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 potassium dichromate is required to make up mL of a 0.10 mol L –1 solution? M(K 2 Cr 2 O 7 ) = g mol –1 Concentration in moles per litre The relationship n = cV requires all volumes to be in litres. To convert mL to L, simply multiply by 10 –3.

What mass of indicator is required to make up 20.0 mL of a 1% solution? Concentration as a percentage Since a percentage solution means so many grams in 100 mL, simply write the concentration as a fraction.

What mass of KMnO 4 should be dissolved in 50 mL of water to make a 3 g L –1 solution? Concentration in grams per litre For this calculation it is important that only one unit of volume is used – but it doesn't matter which one.

What is the concentration, in mol L –1, of a 3 g L –1 solution of KMnO 4 ? M(KMnO 4 ) = 158 g mol –1 In one litre of solution there are 3 g of KMnO 4.