Molarity is the number of moles of solute dissolved in one cubic decimetre (dm³) of solution, measured in mol dm⁻³.
Malay: Kemolaran · Chinese: 摩尔浓度
Molarity is the number of moles of solute dissolved in one cubic decimetre (dm³) of solution, measured in mol dm⁻³. It is a way of expressing concentration in terms of moles rather than grams, which matters because reactions happen in whole-number mole ratios. The defining formula is molarity = moles of solute ÷ volume of solution in dm³, and every calculation in this chapter rearranges that one relationship.
A worked example shows the routine. Dissolve 0.5 mol of sodium hydroxide and make it up to 1 dm³ (1000 cm³) of solution: the molarity is 0.5 ÷ 1 = 0.5 mol dm⁻³. If instead you use 250 cm³, convert to dm³ first (250 ÷ 1000 = 0.25 dm³) so the molarity is 0.5 ÷ 0.25 = 2 mol dm⁻³. Watch the volume unit: cubic centimetres must be divided by 1000 before they enter the formula, and forgetting this is the single most common numerical slip.
The confusion to avoid is between molarity and concentration measured in g dm⁻³. Both are concentrations, but molarity counts moles per dm³ while the mass concentration counts grams per dm³; you convert between them with the molar mass, concentration in g dm⁻³ = molarity × molar mass. A second slip is to confuse molarity with the plain number of moles, molarity already has the volume built in, so it stays the same whether you pour out 10 cm³ or 100 cm³ of the same solution.
Within this chapter molarity is the hub that connects mass, volume and the mole, and it is the quantity a titration is designed to find. Our teachers keep the unit conversions visible in every line so the arithmetic never quietly goes wrong.
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