Units of concentration of solutions

There are four units of concentration that you are likely to come across in high school Chemistry.

1. Concentration in moles per litre, mol L^-1

2. Concentration in grams per litre, g L^-1

3. Percentage concentration, % concentration

4. Concentration in parts per million or milligrams per litre, mg L^-1 or ppm

Concentration, c, in mol L^-1

• The concentration, c, of a solution is measured in moles per litre (mol L^-1 ), and can be calculated using the following equation

• c = n/V which can be re-arranged to n = cV and V = n/c

• c = concentration in moles per litre, mol L^-1

• n = amount of dissolved solute in moles, mol.

• V = volume of solution in litres, L

Determining the concentration of a standard solution from a mass and volume

Some quantitative problems involving concentration may involve the calculation of the amount of substance, n (in mol) from mass, m (in g) and molar mass, M (in g mol^-1 ) first.

This will mean that the n = m/M will need to be used, before the concentration (in mol L^-1) can be calculated using c = n/V

Example calculation

Anhydrous (without water) sodium carbonate can be used to make a standard solution.

3.00 g of anhydrous sodium carbonate, Na₂CO₃ is dissolved in distilled water and the volume of the solution is made up to 250 mL in a volumetric flask. Calculate the concentration of the sodium carbonate solution in mol L^-1.

M(Na₂CO₃) = (2 x 23.0) + 12.0 + (3 x 16.0) = 106 g mol^-1

V(Na₂CO₃) = 250 mL = 0.250 L

Step 1: Calculate the number of moles of Na₂CO₃ using n = m/M.

n(Na₂CO₃) = m/M = 3.00/106 = 0.028301886 mol = 0.0283 mol (3 s.f.)

Step 2: Calculate the concentration of the solution using the unrounded number of moles from step 1.

c(Na₂CO₃) = n/V = 0.028301886/0.250 = 0.113207547 mol L^-1

c(Na₂CO₃) = 0.113 mol L^-1 (3 s.f.)

IMPORTANT: You will need to do one of these calculations in your assessment.

Concentration by mass

Concentration us also commonly expressed in terms of the mass of the solute in a litre of water.

The units for mass concentration are grams per litre, g L^-1

Converting between mol L^-1 and g L^-1 and vice versa

To convert from mol L^-1 to g L^-1 just multiply the concentration in mol L^-1 by the molar mass, M in g mol ^-1

Example 1

Convert 0.113 mol L^-1 sodium carbonate, Na₂CO₃ into g L^-1

c(Na₂CO₃) = 0.113 mol L^-1 & M(Na₂CO₃) = 106 g mol^-1

mass concentration = 0.113 x 106 = 11.978 g L^-1

Example 2

Convert 0.123 g L^-1 sodium carbonate, Na₂CO₃ , into mol L^-1

mass concentration(Na₂CO₃) = 0.123 g L^-1 & M(Na₂CO₃) = 106 g mol^-1

c(Na₂CO₃) = 0.123/106 = 1.16 x 10^-3 mol L^-1 (3 s.f.)

IMPORTANT: You will need to do one of these calculations in your assessment.