Experiment - Bestimmung der Dichte von Luft

A Kilogramm of iron needs by far less space than a Kilogramm of wood. This means the mass of iron must be reduced to a smaller volume. The package is more dense. The density of materials is given by the proportion of mass to volume.

But, does air have a density, too? Does it have a weight at all? In order to find out about that, we carry out the following experiment:

2. Fill the air weight meter with air by means of an air pump. After that ascertain the mass again.

3. In order to determine the volume of the air, the glass tube is filled with water. For this purpose please open the clamp at the upper tubing on the right hand side, suck in the water and - if necessary - complete the filling with water from a second beaker. After that fix the clamp again. Now, let the air come out of the air weight meter and determine the volume of water which is pushed away by the air from the glass tube by means of a standard cylinder.

3) Determination of the water volume pushed away from the tube

Result:

The density of the air is 1,18 g / L

(Experiments carried out by the 8th grade students of the Velbert comprehensive school - teacher: Roland Bergmann)

Extension for advanced levels - chemistry:

Calculation of the air density from the gas law and the average molar mass.

Is the measured result above realistic?

Air can be treated as an ideal gas. The gas law for this case is:

p x V = n x R x T

p = pressure [bar] - assumed: 1 bar
V = volume - set to 1 L
n = quantity [mol]
R = gas constant 0,08314 [L x bar] / [K x mol]
T = temperature [K] - assumed: 295 K = 22°C

Insertion of the values leads to the follwing quantity

n = [p x V] / [R x T] = 1 / [295 x 0,08314] mol = 0,041 mol

If we would like to know about the mass of the respective amount of air, we have to calculate the average molar mass of air.
Question: What is the weight of 1 mol air?

We assume: Air consists of
78% nitrogen (molar mass M = 28 g/mol)
21% oxygen (molar mass M = 32 g/mol)
1% argon (mol mass M = 40 g/mol)

The other trace gases are very important as greenhouse gases or for the chemistry in the atmosphere, but they do not significantly contribute to the molar mass and can be neglected.

The weight of 1 mol air is:

0,78 mol x 28 g/mol + 0,21 mol x 32 g/mol + 0,01 mol x 40 g/mol
= 28,96 g

This means the average molar mass of air is: 28,96 g/mol

Now we are able to calculate the weight of 1 L air (0,041 mol):

m = n x M = 0,041 mol x 28,96 g/mol = 1,187 g

Result:

The calculated density is 1,187 g / L

The Density of Air

Extension for advanced levels - chemistry: