5. Scales and syringes: Weighing air
Experiment
5. Close the stop-cock. Pull the piston to the 50-ml mark.
6. Fix it in this position pressing the nail through the hole in the piston..
6. Weigh the "empty" syringe on a digital pocket scale (Photo 2). Write down the mass.
7. Take the syringe from the scale. Open the stop cock. Re-weigh (Photo 3).
8. Repeat 5. and 6.
9. Dip the syringe with the stop-cock under water. Open it.

Observations (Photo 2 - 3)
1. The difference of masses between the syringe with 50 ml of air and the "empty" syringe is here 33.97 - 33.93 g = 0,04 g = 40 mg.
2. After opening the stop-cock the syringe lifts 40 ml of water.
Calculation
40 ml of air had a mass of 40 mg. So the densitiy of air is 40/40 mg/ml = 1.0 mg/ml = 1.0 g/l = 1.0 kg/m³. (Value in the literature: 1.2 g/l).
Mass of the air in a class room
Density = mass l volume or: Mass = density x volume.
Volume of the class room : breadth x depth x height = 8 m x 8 m x 3,5 m = 224 m³.
Mass of air in the class room: 1.0 kg/m³ x 224 m³ = 224 kg = nearly 4,5 center.
Density of a Styrofoam cube
Volume: 8 cm x 8 cm x 8 cm = 512 cm³ = 512 ml. Mass: 7,28 g. Density 7.28 / 512 g/ml = 0.014 g/ml = 14 mg/ml (14 g/l).