Tartaric acid (C₄H₆O₆) is a weak acid found in grapes and wine. Hydrochloric acid (HCl) is a strong acid. Both acids are dissolved in water to make solutions of equal concentration. Explain why the electrical conductivity of the tartaric acid solution is lower than that of the hydrochloric acid solution of the same concentration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Tartaric acid (C₄H₆O₆) is a weak acid found in grapes and wine. Hydrochloric acid (HCl) is a strong acid. Both acids are dissolved in water to make solutions of equal concentration.
Model answer (5 marks)
Hydrochloric acid is a strong acid and therefore dissociates completely in water:
HCl → H⁺ + Cl⁻
Thus every mole of HCl gives two ions.
Tartaric acid is a weak acid and only partially dissociates:
C₄H₆O₆ ⇌ C₄H₅O₆⁻ + H⁺
An equilibrium is established, so only a small fraction of the acid molecules produce ions.
Because the tartaric acid solution therefore contains fewer ions (lower ion concentration) than the HCl solution of the same concentration, its electrical conductivity is lower.
Electrical conductivity is directly proportional to the number of charge carriers, so the lower ion concentration in the tartaric acid solution gives the lower conductivity.
HCl → H⁺ + Cl⁻
Thus every mole of HCl gives two ions.
Tartaric acid is a weak acid and only partially dissociates:
C₄H₆O₆ ⇌ C₄H₅O₆⁻ + H⁺
An equilibrium is established, so only a small fraction of the acid molecules produce ions.
Because the tartaric acid solution therefore contains fewer ions (lower ion concentration) than the HCl solution of the same concentration, its electrical conductivity is lower.
Electrical conductivity is directly proportional to the number of charge carriers, so the lower ion concentration in the tartaric acid solution gives the lower conductivity.
Examiner tips
- Show the full dissociation of HCl and the equilibrium for tartaric acid; link ion concentration to conductivity.
- Use the term ‘weak acid’ and ‘partial dissociation’ to gain the full marks for the explanation.
Common mistakes
- Saying tartaric acid is a ‘weak base’ or confusing it with a salt.
- Assuming both acids give the same number of ions without mentioning the equilibrium.
Mark scheme (5 marks)
- Hydrochloric acid is fully/completely dissociates (ionises) in aqueous solution
- Tartaric acid only partially dissociates (ionises) in aqueous solution
- A reversible equilibrium is established for tartaric acid / the dissociation of tartaric acid is reversible
- The tartaric acid solution contains fewer ions / a lower concentration of ions than the hydrochloric acid solution at the same concentration
- Electrical conductivity depends on the number/concentration of ions present, so the lower ion concentration in tartaric acid gives lower conductivity
Key terms in this question
strong acid · weak acid · electrical conductivity · ions
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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