You start by diving each quantity given by the atomic wight of each element:
Phosphorus (P) [tex]\frac{3.8}{31} =0.123[/tex]
Hydrogen (H) [tex]\frac{0.371}{1} =0.371[/tex]
Then you divide by the lowest number:
[tex]\frac{0.123}{0.123} = 1[/tex] for phosphorus
[tex]\frac{0.371}{0.123} = 3[/tex] for hydrogen
So the empirical formula will be:
[tex]PH_{3}[/tex]
