28.17 %
In the question we are required to determine the percent by mass of nitrogen in (NH₄)₃PO₄.
In this case;
But; There are three nitrogen atoms in (NH₄)₃PO₄.
Therefore; Mass of nitrogen in (NH₄)₃PO₄ = 14.0 × 3
= 42 g
The relative formula mass of (NH₄)₃PO₄ = 149.0867 g/mol
Thus;
% by mass of N = (42 g ÷ 149.0867 g/mo)× 100%
= 28.17 %
Therefore, the % by mass of N is (NH₄)₃PO₄ is 28.17%