# 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?

Apr 28, 2016

It is likely acetylene.

#### Explanation:

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

$\frac{92.3 \cdot g}{12.011 \cdot g \cdot m o {l}^{-} 1}$ $=$ $7.65 \cdot m o l \cdot C$

$\frac{7.69 .3 \cdot g}{1.00794 \cdot g \cdot m o {l}^{-} 1}$ $=$ $7.65 \cdot m o l \cdot H$

So, clearly, the empirical formula is $C H$.

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:

$\text{Molecular formula}$ $=$ $n \times \left(\text{empirical formula}\right)$

$26 \cdot g \cdot m o {l}^{-} 1$ $=$ $n \times \left(12.011 + 1 , 00794\right) \cdot g \cdot m o {l}^{-} 1$

Clearly, $n$ $=$ $2$, and molecular formula $=$ ${C}_{2} {H}_{2}$, likely acetylene, $H C \equiv C H$.