Question

A compound of B and H is found to have a mass percent composition of 81.1% Boron. Its empirical formula is B2H5. A mass spectrometry experiment tells us that the molecular mass is 53.3 g/mol. What is its molecular formula?

So I feel like this is a trick question, because I thought molecular formula was when it is simplified to its smallest like if it was originally B4H10 then its molecular formula would be B2H5 so I feel like the answer is B2H5 because it cannot be reduced. Am I completely off base on this?

Answer 1

Almost. You have the right idea but you've reversed the facts. The empirical formula is the simplest; the molecular formula is how many of the empirical units you have together.
So the empirical mass is 2*B + 5*H or 2*10.81 + 5*1 = 26.62
The molecular mass is 53.3 from the problem. so 53.3/25.52 = 2.00 so the molecular formula is
(B2H5)2 or B4H10.