Question

A sample of oxygen gas was collected over water. The total pressure was 735 torr, the volume was 555 mL, and the temperature was 31.0ºC. What mass of oxygen was collected? The vapor pressure of water at 31.0ºC is 33.7 torr.

Answer 1

Given:
P of O2 = 735 torr - 33.7 torr
= 701.3 torr
= (701.3/760) atm
= 0.9228 atm
V = 555.0 mL
= (555.0/1000) L
= 0.555 L
T = 31.0 oC
= (31.0+273) K
= 304 K

find number of moles using:
P * V = n*R*T
0.9228 atm * 0.555 L = n * 0.08206 atm.L/mol.K * 304 K
n = 2.053*10^-2 mol

Molar mass of O2 = 32 g/mol

use:
mass of O2,
m = number of mol * molar mass
= 2.053*10^-2 mol * 32 g/mol
= 0.657 g
Answer: 0.657 g

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