An organic compound contains 71.7% carbon, 6.7% hydrogen, 10.4% nitrogen and 11.8% oxygen by mass. Given that its vapour density is 67.5, determine its empirical formula and molecular formula.

From the mass percentages, I have found out that the empirical formula is $\ce{C8H9NO}$. However, to find the molecular formula we usually require the molecular mass.

What does "vapour density" mean, and how can I use it to calculate the molecular mass of the compound?


1 Answer 1


According to Wikipedia, the vapour density of a molecule is "the density of a vapour in relation to that of hydrogen". The density of a gas, $\rho$, is proportional to its molecular mass, $M$:

$$\rho = \frac{m}{V} \propto \frac{m}{n} = M$$

where $m$ is the mass of the gas, $V$ is the volume, and $n$ the amount of gas. Therefore, if we denote your compound by $\ce{X}$:

$$\begin{align} 67.5 &= \frac{\rho(\ce{X})}{\rho(\ce{H2})} \\ &= \frac{M(\ce{X})}{M(\ce{H2)}} \\ M(\ce{X}) &= 67.5 \cdot M(\ce{H2}) \\ &= 67.5 \cdot \pu{2 g mol-1} \\ &= \pu{135 g mol-1} \end{align}$$

Finally, you can use this piece of information to find that the molecular formula is $\ce{C8H9NO}$ as well.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.