Question

A) To a 1L flask that contains air at a pressure of 101 kPa a small amount of liquid is added. The pressure in the flask after the addition of the liquid is 107 kPa. What is the vapor pressure of the liquid?

B)The slope in a plot of lnP_vap vs. (1/T) for an unknown liquid is found to be -1,141. What is the value of ΔH_vap for the liquid in units of kJ/mol?

C)A 5 mL sample of an unknown liquid is placed into a 250 mL flask that has had all of the air removed. The pressure measured in the flask is found to be 32.2kPa. In a seperate trial the student puts 214 mL of the same liquid into the same flask at the same conditions. What will the pressure measure (in kPa) in the second trial?

Answer 1

A)Given,

Total pressure = 107 kPa

Pressure of air = 101 kPa

total pressure = pressure of the air + vapor pressure of the liquid

Vapor pressure of the liquid = Total pressure - Pressure of air

= 107 - 101 kPa

Vapor pressure of liquid = 6 kPa

B)

According to Clausius-Calyperon equation

lnP=- ΔHvap/R+ C

slope = -ΔHvap/R =-1141 where, R=8.314 J/K.mol

ΔHvap =1141*8.314

=9486.3 J/mol

ΔHvap =9.5 kJ/mol

C) 5 mL liquid exerts pressure = 32.2 kPa

1 mL liquid exerts pressure = 32.2/5 = 6.44 kPa

214 mL liquid exerts pressure = 214*6.44

= 1378.16 kPa

214 mL liquid exerts pressure = 1380 kPa