5.10C - Calculating concentrations and volumes from titrations

What a titration result tells you

In an acid-alkali titration, a measured volume of one solution is placed in a conical flask using a pipette. The other solution is added from a burette until the indicator shows the end point. The titre is the volume delivered from the burette.

Concordant titres are accurate titres within the stated tolerance. Exclude the rough trial and any result outside that close group, then add the selected titres and divide by their number.

For calculation questions, the experimental method is usually already done. Your job is to use the useful result, usually a mean titre from concordant accurate titres, rather than a rough trial.

A titration calculation always depends on the balanced equation. If the equation shows a 1:1 ratio, the reacting amounts in moles are equal. If the equation shows a 1:2 ratio, one substance reacts with twice as many moles of the other substance.

For example:

Relationships and units

The main relationship is the same concentration relationship used for solutions:

where:

Titration volumes are usually measured in $\mathrm{cm}^{3}$, but the calculation needs $\mathrm{dm}^{3}$.

So $25.0\,\mathrm{cm}^{3} = 0.0250\,\mathrm{dm}^{3}$ and $18.65\,\mathrm{cm}^{3} = 0.01865\,\mathrm{dm}^{3}$.

You may also need rearranged forms:

Use the full calculator value through the working. Round only at the final answer, unless the question asks for a particular number of significant figures or decimal places.

Calculation route

Most titration calculations follow the same route. Start with the solution where both the concentration and the reacting volume are known. Use $n = c \times V$ to calculate its moles. Then use the mole ratio from the balanced equation to find the moles of the unknown solution.

[DIAGRAM: titration_calculation_route: Titration calculation route - diagram 1; asset_slug: 129_5_10c_titration_calculations_for_unknown_concentration_or_volume_diagram1; file: diagram_assets/129_5_10c_titration_calculations_for_unknown_concentration_or_volume_diagram1.png; recommended_method: deterministic_drawn; description: Monochrome flow diagram showing how titration results lead from mean titre, cm3 to dm3 conversion, known solution moles using n = c x V, balanced-equation mole ratio, then either c = n / V for unknown concentration or V = n / c for unknown volume. Assessed/exact calculation route; all formulae are also explained in nearby prose for accessibility.]

A reliable written method is:

1. Write or use the balanced equation.
2. Convert any volume in $\mathrm{cm}^{3}$ into $\mathrm{dm}^{3}$.
3. Calculate moles of the solution with known concentration and known volume.
4. Use the mole ratio to find moles of the other solution.
5. Calculate the unknown concentration using $c = n / V$, or the unknown volume using $V = n / c$.
6. Give the final answer with the correct unit.

Edexcel calculation habits

For Calculate questions, show enough working that the method can be followed. A correct final answer may gain credit, but clear working protects you if you make one arithmetic slip because later correct steps can often be credited using error carried forward.

Strong titration calculation answers usually include: