Understanding concentration

The concentration of a solution indicates how much solute is dissolved in a particular volume of solution.

The amount of solute may be measured by mass, or by the amount in moles. The amount of solution can be measured as a volume or as a mass.

Common units of concentration are:

• moles per litre, mol L–1

• grams per litre, g L–1

• grams per 100 g (or 100 mL), which is called a percentage

• parts per million, ppm, which is equivalent to mg per litre.

You will need to make up some of your own solutions while doing your practical investigation. You will also need to be able to convert from one unit of concentration to another.What mass of indicator is required to make up 20.0 mL of a 1% solution?

What mass of potassium dichromate is required to make up 250.0 mL of a 0.10 mol L–1 solution?

M(K2Cr2O7) = 294.2 g mol–1

What mass of KMnO4 should be dissolved in 50 mL of water to make a 3 g L–1 solution?

What is the concentration, in mol L–1, of a 3 g L–1 solution of KMnO4? M(KMnO4) = 158 g mol–1

In one litre of solution there are 3 g of KMnO4.

Sometimes we make up solutions by diluting a stronger solution that has already been made up.

What volume of 2.0 mol L–1 H2SO4 should be used to make up 250.0 mL of 0.05 mol L–1 solution?

Instead of plugging numbers into a formula, try using logic:

250 mL of 2.0 mol L–1 H2SO4 in 250 mL = 2.0 mol L–1

125 mL of 2.0 mol L–1 H2SO4 in 250 mL = 1.0 mol L–1

12.5 mL of 2.0 mol L–1 H2SO4 in 250 mL = 0.1 mol L–1

6.25 mL of 2.0 mol L–1 H2SO4 in 250 mL = 0.05 mol L–1

You may need to dilute a solution before analysing it. Once you have determined the concentration of the diluted solution, you will need to calculate the original concentration.

What is the concentration of sodium chloride if 20.0 mL of solution diluted to 100.0 mL has a concentration of 0.127 mol L–1.

To find the concentration of a diluted solution, just reverse the previous formula:

If you are making up very dilute solutions that require successive dilutions, just string the dilutions together: