We’re being asked to determine the element in the phosphorus compound with a formula of X₃P₂.

We are given the mass percent of phosphorus which is 13.7%.

Recall the mass percent formula:

$\boxed{\mathbf{\%}\mathbf{ }\mathbf{mass}\mathbf{=}\frac{\mathbf{mass}\mathbf{ }\mathbf{component}}{\mathbf{total}\mathbf{ }\mathbf{mass}}\mathbf{\times }\mathbf{100}}$

We’re going to determine the identity of X using the following steps:

Step 1. Determine the mass of P and mass of X.
Step 2. Calculate moles of X.
Step 3. Calculate the molar mass of X.
Step 4. Determine the identity of X.

Step 1. Determine the mass of P and mass of X.