Calculating concentrations - Higher
Concentration using mass
The concentration of a solution can be calculated using:
\(concentration\ in\ g/dm^{3}\ = \tfrac{mass\ of\ solute\ in\ g}{volume\ in\ dm^{3}}\)
The soluteThe dissolved substance in a solution. is the substance that is dissolvedA substance is said to be dissolved when it breaks up and mixes completely with a solvent to produce a solution.. The volume is the volume of the solution or the solventThe liquid in which the solute dissolves to form a solution..
Example
5.0 g of sodium hydroxide is dissolved in 250 cm3 of water. Calculate the concentration of the sodium hydroxide solution formed in g/dm3.
250 cm3 = 250 ÷ 1,000 = 0.250 dm3
\(concentration\ = \tfrac{mass}{volume}\)
\(concentration\ = \tfrac{5.0 g}{0.250 dm^{3}}\)
= 20 g/dm3
Concentration using volume
It is often more useful to know the concentration of a reactantA substance that reacts together with another substance to form products during a chemical reaction. in mol/dm3 so that the amountIn chemistry, a measure of the number of particles in a substance. of reactant in a given volume can be calculated.
The concentration of a solution can be calculated using:
\(concentration\ in\ mol\ dm^{3}\ = \tfrac{amount\ of\ solute\ in\ mol}{volume\ in\ dm^{3}}\)
Example
0.50 mol of sodium hydroxide is dissolved in 2.0 dm3 of water. Calculate the concentration of the sodium hydroxide solution formed in mol/dm-3.
\(concentration\ = \tfrac{amount}{volume}\)
\(concentration\ = \tfrac{0.5\ mol}{2.0\ dm^{3}}\)
concentration = 0.25 mol/dm3
Question
100 cm3 of dilute hydrochloric acid contains 0.02 mol of dissolved hydrogen chloride. Calculate the concentration of the acid in mol/dm3.
volume of acid = 100 ÷ 1,000 = 0.1 dm3
concentration of acid = 0.02 ÷ 0.1
= 0.2 mol/dm3