Question

Calculate the vapor pressure of a solution of 92.1g of glycerin C₃H₈O₃ in 184g of ethanol, C₂H₅OH at 40 degrees C. The vapor pressure of pure ethanol at 40 degrees C is 135.3 torr. Assume the solution is ideal.

Answer 1

Vappor pressure of a solution is given by, P

P(solution) = P(pure substance) * Mole fraction of substance ------------------- equation (01)

mole fraction is defined as, X


X = moles A / (moles A + moles B ) ----------------------------- equation ( 02 )


Calculate the No of moles each substance have.

The molar mass of ethanol is 46.08 g/mol
The molar mass of glycerin is 92.11 g/mol

The mass of glycerin is 92.1g

The mass of Ethanol 184g



moles ethanol = 184 g / 46.08 g mol^-1 = 3.99   = 4 moles

moles glycerin =      92.1 g / 92.11 g mol^-1  = 0.999    = 1 moles


Calculate the mole fraction:


X (ethanol)   = 4 moles / (4 moles + 1 moles) = 0.8



from equation 01 :

P(solution) = P(pure substance) * Mole fraction of ethanol

P(solution) = P(Ethanol) * Mole fraction of ethanol

P(solution) = 135.3 torr * 0.8

P(solution) = 108.24 torr

Due to intermolecular forces occurring between glycerin and ethanol, the vapor pressure of a solution should be lower than the vapor pressure of pure substance.