Page 127 - Science Coursebook

9.3 Density calculations

Example: A builder needs 20 000 kg of sand to make
3
mortar. The sand is sold in 1 m bags. How many bags
will he need? (The density of sand is 2500 kg/m .)
3

We need to find the volume of the sand.
mass 20 000
volume = = = 8 m .
3
density 2500
(Notice that, because the mass is in units of kg and the
3
density is in kg/m , the volume is in m .)
3
3
Since each bag has a volume of 1 m , he will need 8 bags.
Question
3 A coin has a mass of 7.0 g. It is made of a metal
alloy of density 5.6 g/cm . Calculate the volume Mixing sand, cement and water to
3
of the coin. make mortar.

Activity 9.3

Measuring the density of a gas

Air is a gas. Gases are much less dense than solids or liquids.
Observe as your teacher measures the density of air.

The rigid plastic bottle once held water. We say that it is empty, but
really it is full of air.
The container is weighed. This includes the mass of the air inside it.
Then the air is pumped out and the tap closed. Now the balance
shows the mass of the container with no air.
Discuss the following:

• How can you calculate the mass of air in the container?
• How could you use water and a measuring cylinder to The plastic container is weighed
when it is full of air, and when
determine the volume of the container? the air has been pumped out.
• How will you calculate the density of the air?

Summary
Density, mass and volume are connected by these three equations:
mass
• density =
volume
• mass = density × volume
mass
• volume =
density

9 Forces in action 125