# Question #a036b

Feb 1, 2015

Methanol has the larger vapor pressure at room temperature because it has a lower molecular weight when compared with ethanol, which implies it has weaker intermolecular forces.

Both methanol ($\text{CH"_3"OH}$) and ethanol ($\text{CH"_3"CH"_2"OH}$) have the capacity to form hydrogen bonds because of the $\text{-OH}$ group, so from this standpoint there is no difference between the two.

However, the difference comes when the weaker London dispersion forces come into play. Ethanol has a molecular weight of $\text{46 amu}$, while methanol has a molecular weight of $\text{32 amu}$.

This difference in molecular weight means stronger London dispersion forces in favor of the heavier molecule - ethanol - which in turn translates into a lower vapor pressure.

As a result, the molecule that exhibits weaker intermolecular forces - methanol - will have the higher vapor pressure.

Ethanol's vapor pressure at ${20}^{\circ} C$ is $\text{5.95 kPa}$, while methanol's vapor pressure at the same temperature is $\text{12.8 kPa}$.

Check out this answer on the topic of vapor pressure and intermolecular forces:

http://socratic.org/questions/how-does-vapor-pressure-related-to-intermolecular-forces