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Chemistry · 10 / 10

Titration: use equivalence

Final-year concepts · Worked example and 4 exercises with solutions, at your pace.

Need a foundation?Units: numbers have dimensionsThe mole: connect mass and particlesBalance a reaction equation
UnderstandSee an exampleGuided exercise3 progressive challenges

The principle, simply

Titration determines an unknown amount using a suitable reaction with known-concentration solution. At equivalence, reactants have been introduced in stoichiometric proportions. Indicators or measurements locate a change with experimental uncertainty.

n=C*V
THE PICTURE TO REMEMBERTitration: use equivalence
  1. 01Reaction coefficients
  2. 02Locate equivalence
  3. 03Amount ratios
  4. 04Concentration and uncertainty

WORKED EXAMPLE

From principle to reasoning

For a 1:1 acid-base reaction, CₐVₐ=CᵦVₑ. 10.0 mL sample, 0.100 mol/L titrant and 12.0 mL equivalence volume give Cₐ=0.120 mol/L. Volumes must share units.

Solve a problem in this subject

  1. Identify species

    Write formulas and useful states. Distinguish atoms, ions, molecules and amount of substance. Total charge is conserved.

  2. Connect quantities

    Use m = nM and C = n/V with consistent units. V is the final solution volume.

  3. Use coefficients

    Balance before building an extent table. Compare initial amounts divided by coefficients to identify the limiting reactant.

  4. Check context

    Remaining amounts cannot be negative. Check conservation, units and assumptions: dilution, medium, completion or equilibrium.

YOUR TURN

01 · Try before checking

A 20.0 mL sample needs 10.0 mL of 0.200 mol/L titrant in a 1:1 reaction. Find concentration.

One hint

Find titrant amount, then divide by sample volume.

Reveal and understand the solution

n=0.00200 mol and C=0.100 mol/L.

Compare method, conditions and answer; identify the exact part to revisit.

IMPROVE THROUGH PRACTICE

Three additional challenges with solutions

Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.

Reflect after practising
02 · Recognise and understandLink to this exercise

In a 1:1 titration, 10 mL acid needs 20 mL of 0.1 mol/L base. Find acid concentration.

A hint to get started

At equivalence caVa = cbVb.

Step-by-step solution
  1. Base amount is 0.1 × 0.020 = 0.002 mol.
  2. The 1:1 reaction consumes equal acid amount.
  3. ca = 0.002/0.010 = 0.20 mol/L.

Which step did you miss? Retry without help, then explain why the method works.

03 · Apply the methodLink to this exercise

Does acid–base equivalence always mean pH = 7?

A hint to get started

Separate stoichiometric amounts from mixture acidity.

Step-by-step solution
  1. Equivalence means reagents were supplied in the reaction’s proportions.
  2. Remaining species can affect pH.
  3. pH = 7 is not universal; reagent nature and temperature matter.

Which step did you miss? Retry without help, then explain why the method works.

04 · Go furtherLink to this exercise

For H₂A + 2OH⁻ → A²⁻ + 2H₂O, 0.004 mol base reaches equivalence. Find initial H₂A.

A hint to get started

Respect the coefficient 2 for OH⁻.

Step-by-step solution
  1. Two moles OH⁻ correspond to one mole H₂A.
  2. n(H₂A) = 0.004/2.
  3. There was 0.002 mol acid; a 1:1 assumption would double it.

Which step did you miss? Retry without help, then explain why the method works.

The trap to avoid

Equivalence is not always pH 7. Different coefficients require different ratios.

Without notes, can you…

  • Explain the principle in your own words?
  • Rebuild the diagram and its connections?
  • Solve the exercise and justify the method?

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