A compound is composed of 85.64% carbon and 14.36% hydrogen. The compound has a formula mass of 42.08 grams. What is the molecular formula?

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Answer 1 · 2016-06-24T16:02:18

$"Molecular formula"= C_3H_6$

As with all of these problems, we assume $100*g$ of unknown compound.

And thus, we determine the elemental composition by the given percentages.

$"Moles of carbon"=(85.64*g)/(12.011*g*mol^-1)=7.13*mol$ .

$"Moles of hydrogen"=(14.36*g)/(1.00794*g*mol^-1)=14.25*mol$ .

Clearly, there are $2$ $"moles"$ of hydrogen per mol of carbon. And thus the empirical formula is $CH_(2)$ .

And $"molecular formula"$ $=$ $nxx("empirical formula")$

Thus,

$42.08*g*mol^-1=nxx(12.011+2xx1.00794)*g*mol^-1$

And thus $n=3$ , and $"molecular formula"$ $=$ $C_3H_6$ .