Question

A 65 WT% HNO3 solution at t = 75oC has PHNO3 = 4670 Pa, and PH2O = 11500 Pa. Given the following data: P*HNO3 = 71990 Pa, and P*H2O = 38540 Pa:

a) Calculate the partial pressure of the solution components PHNO3 and PH2O, and total pressure of the solution assuming the solution behaves ideally.

b) Calculate the actual total pressure of the solution.

c) Indicate if the solution has positive of negative deviations from Raoult’s Law.

d) Indicate what are the molar fractions of HNO3 and H2O in the vapor phase of the solution.

Answer 1

a)    mole fraction of HNO3 = (65/63)/(65/63+35/18) = 0.35 ; Molefraction of water = 0.65

ideal solution Ptotal = P* x Molefraction HNO3 + P* x Molefraction water

          = 71990x0.35 +38540 x0.65 = 46793.5 + 25051 =71844.5 pa

P_HNO3 = 46793.5 Pa

P_H2O = 25051 Pa

b) actual Total pressure = PHNO3 + PH2O = 11500 Pa +46700 Pa, = 58200 Pa

c) the actual pressure is lesser than expected pressure, so it shows negative deviation from raoults law

d) Mole fraction of HNO₃ in vapor phase = PHNO3/Ptotal =46700/58200 = 0.802

Mole fraction of water in vapor phase = 1- 0.802 = 0.2