3.4.1 Concentration in mol/dm3
Concentration can be measured in moles per cubic decimetre
Molar concentration
The concentration of a solution measured in moles of solute per cubic decimetre of solution (mol/dm3\text{mol}/\text{dm}^3mol/dm3).
Solution
A mixture formed when a solute dissolves in a solvent.
- Separate chemistry only, higher tier: concentration in mol/dm3\text{mol}/\text{dm}^3mol/dm3 is assessed only in GCSE Chemistry at higher tier.
- Chemists often give the concentration of a solution in mol/dm3\text{mol}/\text{dm}^3mol/dm3 rather than g/dm3\text{g}/\text{dm}^3g/dm3.
- Measuring in moles is useful because reactions happen in whole-number mole ratios, not in equal masses.
A molar concentration tells you how many moles of solute sit in every cubic decimetre of the solution.
Moles, volume and concentration are linked by one formula
Mole
The amount of a substance, measured in mol, where one mole has a mass in grams numerically equal to its relative formula mass.
- The concentration in mol/dm3\text{mol}/\text{dm}^3mol/dm3 is the moles of solute divided by the volume in dm3\text{dm}^3dm3, or c=nVc = \dfrac{n}{V}c=Vn.
- Rearranging gives the moles as n=c×Vn = c \times Vn=c×V and the volume as V=ncV = \dfrac{n}{c}V=cn.
0.5 mol0.5\ \text{mol}0.5 mol of sodium hydroxide in 250 cm3250\ \text{cm}^3250 cm3 (0.25 dm30.25\ \text{dm}^30.25 dm3) has a concentration of 0.50.25=2 mol/dm3\dfrac{0.5}{0.25} = 2\ \text{mol}/\text{dm}^30.250.5=2 mol/dm3.
Converting between g/dm3 and mol/dm3
Solute
The substance dissolved in a solvent to form a solution.
- To change g/dm3\text{g}/\text{dm}^3g/dm3 into mol/dm3\text{mol}/\text{dm}^3mol/dm3, divide by the relative formula mass of the solute.
- To change mol/dm3\text{mol}/\text{dm}^3mol/dm3 back into g/dm3\text{g}/\text{dm}^3g/dm3, multiply by the relative formula mass.
- Change the volume to dm3\text{dm}^3dm3 before using c=nVc = \dfrac{n}{V}c=Vn, as a volume in cm3\text{cm}^3cm3 gives an answer a thousand times too small.
- Label each concentration with its unit, since mol/dm3\text{mol}/\text{dm}^3mol/dm3 and g/dm3\text{g}/\text{dm}^3g/dm3 are easy to mix up.
Worked example: two concentrations of the same solution
- The sodium hydroxide solution above is 2 mol/dm32\ \text{mol}/\text{dm}^32 mol/dm3.
- With Mr=40M_r = 40Mr=40, this is 2×40=80 g/dm32 \times 40 = 80\ \text{g}/\text{dm}^32×40=80 g/dm3.
- Working the other way, 80 g/dm380\ \text{g}/\text{dm}^380 g/dm3 divided by 404040 returns 2 mol/dm32\ \text{mol}/\text{dm}^32 mol/dm3.
- What does a concentration in mol/dm3\text{mol}/\text{dm}^3mol/dm3 tell you?
- Write the formula linking concentration, moles and volume.
- Find the concentration of 0.2 mol0.2\ \text{mol}0.2 mol of solute in 500 cm3500\ \text{cm}^3500 cm3 of solution.
- Convert 117 g/dm3117\ \text{g}/\text{dm}^3117 g/dm3 of sodium chloride (Mr=58.5M_r = 58.5Mr=58.5) into mol/dm3\text{mol}/\text{dm}^3mol/dm3.
