More chemical calculations - Higher - EdexcelCalculating concentrations
Calculations involving the mole can be used to determine unknown concentrations, volumes and masses in reactions. One mole of any gas occupies 24 cubic decimetres at room temperature and pressure.
The concentrationA measure of the mass or amount of solute dissolved in a given volume of solvent or solution. of a solutionMixture formed by a solute and a solvent. can be measured in g dm-3 or in mol dm-3. It is 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 dm-3 as the amountIn chemistry, a measure of the number of particles in a substance. of reactant in a given volume can be calculated.
Concentration using moles
The concentration of a solution can be calculated using:
the amount of dissolvedA substance is said to be dissolved when it breaks up and mixes completely with a solvent to produce a solution.soluteThe dissolved substance in a solution. in moleThe amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (contains the Avogadro's constant 6.0 ×10²³ number of particles). (mol)
the volume of solution (or solventThe liquid in which the solute dissolves to form a solution.) in cubic decimetres (dm3)
The units for concentration can also be shown as mol dm3 or M (for 'molar'), but these mean the same as mol dm3.
Worked example
0.5 mol of sodium hydroxide is dissolved in 2 dm3 of water. Calculate the concentration of the sodium hydroxide solution formed.
Concentration in mol dm-3 = \(\frac{\textup{amount~of~solute~in~mol}}{\textup{volume~in~dm}^3}\)
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:
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 ÷ 1000). It is often easiest to convert from cm3 to dm3 before continuing with a concentration calculation.
Question
100 cm3 of dilute hydrochloric acid contains 0.02 mol of dissolved hydrogen chloride. Calculate the concentration of the acid in mol dm-3.
Volume of acid = 100 ÷ 1000 = 0.1 dm3
concentration of acid = 0.02 ÷ 0.1
= 0.2 mol dm-3
Converting between units
The relative formula massThe sum of the relative atomic masses of the atoms in a chemical formula., Mr, of the solute is used to convert between mol dm-3 and g dm-3:
to convert from mol dm-3 to g dm-3, multiply by the Mr
to convert from g dm-3 to mol dm-3, divide by the Mr
Worked example
Calculate the concentration of 0.1 mol dm-3 sodium hydroxide solution in g dm-3. (Mr of NaOH = 40)
