Question

Gasoline is primarily a mixture of hydrocarbons and is sold with an octane rating that is based on a comparison with the properties of isootane (C₈H₁₈), which has a molar enthalpy of vaporization of 35.8 kJ/mol and a boiling point of 98.2 degrees celcius. Determine the vapor pressure of isooctane on a very hot summer day when the temperature is 38 degrees celcius.

Answer 1

By using the Clausius-Clapeyron equation we can estimate the vapor pressure at another temperature.

The vapor pressure of iso-ocatne at its boilng point (98.2 ^oC) = 760 mm Hg = 760 torr

P₁ = 760 mm Hg = 760 torr T₁ = 98.2 ^oC = 98.2 + 273 = 371.2 K (converted deg C to kelvin scale)

P₂ = ? T₂ = 38 ^oC = 38 + 273 = 311 K

Molar enthalpy of vaporization of iso-ocatene = = 35.8 kJ/mol = 35.8x10³ J/mol

P₂ =79.8 torr

Vapor pressure of isooctane at 38 ^oC = 79.8 torr= 79.8 mm Hg