Question

Two gases are mixed in a 10.0 L fixed volume flask: 8.0 L of O₂ at 2.00 atm and 2.0 L of N₂ at 3.00 atm.

A.) Calculate the partial pressure for each gas and the total pressure.

B) How much (volume in liters) argon at 5.0 atm must be added to the 10.0 L flask to lower the mole fraction of oxygen to ½ (i.e., to get χ_O2 = 0.5)? Hint – maybe it’s easier to do part C first?

C) What is the total pressure of the mixture from part B (Ar, O₂, N₂)?

Answer 1

we know that

P1V1 = P2V2

so

for 02

2 x 8 = P2 x 10

P2 = 1.6

so

the partial pressure of 02 is 1.6 atm

for N2

3 x 2 = P2 x 10

P2 = 0.6

so

the partial pressure of N2 is 0.6 atm

so

total pressure = 1.6 + 0.6 = 2.2

so

the total pressure is 2.2 atm

C)

we know that

mole fraction = partial pressure / total pressure

given

X02 = 0.5

so

0.5 = 1.6 / total pressure

total pressure = 3.2

so

total pressure of mixture = 3.2

now

total pressure = p02 + pN2 + pAr

so

3.2 = 1.6 + 0.6 + pAr

pAr = 1 atm

now

P1V1 = P2V2

so

5 x V1 = 1 x 10

V1 = 2

so

2 L of argon gas should be added