Chemistry • Year 12 • Module 6 • Lesson 11
IQ2 Mastery: pH Calculations, Mixing & Band 6 Explanations
Lock in the core pH calculation vocabulary, the decision-tree logic, and the five-error checklist before moving to exam-style application.
1. Term–definition match
Ten definitions are listed below in shuffled order. In the right-hand column write the matching term from this list: pH, pOH, strong acid, weak acid, ICE table, Ka, Kb, Henderson-Hasselbalch equation, V(total), simplifying assumption. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | The negative logarithm of the hydrogen ion concentration; pH = −log[H¹º]. | |
| 1.2 | An acid that ionises completely in water; [H¹º] equals the initial concentration multiplied by the number of H¹º donated per formula unit. | |
| 1.3 | An acid that only partially ionises in water; requires an ICE table to find [H¹º]. | |
| 1.4 | An organised layout showing Initial concentrations, Change in concentrations, and Equilibrium concentrations for a reaction. | |
| 1.5 | The acid dissociation constant; equals [H¹º][A¹¯]/[HA] at equilibrium. | |
| 1.6 | The base dissociation constant; equals [BH¹º][OH¹¯]/[B] for a weak base B in water. | |
| 1.7 | The negative logarithm of the hydroxide ion concentration; used as an intermediate step when calculating pH for a base. | |
| 1.8 | The total volume of the combined solutions after mixing, equal to Vacid + Vbase; must be used when calculating the concentration of any excess species after neutralisation. | |
| 1.9 | The assumption that x << c, so (c − x) ≈ c; valid only when x/c × 100% < 5%. | |
| 1.10 | pH = pKa + log([A¹¯]/[HA]); used when both a weak acid and its conjugate base are present in significant amounts (buffer or half-equivalence point). |
2. True or false — with correction
For each statement, circle T or F. If the statement is false, write the corrected version on the line provided. 10 marks (1 for T/F, 1 for the correction where needed)
2.1 For a 0.100 mol/L solution of HCl, [H¹º] = 0.100 mol/L because HCl is a strong acid that ionises completely. T / F
2.2 For a 0.100 mol/L solution of acetic acid (CH3COOH, Ka = 1.8 × 10−5), [H¹º] = 0.100 mol/L. T / F
2.3 When mixing acid and base solutions, the concentration of any excess species must be calculated using V(total) = Vacid + Vbase. T / F
2.4 For a solution of NaOH, the pOH value calculated from [OH¹¯] is the final pH answer. T / F
2.5 H2SO4 is a diprotic strong acid; in dilute solution [H¹º] = 2 × c(H2SO4). T / F
3. Fill in the blank — the IQ2 decision tree
Complete the paragraph below by filling each blank with the correct word or phrase. Use the word bank provided. 8 marks (1 per blank)
Word bank: strong acid | ICE table | [H¹º] = c | weak acid | quadratic | 5% | Henderson-Hasselbalch | pH = 14 − pOH
Every IQ2 pH calculation begins by identifying whether the species is a (1) ___________________ or a (2) ___________________. If the species is on the six strong acid list, the shortcut (3) ___________________ applies directly. If the species is not on this list, a (4) ___________________ must be set up. After solving for x using the square root shortcut, the (5) ___________________ test must be applied: if x/c × 100% is greater than (6) ___________________, the square root answer is invalid and a (7) ___________________ formula must be applied instead. For any base calculation, the final conversion step is (8) ___________________.
4. Function recall — the five IQ2 errors
Answer each in 1–2 sentences using precise lesson terms. 10 marks (2 each)
4.1 What is Error 1, and what is the single fix that prevents it?
4.2 What is Error 2, and why is the simplifying assumption sometimes invalid?
4.3 What is Error 3, and what sanity check detects it immediately?
4.4 What is Error 4, and why does using only Vacid give a concentration that is too high?
4.5 What is Error 5, and why does it occur at the equivalence point or after partial neutralisation?
5. Build a concept map — pH calculation decision tree
Draw labelled arrows between the six terms below to show the decision-tree logic for pH calculations. Each arrow must carry a linking phrase (e.g. “leads to”, “requires”, “is checked by”). Aim for at least 6 labelled arrows. 6 marks
Supplied terms: acid/base type identification · strong acid/base · weak acid/base · direct method ([H¹º] = c) · ICE table · 5% assumption check.
Q1 — Term–definition matches
1.1 pH • 1.2 strong acid • 1.3 weak acid • 1.4 ICE table • 1.5 Ka • 1.6 Kb • 1.7 pOH • 1.8 V(total) • 1.9 simplifying assumption • 1.10 Henderson-Hasselbalch equation.
Q2 — True / false with correction
2.1 True. HCl is a strong acid; complete ionisation means [H¹º] = c exactly.
2.2 False. Correction: CH3COOH is a weak acid; it only partially ionises. [H¹º] must be found using an ICE table: x = √(1.8 × 10−5 × 0.100) = 1.34 × 10−3 mol/L, so pH = 2.87 — not 1.00.
2.3 True. V(total) = Vacid + Vbase always applies when calculating the concentration of any excess species after mixing.
2.4 False. Correction: For a base, pOH is an intermediate step. The final answer requires pH = 14 − pOH. Accepting pOH as pH gives a value below 7 for a base solution, which fails the sanity check.
2.5 True. H2SO4 is diprotic and fully ionises both protons in dilute solution, so [H¹º] = 2 × c(H2SO4).
Q3 — Cloze answers (in order)
(1) strong acid • (2) weak acid • (3) [H¹º] = c • (4) ICE table • (5) 5% • (6) 5% • (7) quadratic • (8) pH = 14 − pOH.
Note: blanks (5) and (6) both accept “5%” — the threshold is the same whether stated as the check name or the critical value.
Q4.1 — Error 1
Error 1 is applying [H¹º] = c (the strong acid shortcut) to a weak acid. The fix is to identify the acid type first by checking the six strong acid list (HCl, H2SO4, HNO3, HClO4, HBr, HI); if the acid is not on the list, it is weak and an ICE table is required.
Q4.2 — Error 2
Error 2 is using x = √(Ka × c) without checking whether the assumption x << c is valid. The assumption is invalid when x/c × 100% ≥ 5%, which occurs when the acid is relatively strong (Ka/c ≥ 0.0025). In that case the quadratic formula must be used instead.
Q4.3 — Error 3
Error 3 is writing the pOH value as the final pH for a base. The sanity check is: pH for any base must be greater than 7 at 25°C. If the number obtained is below 7, the student has forgotten pH = 14 − pOH.
Q4.4 — Error 4
Error 4 is dividing n(excess) by only Vacid or only Vbase instead of V(total). Because the two solutions have been physically combined, the excess species occupies the larger combined volume; dividing by the smaller original volume gives a concentration that is too high by a factor of V(total)/Vcomponent.
Q4.5 — Error 5
Error 5 is applying the ICE table to a mixture that contains both HA and A¹¯ (produced by neutralisation). Once significant amounts of A¹¯ are present the ICE table’s assumption that [A¹¯] starts at zero is violated. The correct method is Henderson-Hasselbalch: pH = pKa + log(n(A¹¯)/n(HA)).
Q5 — Sample concept map
A correct map should include arrows such as:
- acid/base type identification — branches to → strong acid/base
- acid/base type identification — branches to → weak acid/base
- strong acid/base — uses → direct method ([H¹º] = c)
- weak acid/base — requires → ICE table
- ICE table — followed by → 5% assumption check
- 5% assumption check — if ≥5% leads back to → ICE table (quadratic)
Any biologically valid linking phrases are accepted. Award full marks for at least 6 correctly directed arrows.