Higher tier

Concentration

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 Ar of sodium is 23 and the Ar of chlorine is 35.5.
So the Mr 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, dm3. A dm3 is the same as 1 litre or 1000cm3 or 1000ml, they are all the same volume.

Calculating concentration

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.

Examples

1. Glucose sugar has the formula C6H1206, its Mr is 180.
a. What is the concentration in g per dm3 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:

Concentration = mass of solute ÷ volume of solution
The unit for volume asked for in the question is decimetres cubed, dm3, so we need to divide the 500ml volume by 1000 to convert it into dm3. This gives us:
Concentration = 30g/volume of 0.5 dm3 = 60g/dm3
so the concentration of the solution is 60g/dm3, that is 60g per decimetre cubed.
b. What is the concentration in mol per dm3 of a glucose solution made by dissolving 30g of glucose to give a solution with a volume of 500 ml?
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 dm3. First thing to do is to calculate how many moles of glucose we have:
= 0.167 mole
So simply substitute this into the formula, remembering that the volume must be in dm3

Calculating mol, V and C

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 cm3 but you should remember to change these into dm3. The only way to ensure you can successfully answer this type of question is to complete a few practice problems- click the link below!