An hydrocarbon is #92.3%# with respect to carbon. What is its empirical formula? And if the mass of the molecule is #26gmol^-1#, what is its molecular formula?

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Answer 1 · 2016-04-28T14:56:44

It is likely acetylene.

We assume 100 g of compound, and divide thru by the atomic masses:

#(92.3*g)/(12.011*g*mol^-1)# #=# #7.65*mol*C#

#(7.69.3*g)/(1.00794*g*mol^-1)# #=# #7.65*mol*H#

So, clearly, the empirical formula is #CH#.

But we have been given a molecular mass, and it is a fact that the molecular formula is always a multiple of the empirical formula:

#"Molecular formula"# #=# #nxx("empirical formula")#

#26*g*mol^-1# #=# #nxx(12.011+1,00794)*g*mol^-1#

Clearly, #n# #=# #2#, and molecular formula #=# #C_2H_2#, likely acetylene, #HC-=CH#.

An hydrocarbon is #92.3%# with respect to carbon. What is its empirical formula? And if the mass of the molecule is #26*g*mol^-1#, what is its molecular formula?