Given information
wo alcohols, isopropyl alcohol and propyl alcohol, have the same molecular formula, 
. A solution of the two that is two-thirds by mass isopropyl alcohol has a vapor pressure of 
at 
. A solution that is one-third by mass isopropyl alcohol has a vapor pressure of 
at .
Step-by-step explanation
Vapor pressure is a measure of the tendency of a material to change into a gaseous and vapor state.
Calculate the total pressure of the solution:
The total pressure of the solution can be determined by the sum of the partial pressure of the components present in the mixture.
Therefore, the total pressure of the mixture of the isopropyl alcohol and propyl alcohol is calculated by,
Where,
-
is the vapor pressure of the solution. -
is the mole fraction of isopropyl alcohol. -
is the vapor pressure of pure isopropyl alcohol. -
is the mole fraction of propyl alcohol. -
is the vapor pressure of pure propyl alcohol.
Putting the values in the above-mentioned equation for the first solution, we get
Putting the values in the above-mentioned equation for the second solution, we get
Multiplying equation 
with 
and subtracting it from equation 
we get,
