An organic compound containing 78.5% carbon, 13.1% nitrogen, 8.4% hydrogen. If the molar mass of the compound is 107 g/mol, what is the empirical and molecular formulas?

1 Answer
anor277
Jan 10, 2017

Empirical formula = Molecular formula = C7H9N.

Explanation:

With all these problems, we assume a mass of 100⋅g and divide the individual elemental masses thru by the atomic mass, i.e.

C: 78.5⋅g12.011⋅g⋅mol−1=6.54⋅mol.

H: 8.4⋅g1.00794⋅g⋅mol−1=8.33⋅mol.

N: 13.1⋅g14.01⋅g⋅mol−1=0.935⋅mol.

We divide thru by the smallest molar quantity, that of nitrogen, to get an empirical formula of C7H9N. The hydrogen ratio took a bit of rounding up.

Now the molecular formula is always a multiple of the empirical formula. So we solve for n in the equation n×(7×12.011+9×1.00794+1×14.01)⋅g⋅mol−1=107⋅g⋅mol−1.

Clearly, n=1, and the molecular formula ≡ C7H9N.

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