A compound has a gram formula mass of 56 grams per mole. What is the molecular formula for this compound?

(1) CH2 (3) C3H6

(2) C2H4 (4) C4H8

The answer is (4). Since you are given choices, you just need to calculate the molecular mass of each choice and see which one corresponds to the given molecular mass. The molecular mass of C4H8 is 12.01 (4) + 1.01 (8) = 48.04 + 8.08 = 56.12g/mole, so rounded to the nearest whole, it's 56 g/mole