Chemical change

Acids and bases

Arrhenius, Lowry-Brønsted. Strong vs weak. pH = −log[H₃O⁺]. Hydrolysis of salts.

CAPS

Grade 12 · Physical Sciences (Chemistry) · Chemical change

Chemistry is Physical Sciences Paper 2.

The paper
Grade 12 Paper 2 covers quantitative chemistry, energy and rates, chemical equilibrium, acids and bases, electrochemical cells, and organic nomenclature, reactions and polymers.
Working
Balance before you use a mole ratio. Show n = mM or c = nV with units. An IUPAC name needs the number on the functional group. A reaction that is not balanced cannot support a stoichiometric answer.
The relation
pH=−log [H₃O⁺]. pH definition
How a marker reads it
pH to two decimals. Write −log with the square brackets. “It is strong because pH is 2” is a trap.

Paper 2 is 150 marks. Tables of ions and the data sheet are part of the exam. Significant figures follow the data you were given.

Diagram

Worked example

[H₃O⁺] = 2.5 × 10⁻³ mol·dm⁻³. Find pH. Strong or weak cannot be decided from this number alone.

Before any working — hunt first

Given

Numbers and units already in the sentence: 2.5 · 10⁻³. Copy each one into a column. A name without a number is not given.

Missing

After you copy the given, the letter that still has no number is missing. The paper is pointing at: pH. Circle that word — that is the missing variable.

Find

We are required to find: pH. Write that as a symbol on the left, with a question mark, before you touch a formula.

Calculate

Write pH = −log [H₃O⁺] with empty letters. Cross out every letter that already has a number. Calculate the letter that is still standing — that gives pH.

Simplified approach

pH = −log(2.5 × 10⁻³). Use the log button. Strong/weak needs concentration of the acid, not only [H₃O⁺].

Culture note

Amasi in a township fridge is weakly acidic — same pH maths, different strength story.

Formulas

pH=−log [H₃O⁺]

pH definition

K_w=[H₃O⁺][OH⁻]=1×10⁻¹⁴

Ionic product of water at 25 °C

Exam tricks you must know

These are the moves markers look for — not shortcuts that skip the method, but the habits that earn the method mark.

pH to two decimals, with −log and square brackets — “it is strong because pH is 2” is a trap
How:Write pH = −log[H₃O⁺] as the formula line. Substitute the concentration with a division line if you had to convert cm³. Box the answer to two decimal places. Strength is about complete ionisation, not the pH number alone.
When to use it:Strong/weak acids, K_w, and titration pH.
Never change a subscript to balance — only coefficients
How:The given formula is a substance. Fe₂O₃ is not FeO. Count atoms on each side. Put a number in front (coefficient) until both sides match. Changing a subscript invents a different compound and zeros the question.
When to use it:Balancing equations and mole-ratio questions.
Never write “the catalyst increases yield” — that sentence loses the mark in every province
How:A catalyst speeds both the forward and reverse rates equally. Equilibrium is reached faster; the yield (position of equilibrium) does not change. Write: “increases rate, yield unchanged.” On Haber, add the compromise sentence: rate versus yield.
When to use it:Rates, equilibrium, Haber and Contact processes.

Step 1 / 5

Write the definition

pH = −log [H₃O⁺]. Copy it before the calculator.

Acids and bases / titration-ph-curve — Diagram

Acid-base titration and pH curve — Diagram

pH=−log [H₃O⁺]

Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.

4. Step-by-step working

01
Write the definition
pH = −log [H₃O⁺]. Copy it before the calculator.
pH=−log [H₃O⁺]
02
Substitute
pH = −log(2.5 × 10⁻³) = 2.60 (two decimal places, CAPS).
pH=2.60
03
Step F — Box the answer with its unit
pH = 2.60. Strength cannot be told from [H₃O⁺] alone.

More worked examples

Board walkthroughs on this topic. Open one, copy the working, then try the next.

Example 1 / 20

Calculate the pH of 0.0010 mol·dm⁻³ hydrochloric acid. HCl is a strong acid.

4. Step-by-step working

01
Identify the given
[H₃O⁺]=1.0×10⁻³ mol·dm⁻³.
[H₃O⁺]=10⁻³
02
Apply the law
pH=−log(10⁻³)=3.
pH=3

How we got there

Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.

Explain in another language · 11

1. What is the question asking?

Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: Calculate the pH of 0.0010 mol·dm⁻³ hydrochloric acid. HCl is a strong acid.

Before any working — hunt first

Given

Numbers and units already in the sentence: 0.0010 mol·dm⁻³. Copy each one into a column. A name without a number is not given.

Missing

After you copy the given, the letter that still has no number is missing. The paper is pointing at: the pH of 0. Circle that word — that is the missing variable.

Find

We are required to find: the pH of 0. Write that as a symbol on the left, with a question mark, before you touch a formula.

Calculate

Write [H₃O⁺] = 10⁻³ with empty letters. Cross out every letter that already has a number. Calculate the letter that is still standing — that gives the pH of 0.

In other words — as if you have never seen this before

If you have never seen this before: A strong acid ionises completely, so [H₃O⁺] equals the acid concentration. pH is the negative log of that concentration. We will name a method in words, write the formula with no numbers, then substitute one line at a time.

2. What do we already know?

[HCl]=0.0010 mol·dm⁻³=1.0×10⁻³. Strong acid.

3. Which method — and why this one?

A strong acid ionises completely, so [H₃O⁺] equals the acid concentration. pH is the negative log of that concentration.

4. Step-by-step working

01 / 05
Identify the given
[H₃O⁺]=1.0×10⁻³ mol·dm⁻³.
[H₃O⁺]=10⁻³
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
02 / 05
Apply the law
pH=−log(10⁻³)=3.
pH=3
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
03 / 05
Sense-check
pH below 7 is acidic. 0.0010 is not the pH.
pH=3, not 0.001
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
04 / 05
Step F — Box the answer with its unit
pH=3
Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.
05 / 05
Step H — What the marker is looking for
On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.
Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.

5. The answer

pH=3

Copy onto your answer sheet — in this order

Formula: [H₃O⁺]=10⁻³

Answer: pH=3

6. Check it like a marker

Writing 0.0010 as the pH, or 11 because someone subtracted from 14, both miss the log.

7. A trap first-timers fall into

First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”

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