The volumes in a titration tell you the mole ratio between acid and alkali. Read the ratio from the data and you do not need to guess whether the acid is monoprotic or diprotic.
This lesson is part of acid-base reasoning beyond memorised labels in SPM Chemistry. The standard method is in calculating acid-base quantities.
How do I find the ratio from the data?
Work out the moles of each substance first. Then compare them.
- Moles of alkali = concentration × volume in dm³.
- Moles of acid = concentration × volume in dm³.
- Ratio = moles of alkali ÷ moles of acid.
A ratio of 1 means the acid is monoprotic. A ratio of 2 means the acid is diprotic. You can then write the equation with the correct coefficients.
Worked example: two acids, one alkali
A student titrates 25.0 cm³ of 0.100 mol dm⁻³ sodium hydroxide with two acids, each of concentration 0.100 mol dm⁻³.
| Acid | Volume needed |
|---|---|
| Acid M | 25.0 cm³ |
| Acid N | 12.5 cm³ |
Moles of NaOH = 0.0250 × 0.100 = 0.00250 mol in both cases.
Acid M: moles = 0.0250 × 0.100 = 0.00250 mol. The ratio is 0.00250 ÷ 0.00250 = 1, so each acid particle reacts with one OH⁻. M is monoprotic, for example HCl: HCl + NaOH → NaCl + H₂O.
Acid N: moles = 0.0125 × 0.100 = 0.00125 mol. The ratio is 0.00250 ÷ 0.00125 = 2, so each acid molecule reacts with two OH⁻. N is diprotic, for example H₂SO₄: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O.
The same concentration gave half the volume because each N molecule carries twice as many H⁺ to neutralise.
What goes wrong if I assume 1 : 1?
Suppose a question gives 20.0 cm³ of an unknown acid, neutralised by 25.0 cm³ of 0.160 mol dm⁻³ NaOH, and says the acid is sulfuric acid. The alkali has 0.0250 × 0.160 = 0.00400 mol.
Using M₁V₁ = M₂V₂ gives an acid concentration of 0.160 × 25.0 ÷ 20.0 = 0.200 mol dm⁻³. The correct ratio is 1 : 2, so the acid has 0.00200 mol, which is 0.00200 ÷ 0.0200 = 0.100 mol dm⁻³. The shortcut overstated the concentration by a factor of two.
The mistake to avoid
The common mistake is to choose the formula before reading the data. Reverse the order: find the ratio or read it from the formula, then choose the method.
| Step | Shortcut habit | Reasoning habit |
|---|---|---|
| First | Pick M₁V₁ = M₂V₂ | Write the equation |
| Second | Substitute numbers | Find moles of each substance |
| Third | Report the answer | Check the ratio makes chemical sense |
The graph evidence comparison lab helps you compare how data changes with different acids.
Check yourself
25.0 cm³ of 0.150 mol dm⁻³ potassium hydroxide needs 18.75 cm³ of an acid of concentration 0.100 mol dm⁻³ for complete neutralisation. Is the acid monoprotic or diprotic?
Answer
Moles of KOH = 0.0250 × 0.150 = 0.00375 mol. Moles of acid = 0.01875 × 0.100 = 0.001875 mol.
Ratio = 0.00375 ÷ 0.001875 = 2. Each acid molecule reacts with two OH⁻, so the acid is diprotic.
What to study next
Continue with explaining why dilution changes concentration but not the original solute amount. Then mix the four ideas in the integrated practice set.
To have a teacher read your titration working with you, see online one-to-one Chemistry tuition.