# The empirical formula is C_5H_4 and its molar mass is 128.16 g/mol. What is the molecular formula?

Feb 14, 2016

The molecular formula is ${C}_{10} {H}_{8}$. How do we know?

#### Explanation:

The molecular formula is ALWAYS a mulitple of the empirical formula. Of course, the multiple might be $1$.

So $\text{MF} = {\left(E F\right)}_{n}$ $=$ ${\left(5 \times 12.011 \cdot g \cdot m o {l}^{-} 1 + 4 \times 1.00794 \cdot g \cdot m o {l}^{-} 1\right)}_{n}$ $=$ $\left(64.08\right) g \cdot m o {l}^{-} 1 \times n = 128.16 \cdot g \cdot m o {l}^{-} 1.$

Clearly, $n = 2$, and molecular formula $=$ ${C}_{10} {H}_{8}$, probably napthalene.

Capisce?