Higher tier

Many chemical reactions are carried out using solutions of substances. In order to calculate the
masses of the reactants and products
that take part in these chemical reactions we need to know the
concentration of these solutions.
Remember a solution is made by dissolving a substance, the solute
in a solvent. The solvent is usually water.

When we talk about the concentration of a solution
we mean how much solute has been dissolved in a
certain volume of water (solvent).

As an example consider the solutions made by dissolving salt, sodium chloride in water.

Sodium chloride
is NaCl. The A_{r} of sodium is 23 and the A_{r} of chlorine is 35.5.

So the M_{r} of sodium chloride (NaCl)

= 23 + 35.5 = 58.5

So 1 mole of sodium chloride = 58.5 grams.

Most people in everyday life measure volumes in litres but this is not the SI unit used in science.
The unit used for measuring volumes in science is the decimetre cubed, dm^{3}. A dm^{3} is the same
as 1 litre or 1000cm^{3} or 1000ml, they are all the same volume.

The units of concentration will depend upon the units used to measure the masses of the solute and the solvent. You need to be confident in using units e.g.

This gives you two ways for expressing the concentration of solutions. You just need to take care and read any questions carefully to ensure you use the correct formula to ensure you end up with the correct units.

1. Glucose sugar has the formula C_{6}H_{12}0_{6}, its M_{r} is 180.

a. What is the concentration in g per dm^{3} of a glucose
solution made by dissolving 30g of
glucose to give a solution with a total volume of 500 ml?

We will use the formula below to solve this problem since the problem gives mass of glucose in grams:

b. What is the concentration in mol per dm

This time itâ€™s the same problem but the units are different, the units of concentration needs to be in mol (short for moles) per dm

We can take our two formulae from above for calculating concentrations and rearrange them if required to calculate either volumes (V) or number of moles (n):

You should be able to use these formula to calculate either concentrations, volumes or moles. You should take care that
you use the correct units. Questions in your exam may have volumes in ml or cm^{3} but you should remember to
change these into dm^{3}. The only way to ensure you can successfully answer this type of question is to complete a few practice problems- click the link below!