You are to make up a reaction mixture with a specified final volume. You are given a stock solution of a component at a certain concentration, and you have to calculate the volume to add to the reaction mixture to obtain a particular final concentration.
You should aim to do these calculations in your head or on a scrap of paper (i.e. without a calculator). The following logic may be helpful:
You are diluting a stock solution.
The extent of the dilution is the final concentration divided by the stock concentration.
This is the same as the volume of added stock divided by the final volume.
The volume required is therefore:
final volume x (final concentration/stock concentration)
Always do a ‘sanity check’ on your answer. Is it the right order of magnitude? E.g. for a dilution factor of 8.5, round this up to 10 and check whether your answer is somewhere near one-tenth of the final volume (rather than one hundredth).