Question

What is the vapor pressure of a solution made by adding 25.0 g of glucose, C6H12O6 (molar mass = 180. g/mol) to 145 g of water and heating the solution to 60 °C? The vapor pressure of pure water at 60 °C is 149 torr. The vapor pressure of pure glucose at 60 °C is negligible.

(a) 2.53 torr

(b) 127 torr

(c) 143 torr

(d) 146 torr

Answer 1

answer : (d) 146 torr

solution :

glucose mass = 25 g

water mass = 145 g

glucose molar mass = 180 g / mol

water molar mass = 18 g /mol

glucose moles = 25 / 180 = 0.138

water moles = 145 / 18 = 8.056

total moles = 0.138 + 8.056

                  = 8.194

mole fraction of glucose = moles of glucose / total moles

                                       = 0.138 / 8.194

                                       = 0.0168

water vapour pressure (Po) = 149 torr

raoults law :

(Po -Ps ) / Po = mole fraction of glucose

(149 - Ps) / 149 = 0.0168

Ps = 146.5 torr