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QUESTION

# Which liquid, methanol or ethanol, has the larger vapor pressure value at room temperature?

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

Both methanol ("CH"_3"OH") and ethanol ("CH"_3"CH"_2"OH") have the capacity to form 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