A hydrocarbon is composed of 85.57% carbon and 14.43% hydrogen and a molar mass of 28.06 mol. What is the molecular formula?

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Answer 1 · 2016-12-16T17:48:50

The stuff is $"ethylene"$ , $H_2C=CH_2$ .

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

And thus...........

$"Moles of carbon"=(85.57*g)/(12.011*g*mol^-1)=7.12*mol$

$"Moles of hydrogen"=(14.43*g)/(1.00794*g*mol^-1)=14.32*mol$

We divide thru by the smallest molar quantity (that of carbon) to get an $"empirical formula"$ of $CH_2$ .

Now it is a fact that the molecular formula is a whole number multiple of the empirical formula:

i.e. $28.06*g*mol^-1=nxx(12.011+2xx1.00794)*g*mol^-1$ .

Clearly, $n=2$ , and the $"molecular formula"$ is $C_2H_4$ .