Mass concentration and concentration by amount of substance

Two ways of expressing concentration

The concentration of a solution indicates the amount of solute dissolved per litre of solution. There are two common ways of expressing it: by mass, or by amount of substance.

Concentration by mass (mass concentration)

Denoted by cm, it indicates the mass of solute dissolved per litre of solution:

cm = m / V (en g/L), avec m la masse de soluté (en g) et V le volume de solution (in L, not in mL!)

Example: 5.0 g of salt is dissolved in water to make 500 mL (or 0.500 L) of solution.

cm = 5,0 / 0,500 = 10,0 g/L

Concentration in terms of amount of substance (molar concentration)

Denoted by c, it indicates the amount of substance of solute dissolved per litre of solution:

c = n / V (in mol/L), avec n la quantité de matière de soluté (in moles) et V le volume de solution (en L)

Example: a solution contains 0.10 mol of solute in 200 mL (0.200 L) of solution.

c = 0,10 / 0,200 = 0,50 mol/L

Converting between the two

As n = m/M, the two concentrations can be related by cm = c × M (where M is the molar mass of the solute, in g/mol).

Common mistake

The number one mistake is forgetting to convert the volume to litres: a volume given in mL MUST be divided by 1000 before being used in c = n/V or cm = m/V. For example, dividing 5 g by 500 (instead of 0.500) gives a result that is 1,000 times too small.