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4.5.6Unit 4. Rates, Equilibria and Further Organic Chemistry

Analysing pH Data — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 38 of 56 Unit quiz

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Part 1 of 5Finding Ka from pH

A measured pH of a weak acid solution of known concentration gives its Ka.

1
Convert the pH into hydrogen ion concentration using [H⁺] = 10⁻ᵖᴴ.
2
Because each dissociation gives one H⁺ and one A⁻, the concentration of A⁻ equals [H⁺].
3
Assume that the equilibrium concentration of HA equals the initial concentration, as dissociation is very small.
4
Substitute into Ka = [H⁺]² / [HA] and calculate, giving units of mol dm⁻³.

Part 2 of 5Interpreting Data

Strong acid dilution
  • Strong acid dilution Diluting a strong acid by a factor of ten raises the pH by exactly one unit.
Weak acid dilution
  • Weak acid dilution Diluting a weak acid by ten raises the pH by less than one unit, because it dissociates more as it is diluted.
Degree of dissociation
  • Degree of dissociation Percentage dissociation equals [H⁺] divided by the initial acid concentration, multiplied by 100.
Reading graphs
  • Reading graphs On a graph of pH against volume, a steeper region shows where the pH changes fastest with each addition.

Part 3 of 5Using pH Data in Practice

Using pH Data in Practice
pH meter
  • pH meter A calibrated pH meter gives readings to two decimal places and is more precise than an indicator.
Calibration
  • Calibration The probe is calibrated with buffer solutions of known pH before use to ensure accuracy.
Half-neutralisation
  • Half-neutralisation When a weak acid is half neutralised, [HA] = [A⁻], so pH equals pKa.
Sources of error
  • Sources of error Using a non-calibrated probe or reading before the value settles gives incorrect Ka values.

Part 4 of 5Worked Example

Question

  • A 0.0500 mol dm⁻³ solution of a weak acid HA has pH 3.20.
  • Calculate Ka.

  1. [H⁺] = 10-3.2 = 6.31 × 10⁻⁴ mol dm⁻³, and [A⁻] has the same value.
  2. Ka = [H⁺]² / [HA] = (6.31 × 10⁻⁴)² / 0.0500.
  3. Ka = 3.98 × 10⁻⁷ / 0.0500 = 7.96 × 10⁻⁶ mol dm⁻³.

Answer Ka = 7.96 × 10⁻⁶ mol dm⁻³.

Part 5 of 5Exam Checklist & Tips

4.5.6 – Key Points for the Exam

  • Degree of dissociation Fraction of acid molecules that have dissociated.
  • Calibration Adjusting an instrument using standards.
  • Half-neutralisation Point at which half the acid has been converted to its salt.
  • Ka Acid dissociation constant.

Common Mistakes

  • Using the pH directly as [H⁺] in Ka.
  • Forgetting to square the hydrogen ion concentration.
  • Saying dilution changes the Ka of the acid; Ka depends on temperature only.
  • Reading pH to too many figures from an imprecise indicator.

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