Question

An organic compound contains 39.9 carbon,6.9 hydrogen and 53.2 oxygen
(A) calculate the empirical formula
(B)if the vapour density is 30,find it's molecular formula

Answer 1

I assume you meant 39.9% C, 6.9% H, and 53.2% O.If you take a 100 g sample that gives you 39.9 g C, 6.9 g H and 53.2 g O.
mols C = g/atomic mass = 39.9/12 = 3.325
mols H = 6.9/1 = 6.9
mols O = 53.2/16 = 3.325
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You want to find the ratio of these to each other with the smallest being no less than 1.0. The easy way to do this is to divide the smallest number by itself, then divide the other two by the same small number. Dividing by 3.325 gives us
mols C = 3.325/3.325 = 1.0
mols H = 6.9/3.325 = 2.07
mols O = 3.325/3.325 = 1.0
The empirical formula; therefore, is C1H2O1.
The empirical formula mass is 12 + 2 + 16 = 30
b. The molecular formula is 30 and the molecular formula is CH2O
Probably this formula (Cwater) is formaldehyde, HCHO