You will need

- the density of air molar mass of air quantity of air, the volume occupied by the air

Instruction

1

Let us known the volume V, which takes the air. Then, by the well-known formula m = p*V, where p is the density of

**air**, we can find the mass of**air**in this volume.2

Density

At sea level at a temperature of 0oC the density

**of air**depends on its temperature. The density of dry**air**is calculated using the Clapeyron equation for an ideal gas according to the formula: p = P/(R*T) where P is the absolute pressure, T is the absolute temperature in Kelvin and R is the specific gas constant for dry**air**(R = 287,058 j/(kg*K)).At sea level at a temperature of 0oC the density

**of air**equal to at 1.2920 kg/(m^3).3

If we know the amount of

**air**, its mass can be found by the formula: m = m*V, where V is the amount of substance in moles, and M is the molar mass of**air**. Average relative molar mass**of air**equal to of 28.98 g/mol. Thus, substituting it in the formula, you get a lot**of air**in grams.# Advice 2 : How to find the molar mass of air

Molar mass is the mass of one mole of a substance, i.e. the value showing what quantity of a substance contains 6,022*10 (in degree 23) of particles (atoms, molecules, ions). But if we are not talking about pure substance and mixtures of substances? For instance, vital to people the air, because it represent a mixture of a great many of gases. How to calculate its molar mass?

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.

Instruction

1

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.

2

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%.

- 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%.

3

To facilitate calculations, rounded values of the concentrations:

- for nitrogen – up to 76%;

- for oxygen – up to 23%;

- for argon gas – to 1.3%.

- for nitrogen – up to 76%;

- for oxygen – up to 23%;

- for argon gas – to 1.3%.

4

Make a simple calculation:

28* 0,76 + 32* 0,23 + 40*0, 013 = 29,16 grams/mol.

28* 0,76 + 32* 0,23 + 40*0, 013 = 29,16 grams/mol.

5

The obtained value is very close to that specified in the handbooks: of 28.98 g/mol. The discrepancy is due to rounding.

6

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.

7

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.

8

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.

9

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.

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.

Useful advice

The equation Mendeleev-Clapeyron describes the state of an ideal gas, which air is, of course, is not. But the values of pressure and temperature close to the normal, the error is so small that can be neglected.