NUMBAT OER - Open Educational Resources

3. Molarity

A concentration of a solution expressed in, say, grammes per litre (g l-1) tells us how much solute is dissolved, but it does not provide information about the reactivity of the solute. Expressing the mass of the solute as a proportion of its atomic or molecular mass is a way of providing this information. An example:

The salt sodium nitrate (NaNO3) has a molecular mass of 85 (Na = 23, N = 14, O = 16 relative to a carbon atom of mass 12). A mass of sodium nitrate of 85 g is termed one 'mole' of the salt (ie 1 mol is the molecular mass in grammes). A sodium nitrate solution of 85 g l-1 can be expressed as 1 mol l-1. Similarly, a solution containing 85 mg m-3 would be expressed as 1 mmol m-3, and a solution of 85 μg l-1 would be 1 μmol l-1.

You may see a solution of one mole per litre (1 mol l-1) described as a 'molar solution'. The unit may be abbreviated as 1 M, and subsidiary units may be found such as millimolar (mM) and micromolar (μM).