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Density

definition

Density is mass of a substance contained it it’s unit volume.

Density is usually represented by rho (ρ)

The SI Unit of density is kilogram per cubic meter, that is; (kgm-3)

A common unit of density is grams per cubic centimeter (gcm-3) which is very common in day to day measurement of density.

Formula for density

we can use symbols alone to write expressions about density when solving problems involving density.

we should be able to convert densities expressed in Kilogram per cubic meter(kgm-3) into grams per cubic centimeter (gcm-3). In many cases, conversion from on unit of measurement to another is usually necessary. Let say we want to change 1kgm-3 into grams per cubic centimeter (gcm-3) . 1kgm-3 means that:

Now we convert 1 kg into grams remembering that, 1 kg =1000 grams and 1 cubic meter into cubic centimeters;

remembering that 1m3 = 1000000 cm3

hence 1 kgm-3 = 0.001 gcm-3

then dividing by 0.001 gcm-3 on both sides:

hence 1 gcm-3 is equivalent to 1000 kgm-3

Example

The density of a substance in a lab is expressed as 5g/cm3. Express it’s density in SI Unit.

solution

The SI unit of density is kilogram per cubic meter. We therefore change grams into kilograms and cubic centimeter into cubic meter.

expressing density in terms of grams and cubic centimeters:

1 000,000 cm3 = 1 m3 hence :

hence

=5000 kgm-3

Practice Questions

  1. A glass block measures 180mm by 80 mm by 20 mm. It’s mass is 280 g. Determine it’s density in SI Units
  2. A certain metal has it’s density given as 1.9gcm-3. If 50000 kg of such metal was purchased by a company. what volume did it occupy?

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