When a 14.2-g sample of mercury (II) oxide is decomposed into its elements by heating, 13.2 g Hg is obtained. What is the percent composition of the compound?
93.0% Hg, 7.0% O.
Mercuric oxide decomposes to the elements mercury and oxygen. Dividing the weight of the mercury (13.2 g) by the weight of the oxide before heating (14.2 g), and using three significant digits to match the weight measurements, we get 0.930 = 93.0% for the proportion of mercury. The remainder is oxygen.