Answer
$$82.2 \% $$
Work Step by Step
Percent composition by mass of a compound is the representation of how much each type of atom contributes to the total mass of the compound.
For example, $NH_3$ has two types of atoms, N and H. The total mass of Nitrogen in this compound is ( 14.01 $\times$ 1 = 14.01 amu), and the same of Hydrogen is ( 1.008 $\times$ 3 = 3.024 amu).
The total mass of ammonia is $ NH_3 $ : ( 3.024 )+ ( 14.01)= 17.03 amu.
Now, to find how much each atom contributes, we divide them by the total mass, and muliply by 100% to get the percent form.
$$\frac{3.024}{17.03} \times 100\% = 17.8 \% $$
$$\frac{14.01}{17.03} \times 100\% = 82.2 \% $$
