Amounts and concentrations

Many students have great difficulty in distinguishing clearly between amounts of a substance, and the concentration of a substance in a solution, or the proportion of a substance in a mixture.

An amount is a definite quantity of a solid, liquid or gas. Examples are: 10 mg anhydrous potassium nitrate; 1 g of 10m sulphuric acid; 25 ml of 2m sulphuric acid; 10 L hydrogen (at STP); 3 ml of 0.9% (w / v) sodium chloride in water; 2 × 107 staphylococci; 5 mmoles ammonium sulphate.

The relative molecular mass (abbreviated RMM or mol. wt) of a substance, expressed in grams, is one mole of that substance. A mole (abbreviated mol) is a definite quantity of a substance, even if the RMM of that substance is not known, because the number of molecules in one mole of any pure substance is always 6.02 × 1023 (Avogadro's number). This is not the same as the number of atoms in a molecule; for example oxygen or hydrogen gases each have 2 atoms per molecule, methane has 5 atoms per molecule while glucose has 24 atoms in its molecule.

A concentration is an expression of proportion, whether this is for example the strength of a solution or the percentage of an organism that is nitrogen. Knowing a concentration is very important, but the concentration, of itself, does not tell the amount of material.