Chemical concentration is usually measured in molecular weight equivalents (mole) dissolved in a litre of liquid. This is called the molarity and has the unit mol/L (mol L-1) or molar (M). Normality is another common measure of concentration and refers to the molar concentration divided by an equivalence factor. It is indicated by the unit N. Another measure is moles per kilogram, or molal, which has the unit m.
Do not italicise the units M (molar) or N (normal), but do italicise m (molal):
2 M NaCl 5 mM CuSO4 1 N NaOH 2 m CaCl2
Do not add a plural s to mole when written in full:
In reactions and equations, concentration can be denoted by square brackets:
However, do not use this abbreviation in running text or with the spelt-out name of the chemical:
The hydrochloric acid concentration was reduced
not
The [HCl] was reduced or The [hydrochloric acid] was reduced
See also Relative quantities for information on expressing measures of concentration such as weight/weight and parts per million).