 Welcome to this WISC Online Learning Module, Percent Composition of Compounds. The formula of a compound shows its composition in terms of what elements it contains and how many of each. It's often useful to know the composition of a compound in terms of the masses of its elements. For example, Mercury II Oxide decomposes to its elements Mercury and Oxygen. The mass of Mercury obtained as a product from the decomposition can be calculated from the mass percent of Mercury in Mercury Oxide. The mass percent of an element in a compound is calculated from the mass of the element present in one mole of the compound, divided by the mass of one mole of the compound, and converted to percent. Let's look at how this works using the hypothetical compound A2B3. Now let's go back to Mercury Oxide. Watch as we calculate the mass percent of Mercury. How much Mercury will be obtained as product from the decomposition of 7.65 grams of Mercury Oxide? Watch as we perform the calculations. What is the mass percent of each element in sodium phosphate? Watch as we perform the calculations. Now it's your turn. Complete the necessary calculations to determine the percent composition of the following compounds. You have completed this activity, percent composition of compounds.