Relative formula masses can be calculated and used in conservation of mass calculations. Calculations can be carried out to find out concentrations of solutions.

A solution forms when a solute dissolves in a solvent. The concentration of a solution is a measure of how 'crowded' the solute particles are. The more concentrated the solution, the more particles it contains in a given volume.

The concentration of a solution can be calculated using:

- the mass of dissolved solute in grams, g
- the volume of solution (or solvent) in cubic decimetres, dm
^{3}

The units for concentration can also be shown as g dm^{-3}, but this means the same as g/dm^{3}.

8 g of sodium hydroxide is dissolved in 2 dm^{3} of water. Calculate the concentration of the sodium hydroxide solution formed.

concentration =

concentration =

concentration = 4 g/dm^{3}

Apparatus used to measure volumes is usually marked in cm^{3} or ml. Although these are different units, they describe the same volume. For example, 250 ml = 250 cm^{3}.

Volumes used in concentration calculations must be in dm^{3}, not in cm^{3} or ml. It is useful to know that 1 dm^{3} = 1000 cm^{3}. This means:

- divide by 1000 to convert from cm
^{3}to dm^{3} - multiply by 1000 to convert from dm
^{3}to cm^{3}

For example, 250 cm^{3} is 0.25 dm^{3} (250 ÷ 1000). It is often easiest to convert from cm^{3} to dm^{3} before continuing with a concentration calculation.

- Question
100 cm

^{3}of dilute hydrochloric acid contains 0.5 g of dissolved hydrogen chloride. Calculate the concentration of the acid in g/dm^{3}.volume of acid = 100 ÷ 1000 = 0.1 dm

^{3}concentration of acid =

= 5 g/dm

^{3}

Rearranging the equation for concentration allows the mass of solute to be calculated:

mass of solute in g = concentration in g/dm^{3} × volume in dm^{3}

A solution of sodium chloride has a concentration of 10 g/dm^{3}. What mass of sodium chloride is dissolved in 2 dm^{3} of the solution?

mass of solute in g = concentration in g/dm^{3} × volume in dm^{3}

= 10 g/dm^{3} × 2 dm^{3}

= 20 g

The concentration of a solution can be changed:

- concentration can be increased by dissolving more solute in a given volume of solution - this increases the mass of the solute
- concentration can be increased by allowing some of the solvent to evaporate - this decreases the volume of the solution