Two alcohols, isopropyl alcohol and propyl alcohol, have the same molecular formula, C3H8O. A solution of the two that is two-thirds by mass isopropyl alcohol has a vapor pressure of 0.110 atm at 313 K. A solution that is one-third by mass isopropyl alcohol has a vapor pressure of 0.089 atm at 313 K. 1. Calculate the vapor pressure of each pure alcohol at this temperature. 2. Explain the difference given that the formula of propyl alcohol is CH3CH2CH2OH and that of isopropyl alcohol is (CH3) 2CHOH.

1.

Since both components of these solutions have the same molar mass, mole fractions would be the same as mass fractions.

0.110 atm = (2/3) (Pi) + (1/3) (Pn) [1]

0.089 atm = (1/3) (Pi) + (2/3) (Pn) [2]

2*[1] - [2]:

(2) (0.110) - 0.089 atm = Pi

Pi = 0.131 atm

2*[2] - [1]:

(2) (0.089) - 0.110 atm = Pn

Pn = 0.068 atm

2.

The hydroxyl (-OH) group on the end of a longer 1-propanol molecule makes it more polar than IPA. It follows that the intermolecular forces between 1-propanol are stronger than those of IPA and thus the vapor pressure of 1-propanol should be lower than IPA.