2O2.
In H2O the proportion of H :O = 2:16 or 1:8
In H2O2 the proportion of H :O = 2:32 or 1:16
The ratio of the mass of oxygen in each is 8 :16 or 1:2 (a small-whole-number ratio).