Concentration of solution - Higher

A solution forms when a solute dissolves in a solvent. The concentration of a solution is a measure of the amount of solid dissolved in 1 dm3 of solution. When the mass of solute dissolved is measured in g, the units for concentration are g/dm3.

curriculum-key-fact
concentration (g/dm3) = \frac{mass\ of\ solute\ (g)}{volume\ (dm^3)}
Question

8 g of sodium hydroxide is dissolved in 2 dm3 of water. Calculate the concentration of the sodium hydroxide solution formed.

concentration = \frac{mass\ of\ solute\ in\ g}{volume\ in\ dm^3}

concentration = \frac{8\ g}{2\ dm^3}

= 4 g/dm3

Volume units

Apparatus used to measure volumes is usually marked in cm3 or ml. Although these are different units, they describe the same volume. For example, 250 ml = 250 cm3.

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

  • divide by 1,000 to convert from cm3 to dm3
  • multiply by 1,000 to convert from dm3 to cm3

For example, 250 cm3 is 0.25 dm3 (250/1,000). It is often easiest to convert from cm3 to dm3 before continuing with a concentration calculation.

Question

100 cm3 of dilute hydrochloric acid contains 0.5 g of dissolved hydrogen chloride. Calculate the concentration of the acid in g/dm3.

volume of acid in dm3 = \frac{100}{1000} = 0.1 dm3

concentration = \frac{mass\ of\ solute\ in\ g}{volume\ in\ dm^3}

= \frac{0.5}{0.1}

= 5g/dm3