The composition of dry air at sea level is 78.03% N2, 20.99% O2, and 0.033% CO2 by volume. (a) calculate the average molar mass of this air sample (b) calculate the partial pressures of N2, O2, and CO2 in atm. (at constant temperature and pressure, the volume of a gas is directly proportional to the number of moles of the gas). Also what should be used for constant temperature and pressure?

the average molar mass of this air sample can be calculated as

addition of the product of the average molar weights of the component gases and their percentage compositions

1. Average Molar mass of Air = 0.7803 x 28 + 0.2099 x 32 + 0.00033 x 44 = 28.58g/mol

2. The partial pressures of N2, O2, and CO2 in atm.

From Ideal gas law, at stp, Volume of air V=22.4L/mol

PV = nRT

Since, at constant temperature and pressure, the volume of a gas is directly proportional to the number of moles of the gas

Total Pressure P=1atm

Partial Pressure p = mol fraction x P

Volume of N2 = 0.7803 x 22.4L = 17.47L, Partial Pressure = 0.7803atm

Volume of O2 = 0.2099 x 22.4L = 4.68L Partial Pressure = 0.209atm

Volume of CO2 = 0.00033 x 22.4L = 0.00739L, Partial Pressure = 0.033atm