This section covers molar concentrations covered in S1.4 but also much of the content of R2.1 - The amount of chemical change which explains why an extra 8 hours have been allotted to it. It is added here as the ability to understand and use chemical equations and calculate quantities underpins much of the chemistry covered in the Structure strands. R2.1 is covered again separately on R2.1 Amount of chemical change.