A key part of practical chemistry is the preparation and making up of solutions.
Most solutions in chemistry are made up in terms of molarity, while in Biology it is more common that they are made as percent solutions. You will need to be able to make up both molar and percentage solutions either starting with the solid or by starting with a more concentrated stock solution.
This part of the course will show you how to do this.
How much is a mole?
A mole of a substance is the number of grams of it equal to its formula mass (relative molecular mass). The actual definition is a bit more complicated but you don’t need to know the details.
So what are molar solutions?
A 1 molar solution as 1 mole dissolved in 1 litre of solution. So, 1 molar solution of copper sulphase could be:
248g of Copper sulphate in 1 litre of solution
24.8g of Copper sulphate in 100cm3 of solution
124g of Copper sulphate in 1 litre of solution
4.96g of Copper sulphate in 20cm3 of solution
So, if there is 0.1 mole in a litre, the solution is 0.1 molar – written as 0.1 mol l-1. The following are also 0.1 molar solutions of copper sulphate:
24.8g of Copper sulphate in 1 litre of solution
2.48g of Copper sulphate in 100cm3 of solution
12.4g of Copper sulphate in 500cm3 of solution
0.496g of Copper sulphate in 20cm3 of solution
Working out the moles
To work out the number of moles in a solution you need to multiply or divide to get the volume up or down to 1 litre. Take this solution:
0.992g of Copper sulphate in 20cm3 of solution
- Work out how many times 20 goes into 1000
= 50
- Multiply the mass by this 0.992 x 50
= 49.6
- Divide this by the formula mass 49.6/248
= 0.2
So the solution is 0.2 mol l-1
When you are making up solutions, you need to do this in reverse. For example, if you need to make up
500cm3 of a 0.2 mol l-1 Copper sulphate solution
- 1000cm3 of a 1 mol l-1 solution needs 248g
- So 500cm3 of a 1 mol l-1 solution needs half that = 124g
- And 500cm3 of a 0.2 solution needs 0.2 x 124 = 24.8g
So you dissolve 24.8g of Copper sulphate in water and make up to 500cm3.
Percent solutions
Some solutions, particularly indicators and most biochemicals are made as percentage solutions. A 1% solution has 1g in 100cm3 of solution. For example, if you need to make up
200cm3 of a 0.5% solution of starch (or anything else)
- 0.5% = 5g in 100cm3
- So for 200cm3 you need 1g.
Make a slurry of 1g of starch with a little cold water. Add boiling water to dissolve and make up to 200cm3 with cold water.
