Converting Units - Higher

The concentration of a solution can be measured in g/dm3 or in mol/dm3.

Volume units

Volumes used in concentration calculations must be in dm3, not in cm3. It is useful to know that 1 dm3 = 1,000 cm3. This means:

  1. divide by 1,000 to convert from cm3 to dm3
  2. multiply by 1,000 to convert from dm3 to cm3

For example, 500 cm3 is 0.5 dm3 (500 ÷ 1,000). It is often easiest to convert from cm3 to dm3 before continuing with a concentration calculation.

Converting between units

The relative formula mass, Mr, of the solute is used to convert between mol/dm3 and g/dm3:

To convert from mol/dm3 to g/dm3, multiply by the Mr.

Example

Calculate the concentration of 0.100 mol/dm3 sodium hydroxide solution in g/dm3. (Relative formula mass: NaOH = 40.0)

concentration = 0.100 × 40.0

= 4.00 g/dm3

Question

Calculate the concentration of 7.30 g/dm3 hydrochloric acid in mol/dm3. (Relative formula mass: HCl = 36.5)

Concentration = 7.30/36.5

= 0.200 mol/dm3