You will need
  • - accurate laboratory scales;
  • - round-bottomed flask with a ground joint and stopcock;
  • - vacuum pump;
  • - pressure gauge with two valves and connecting hoses;
  • thermometer.
First of all, consider the admissible computational error. If you don't need high accuracy, limit yourself to only the three most significant components: nitrogen, oxygen and argon, and take a "rounded" value of their concentrations. If you need more accurate result, use in calculations and carbon dioxide and can do without rounding.
Suppose you want the first option. Write the molecular masses of these components and their mass concentration in the air:

- nitrogen (N2). Molecular mass 28, mass concentration of 75,50%;
- oxygen (O2). Molecular mass 32, the mass concentration of 23.15%;
- argon (Ar). Molecular mass 40, mass concentration of 1.29%.
To facilitate calculations, rounded values of the concentrations:

- for nitrogen – up to 76%;
- for oxygen – up to 23%;
- for argon gas – to 1.3%.
Make a simple calculation:

28* 0,76 + 32* 0,23 + 40*0, 013 = 29,16 grams/mol.
The obtained value is very close to that specified in the handbooks: of 28.98 g/mol. The discrepancy is due to rounding.
Unable to determine the molar mass of air and with the help of simple laboratory experience. To do this, measure the mass of the flask with her air.
Write down the result. Then, by connecting the hose of the flask to the pressure gauge, open the faucet and turned on the pump, begin to pump the air out of the flask.
Wait for a while (so the air in the flask was heated to room temperature), note the reading of manometer and thermometer. Then, closing the valve on the bulb, disconnect the hose from the pressure gauge, and weigh the flask with the new (reduced) amount of air. Write down the result.
Next you will come to the aid of the universal equation Mendeleev-Clapeyron:

PVm = MRT.

Write it down in a somewhat modified form:

∆PVm = ∆MRT, and you are aware of, and change of air pressure ∆P and the change of the air mass ∆M. the Molar mass of air m is calculated, is elementary: m = ∆MRT/∆PV.