Question

For each of the following pairs of liquids, which species has the lower vapour pressure at room temperatures, and state the reason why (i.e. which is closer to its boiling point when at room temperature?):

a. water (H2O) or methanol (CH3OH)

b. 1-pentanol (CH3 (CH2 )4OH) or 1-hexanol (CH3 (CH2 )5OH)

c. methylchloride (CH3Cl) or chloroform (CHCl3 )

I got the answers for most but I do not know how to compare them when they have similar intermolecular forces. For example, for "a" water and methanol have the same intermolecular forces, but yet water has the lower vapour pressure.

Answer 1

Answer (a): Methanol has more weaker IMF's than water accordingly the methanol particles are held less firmly together to each other methanol has a lower enthalpy of vaporization, more fragile IMF's, and a higher fume pressure than water.

(b): The boiling point of 1-hexanol and 1-pentanol are 157 and 138 degree celsius .The higher the vapor pressure of a liquid at a given temperature, the lower the normal boiling point (i.e., the boiling point at atmospheric pressure) of the liquid.Hence, 1-hexanol has lower vapour pressure.

(c): Chloroform has lower vapour pressure than methylchloride since, the boiling point of chloroform is 61.2 and of methyl chloride is -24.2 degree celsius.