1.4.5—Molar concentration

Molar concentration is the amount of solute in moles per cubic decimetre of solution. Square brackets can denote molar concentration.

$n = VC therefore C = n/V$

Use V in dm³ when calculating C in mol dm⁻³. Rearrange the same relationship to find amount or volume, and keep the solution volume distinct from the solute mass.

Convert volume before substitution: 500 cm³ = 0.500 dm³, so 0.250 mol in that solution gives C = 0.500 mol dm⁻³. The denominator is the final solution volume, not the volume or mass of solute alone.

$For dilution with no solute loss or reaction: n_before = n_after, so c₁V₁ = c₂V₂$

Adding solvent increases the final solution volume while the amount of solute stays constant, so concentration decreases. Use the final solution volume—not the solvent volume added—and do not apply c₁V₁ = c₂V₂ when solute reacts or is removed.

How it is assessed

Questions calculate concentration from solute amount and solution volume, including a titration context.

Command terms

calculate

What earns marks

Obtain moles when needed, divide by the stated equivalence or solution volume in dm³, and report mol dm⁻³ with the correct significant figures.

Watch for

Using cm³ without conversion to dm³, or dividing by solute mass instead of solution volume.

Representative question

[Maximum number: 2]

Calculate the molar concentration of the resulting solution of lithium hydroxide.

$n(\mathrm{Li})=\frac{0.200\ \mathrm{g}}{6.94\ \mathrm{g\ mol^{-1}}} =0.0288\ \mathrm{mol}$ $n(\mathrm{LiOH})=n(\mathrm{Li})=0.0288\ \mathrm{mol}$ $[\mathrm{LiOH}] =\frac{0.0288\ \mathrm{mol}}{0.5000\ \mathrm{dm^3}} =0.0576\ \mathrm{mol\ dm^{-3}}$

Award [2] for the correct final answer.

Retrieve the quantitative chain: count entities with mN_A, sum relative masses from formulae, convert mass with n=m/M, derive formula ratios, use n=VC for solutions, and apply gas-volume ratios at the same temperature and pressure.

Before finalising, check the requested entity, formula subscripts, units, dm³ conversion, balanced-equation coefficients, and any limiting reactant.

1.4.5—Molar concentration

Molar Concentration

Calculating Concentration

Assessment in practice

The Mole Summary

Question 1