NCEA 91392: equilibrium principles in aqueous systems (Level 3 Chemistry)

What NCEA 91392 assesses

NZQA limits aqueous systems in the standard (version 2) to sparingly soluble ionic solids and to acidic and basic solutions in which proton transfer occurs. You use qualitative descriptions or calculations involving:

• relative concentrations of dissolved species
• sparingly soluble ionic solids: relating solubility to Ks, the common ion effect, complex ion formation, the reaction of a basic anion with added acid, and predicting precipitation or dissolution
• acidic and basic solutions, including buffers, with acid and base strength, Ka and pKa
• the concentration of species present in weak acidic and basic solutions, and relating it to pH and conductivity
• titration curves for acid-base systems, including the selection of indicators (titrations of weak acids with weak bases are excluded)

The three grades go up in steps. Achieved describes, identifies and gives an account of aqueous systems using equilibrium principles. Merit uses equilibrium principles to explain properties of aqueous systems. Excellence elaborates, justifies, relates, evaluates, compares and contrasts, or analyses properties of aqueous systems in terms of equilibrium principles. Every grade needs correct chemistry vocabulary, symbols and conventions, and Excellence needs them used consistently. NZQA's 2025 report says that at Excellence you need to apply ideas in unfamiliar contexts and read each question for what it asks, not for what you hoped it would ask.

The NCEA 91392 exam in 2026 and 2027

NCEA 91392 is one of three Level 3 Chemistry externals, with 91390 (thermochemistry) and 91391 (organic compounds). The 2026 exam is on Friday 20 November, in the afternoon session. NZQA's specifications for both years say the exam is on printed paper and 91392 is a 5-credit external.

• Three questions: you answer all three. The format is the same for each Level 3 Chemistry standard, and the questions may be set in contexts that are unfamiliar to you.
• Calculator: an approved scientific or graphics calculator is required.
• Resource booklet: it has the formulae you need, such as n = cV, a periodic table with symbols, atomic numbers and molar masses, and a list of nine complex ions with their formulae.
• Calculations: show all working, use an appropriate number of significant figures (usually three) and include the correct units.
• Practical work: NZQA says you should be familiar with relevant practical work, and the content is set by the explanatory notes of each standard.
• What you must recognise: strong acids HCl, HBr, HNO₃ and H₂SO₄; strong bases KOH and NaOH; weak acids HF, CH₃COOH and NH₄⁺; weak bases NH₃, CH₃NH₂ and CH₃COO⁻. Values of Kb or pKb are not provided, but can be derived and used. NZQA also says knowledge of nine listed complex ions is assumed, and the list with formulae is in the Resource Booklet.

How NCEA 91392 is marked

Three questions, each marked N0 to E8, give a total out of 24. NZQA's report says all three needed both explanations and calculations to reach Excellence: solubility equilibria, buffers and a titration curve in 2025.

NCEA 91392 cut scores

Excellence reached 20 in 2025 after sitting at 19 in the four years before it, and Achieved rose to 9. The scores are where each grade begins, out of 24.

Cut scores 2025

2025 schedule: Achieved from 9, Merit from 14, Excellence from 20. Compared with 2024, Achieved went up by 1 and Excellence went up by 1.

Cut scores 2024

2024 schedule: Achieved from 8, Merit from 14, Excellence from 19. Compared with 2023, Achieved came down by 1.

Cut scores 2023

2023 schedule: Achieved from 9, Merit from 14, Excellence from 19. Compared with 2022, Achieved went up by 1.

Cut scores 2022

2022 schedule: Achieved from 8, Merit from 14, Excellence from 19. Compared with 2021, Merit went up by 1.

Cut scores 2021

2021 schedule: Achieved from 8, Merit from 13, Excellence from 19.

Worked examples at each grade

Achieved: solubility product and prediction

For a sparingly soluble salt MX₂ with Ks = 4.0 × 10⁻¹², find its solubility. If s mol L⁻¹ dissolves, [M²⁺] = s and [X⁻] = 2s:

Now predict precipitation. Mix 25.0 mL of 0.0010 mol L⁻¹ AgNO₃ with 25.0 mL of 0.0010 mol L⁻¹ NaCl, with Ks(AgCl) = 1.8 × 10⁻¹⁰ given. The volume doubles, so each concentration halves to 5.0 × 10⁻⁴ mol L⁻¹:

The ionic product is larger than Ks, so a precipitate forms. NZQA's 2025 report lists failing to dilute the ions when two solutions are mixed as a Not Achieved error.

Merit: weak acids and buffers

Find the pH of 0.100 mol L⁻¹ ethanoic acid, Ka = 1.74 × 10⁻⁵. The acid is weak, so [H₃O⁺] = [CH₃COO⁻] = x, and [CH₃COOH] stays close to 0.100:

See it in the pH Calculator. For a buffer of 0.30 mol L⁻¹ ethanoate and 0.20 mol L⁻¹ ethanoic acid:

To explain how it works: when a strong base is added, the OH⁻ reacts with CH₃COOH, which is present in large amounts, so [H₃O⁺] and the pH barely change. When acid is added, the large amount of CH₃COO⁻ removes the H₃O⁺. Each added substance is used up by one component of the buffer pair.

Excellence: reading a titration curve

Which indicator should be used depends on the pH at the equivalence point, and that depends on what is being titrated. NZQA's 2025 report says candidates had rote-learned the weak acid and strong base case, where the equivalence point is above 7, and did not notice that the curve in the exam was a weak base titrated with a strong acid. In that case, at equivalence the flask holds only the conjugate acid. For 0.100 mol L⁻¹ NH₃ with 0.100 mol L⁻¹ HCl, equal volumes give 0.0500 mol L⁻¹ NH₄⁺, which has Ka = 5.6 × 10⁻¹⁰:

The equivalence point is acidic, at pH 5.28, not basic, so an indicator that changes colour below 7, such as methyl red, suits it better than one that changes above 7. Excellence compares the pKa of the indicator with the pH range of the steep part of the curve, and does the calculation for the right system.

Test your understanding of buffers before you move on:

Exam answers

The answers and a worked mark scheme come with today's paper once it is marked. We do not publish official or past paper answers. Every question on the daily paper is our own.

Use the daily paper as a practice exam

The Level 3 Chemistry track above is a short timed mock of 8 questions, not a full exam and not written by NZQA (NCEA Level 3 Chemistry). It has a topic on acids, bases and buffers, which supports the acid and buffer part of 91392. It has no topic on solubility equilibria, complex ions or titration curves, and it includes Kp, rate equations and electrode potentials, which are not part of 91392. For titrations, use our Titration Lab.

Is NCEA 91392 changing?

The 2027 specification keeps the same acid, base and complex ion expectations, with the complex ion list in the resource booklet. Year 13 moves to NZACE in 2030.

Common mistakes in NCEA 91392

• Wrong charges on ions. NZQA's 2025 report lists incorrect charges as a Not Achieved error.
• Not diluting when mixing. Work out the new concentration of each ion in the combined volume.
• Rote-learning one titration. Decide which acid and base are in the flask and in the burette before you calculate the pH at equivalence.
• Saying "charged particles" for conductivity. Name the ions.
• Mixing up degrees of dissociation. Compare solutions by how much of the acid ionises, and by the [H₃O⁺].

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