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#.