 The concentration of a solution is a measure of the number of particles of a solute, a dissolvable solid, which are in the solvent, the liquid in which the solute is dissolved. What might you say about the number of particles in a concentrated solution compared with a dilute solution? Pause, think, and continue when ready. The number of particles in a concentrated solution is greater than the number of particles found in a dilute solution. Think about when you sweeten a drink by adding sugar. The more sugar you add, the sweeter the drink tastes. We can calculate this using two formulae and accompanying formula triangles. The first way of calculating concentration is to use moles and amount of a chemical substance and divide it by its volume. Here's the formula triangle for this. Most of the time, volumes are measured in cubic centimetres, so 1000 cubic centimetres is equal to 1 decimetre cubed, or 1 litre. So, how many decimetre cubed is 500 cubic centimetres? Pause, think, and continue when ready. To get the answer, you need to divide 500 cubic centimetres by 1000. So 500 cubic centimetres is the same as 0.5 decimetre cubed. Did you get it right? You can find any of these quantities by covering up the one you're looking for with your finger. For example, if you cover moles, you can find this by multiplying the concentration by the volume. This is because any two terms on the bottom of a formula triangle are always multiplied together. If you want to find the concentration, you cover this and divide the number of moles by the volume. Let's try this now. If you have two moles of salt and dissolve it in 2 litres of water, use the formula triangle to calculate the concentration of the salt solution. Don't forget the units. Pause, write down your working out, and continue when ready. Two moles divided by 2 litres is equal to 1 mole per decimetre cubed. Did you get it right? And did you include your units? The second way of calculating the concentration is very similar, except sometimes we use a mass of a substance divided by volume rather than a mole of a substance. The formula is mass divided by volume and has the unit grams per decimetre cubed rather than moles per decimetre cubed. Its formula triangle looks like this. Let's try a challenge. If you know that you need a 10 gram per decimetre cubed salt solution and have to dissolve it to a volume of 250 cubic centimetres, what mass of salt will you need to dissolve? Pause and write down your working out. Don't forget that you have to convert the volume from cubic centimetres to decimetre cubed. Continue when ready. The working out is shown now. Did you get it right? In summary, there are two ways of calculating concentration. One involves moles, the amount of solute, the other, the mass of the solute. When calculating the concentration, both ways will involve dividing it by the volume. The formula triangle can be used to calculate any quantity by covering up the one you need to find and using a multiplication or a division as shown in the triangle.