Question

Question #a036b

Answer 1

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 (`"CH"_3"OH"`) and ethanol (`"CH"_3"CH"_2"OH"`) have the capacity to form hydrogen bonds because of the `"-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 `"46 amu"`, while methanol has a molecular weight of `"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^@C` is `"5.95 kPa"`, while methanol's vapor pressure at the same temperature is `"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