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A compound with a formula mass of 42.08 amu is found to be 85.64% carbon and 14.36 % hydrogen by mass. find its molecular formula.

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Take a 100 g sample. This will give you
14.36 g H and 85.64 g C. Convert those to moles.
14.36/1 = ??
85.64/12 = ??

Now find the ratio of the two elements to each other. The easy way to do this is to divide the smaller number by itself (thereby assuring that you will have 1.00) for that number. Then divide the other number by the same small number. Round to whole numbers. That will give you the empirical formula.
To find the molecular formula, add the atomic masses to find the empirical formula mass. Then 42.08/empirical formula mass = ?? and round to a whole number. I will call that x, then the molecular formula is (CyHz)x.

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