Question

Which one of the following has the lowest vapor pressure at 25 degrees celsius? a) CH3OH b) HOCH2CH2OH c) CH3OCH3 d) CH3CH2OH

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 (CH3OH) and ethanol (CH3CH2OH) have the capacity to formhydrogen 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.

Basing on the above explanation ,

The order of vapour pressure is as follows

b > a > d > c