Question

The compound with the common name isooctane (2,2,4-trimethylpentane) has the structure shown below. It defines the 100 value on the octane rating used to grade gasoline. Its normal boiling point is 99°C and its heat of vaporization is 35.2 kJ/mol. What is the vapor pressure of isooctane at 25°C in torr?

Answer 1

Boiling point is defined as the temperature at which the vapor pressure of the substance equals the atmospheric pressure (1 atm = 760 torr).

Hence, given that the normal boiling point of isooctane is 99 C,we can say that isooctane has vapor pressure of 760 torr at 99 C.

Hence,

Now, we have to find the vapor pressure at 25 C given that the heat of vaporization is 35.2 kJ/mol

Hence,

The gas constant, R = 8.314 J K^-1 mol^-1

Let the vapor pressure at 25 C be .

Now, the two vapor pressure at different temperatures are related by the Clausius-Clapeyron equation as given below.

Hence, putting in all the known values, we can find unknown vapor pressure as follows:

Hence, the vapor pressure of isooctane at 25 C is about 45.1 torr. (Rounded to three significant figures).