The analysis of an organic compound showed that it contains 0.175 mol C, 0.140 mol H, and 0.035 mol N. Its molecular is about 160 g/mol. What is the subscript on in the chemical formula?

1 Answer
anor277
Sep 2, 2016

#"molecular formula"=C_10H_8N_2#

Explanation:

We divide thru the molar quantities by the SMALLEST such molar quantity, that of nitrogen:

#C: (0.175*mol)/(0.035*mol)# #=# #5#

#H: (0.140*mol)/(0.035*mol)# #=# #4#

#N: (0.035*mol)/(0.035*mol)# #=# #1#.

And thus we get an empirical formula of #C_5H_4N#.

But the molecular formula is always a whole number mulitple of the empirical formula:

#"(molecular formula)"="n"xx"(empirical formula)"#

And then solve for #"n".#

#160*g*mol^-1# #=# #nxx(5xx12.011+4xx1.0076+14.01)*g*mol^-1#.
By this calculation, the empirical mass is #78*g*mol^-1#. This is tolerably close to half the measured molecular mass. Thus #n~=2#. This often occurs if the molecular mass has to be estimated by more traditional means than mass spectroscopy, i.e. boiling point elevation, or by the isopiestic method.

Thus #"molecular formula"=C_10H_8N_2#

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