Answer
Ammonia ($NH_3$) has the higher percent composition of N by mass.
Work Step by Step
1. Calculate each molar mass:
$ (NH_2)_2CO $ : ( 1.008 $\times$ 4 )+ ( 12.01 $\times$ 1 )+ ( 14.01 $\times$ 2 )+ ( 16.00 $\times$ 1 )= 60.06 g/mol
$ NH_4NO_3 $ : ( 1.008 $\times$ 4 )+ ( 14.01 $\times$ 2 )+ ( 16.00 $\times$ 3 )= 80.05 g/mol
$ HNC(NH_2)_2 $ : ( 1.008 $\times$ 5 )+ ( 12.01 $\times$ 1 )+ ( 14.01 $\times$ 3 )= 59.08 g/mol
$ NH_3 $ : ( 1.008 $\times$ 3 )+ ( 14.01 $\times$ 1 )= 17.03 g/mol
2. Find the nitrogen composition in each.
$ (NH_2)_2CO $ : $$\frac{( 14.01 \times 2 )}{60.06} \times 100\% = 46.65 \% $$
$ NH_4NO_3 $ $$\frac{( 14.01 \times 2 )}{80.05} \times 100\% = 35.00 \% $$
$ HNC(NH_2)_2 $ : $$\frac{( 14.01 \times 3 )}{59.08} \times 100\% = 71.14 \% $$
$ NH_3 $ $$\frac{( 14.01 \times 1 )}{17.03} \times 100\% = 82.27 \% $$
