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#.