A compunds whose empirical formula is xf3 consists of 65% f by mass. what is the atomic mass of x

Empirical formula of compound is XF3

Compound consist of 65% F

In 100g of compound there is 65 g of F

= 65 / 19 moles of Fluorine = 3.421 moles

So moles of X = 3.421 / 3 = 1.140 moles

And in 100 g X consist of 35 g

So the molar mass of X = 35 / 1.140 = 30.71 g = 31 approximately

And it is the mass of phosphorus

So the empirical formula for the compound is PX3