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IB Chemistry HL 3.1.5 Ion Product of Water Kw Questions

Use real HL exam questions to apply Kw at different temperatures and calculate or interpret hydrogen- and hydroxide-ion concentrations.

Syllabus
First assessment 2025
Course
Chemistry HL
Level
HL

Exam points

  • Apply Kw = [H3O+][OH−] at the stated temperature to calculate an unknown ion concentration.
  • Interpret acidic, neutral and basic solutions by comparing hydrogen-ion and hydroxide-ion concentrations, including temperature-dependent pure-water neutrality.

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Study 3.1.5—Ion product of water (Kw) conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

IB Chemistry HL 3.1.5 Ion Product of Water Kw Questions question 1

[Maximum number: 6]

Iron rusts in the presence of oxygen and water. Rusting is a redox process involving several steps that produces hydrated iron(III) oxide, Fe2O3∙nH2O\mathrm{Fe}_{2} \mathrm{O}_{3} \bullet \mathrm{nH}_{2} \mathrm{O}, as the final product. The half-equations involved for the first step of rusting are given below.

Half-equation 1: Fe(s)→Fe2+(aq)+2e−\quad \mathrm{Fe}(\mathrm{s}) \rightarrow \mathrm{Fe}^{2+}(\mathrm{aq})+2 \mathrm{e}^{-}

Half-equation 2: O2(aq)+4e−+2H2O(l)→4OH−(aq)\quad \mathrm{O}_{2}(\mathrm{aq})+4 \mathrm{e}^{-}+2 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow 4 \mathrm{OH}^{-}(\mathrm{aq})

Question (a)

(a)

State a chemical equation for the partial dissociation of water into ions, including state symbols.

[ 1 ]

Question (b)

(b)

The dissociation of water into ions is reversible. State the expression for the ionic product constant of water.

[ 1 ]

Question (c)

(c)

The ionic product constant of water was measured at three different temperatures.

Table for Question (c) — IB Chemistry HL

Deduce whether the ionization of water is exothermic or endothermic, giving your reason.

[ 2 ]

Question (d)

(d)

Use the data in part (iii) to determine the pH of water at 373 K , correct to two decimal places.

[ 2 ]
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