← Module 5: Equilibrium and Acid Reactions

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Inquiry Question 5: How are acids and bases defined and how do they behave in aqueous solution?

Conduct an investigation to perform titrations of strong acid and strong base, weak acid and strong base, and weak base and strong acid, and analyse the data to determine concentration, pH at the equivalence point, and appropriate indicator selection

Q: 2021 HSC HSC: A 25.0 mL sample of vinegar (containing ethanoic acid, CH₃COOH) was titrated against 0.100 mol/L NaOH. The mean titre was 22.40 mL. Calculate the concentration of ethanoic acid in the vinegar and identify a suitable indicator for the titration. Justify your indicator choice.

A 5 mark answer needs the stoichiometry, the concentration calculation, the indicator name, and a justification linked to the equivalence pH. **Equation.** $$CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O$$ One mole of acid reacts with one mole of base. **Moles of NaOH used.** $$n(NaOH) = 0.100 \times 0.02240 = 2.24 \times 10^{-3} \text{ mol}$$ **Moles of acid in the 25.0 mL sample.** From 1:1 stoichiometry, $n(CH_3COOH) = 2.24 \times 10^{-3}$ mol. **Concentration of ethanoic acid.** $$c(CH_3COOH) = \frac{2.24 \times 10^{-3}}{0.0250} = 0.0896 \text{ mol/L}$$ **Indicator.** This is a **weak acid / strong base** titration, so the equivalence pH is **above 7** (the conjugate base $CH_3COO^-$ is basic). The salt $CH_3COONa$ hydrolyses, giving an equivalence pH around 8.7. **Phenolphthalein** (colour change pH 8.3 to 10.0) is appropriate because its colour change range straddles the equivalence pH. Methyl orange (pH 3.1 to 4.4) would change colour too early and give a falsely low titre. Markers reward (1) the balanced equation, (2) correct stoichiometric calculation to 3 sig figs, (3) identifying the curve type, (4) naming an indicator with its pH range, (5) explicit justification linking range to equivalence pH.

Q: 2018 HSC HSC: Distinguish between the terms equivalence point and end point in an acid-base titration.

The **equivalence point** is the point in a titration at which **stoichiometrically equivalent** amounts of acid and base have been added. For a 1:1 reaction, moles of acid added equals moles of base. The equivalence point is a property of the chemistry of the titration. The **end point** is the point at which the **indicator changes colour**. It depends on the indicator chosen, not on the chemistry alone. A well-chosen indicator has its colour change range straddling the equivalence pH, so the end point coincides closely with the equivalence point. A poorly chosen indicator can make the end point occur before or after the equivalence point, giving an inaccurate titre. Markers reward (1) a clear definition of each, (2) the contrast that equivalence is stoichiometric and end point is indicator-dependent, (3) the connection (a good indicator makes them approximately coincide).

A focused answer to the HSC Chemistry Module 5 dot point on titrations. The four titration curve shapes, equivalence vs end point, indicator selection rules, calculating unknown concentrations from titration data, and worked HSC past exam questions.

Generated by Claude OpusReviewed by Better Tuition Academy7 min answerUpdated 2026-05-18

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What this dot point is asking

NESA wants you to interpret titration data, distinguish equivalence point from end point, recognise the four canonical titration curve shapes, select an appropriate indicator, and calculate an unknown concentration from titration data. The shape of every curve is set by the acid-base chemistry of the species involved and the pH math you used earlier in the module. Expect a 5-7 mark calculation in Section II that often combines stoichiometry with indicator justification.

The answer

Titration basics

A titration is a volumetric analysis in which a solution of known concentration (the titrant, usually in the burette) is added gradually to a solution of unknown concentration (the analyte, usually in a conical flask) until the reaction is complete.

The equivalence point is reached when stoichiometrically equivalent moles of acid and base have been mixed. The end point is when the indicator changes colour. Good experimental design makes these coincide.

Calculation pattern

For an acid-base titration with 1:1 stoichiometry:

c_{\text{unknown}} = \frac{c_{\text{titrant}} \times V_{\text{titrant}}}{V_{\text{unknown}}}

For diprotic acids or bases, include the stoichiometric ratio. For example, $H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O$ : moles of NaOH equals 2 times moles of $H_2SO_4$ .

Comparing titration curves

The shape of a titration curve depends on the strengths of the acid and base involved. The table below traces pH against the fraction of titrant added (acid being titrated with base), making the differences explicit.

Volume added	Strong acid / strong base	Weak acid / strong base	Weak base / strong acid
0%	~1.0	~2.9	~11.1
50% (mid-point)	~1.5	4.74 (= $pK_a$ )	9.26 (= 14 - $pK_b$ )
100% (equivalence)	7.0	8.7	5.3
150%	~12.5	~12.5	~1.5

Note that the 50% point of a weak acid (or weak base) titration occurs at $pH = pK_a$ (or $pH = 14 - pK_b$ ). This is the heart of the buffer region (see buffer systems). After equivalence, all three curves converge toward the pH of the excess titrant.

xychart-beta title "Titration curves: 0.10 M acid + 0.10 M NaOH" x-axis "Volume base added (mL, 25 mL = equivalence)" 0 --> 50 y-axis "pH" 0 --> 14 line [1.0, 1.1, 1.2, 1.4, 1.7, 2.4, 4.0, 7.0, 10.0, 11.6, 12.0, 12.3, 12.5, 12.6, 12.7] line [2.9, 3.8, 4.2, 4.5, 4.7, 4.9, 5.3, 8.7, 11.5, 11.9, 12.1, 12.3, 12.5, 12.6, 12.7]

The upper line is strong acid / strong base (steep, equivalence pH 7). The lower line is weak acid / strong base (gentler approach, buffer plateau around $pK_a$ = 4.74, equivalence pH 8.7).

Why each curve has the equivalence pH it does

Strong acid / strong base (HCl + NaOH). Equivalence pH 7 because the salt (NaCl) does not hydrolyse. The transition is very steep, so several indicators work.

Weak acid / strong base ( $CH_3COOH$ + NaOH). Equivalence pH > 7 because the conjugate base ( $CH_3COO^-$ ) hydrolyses water to give $OH^-$ . The buffer plateau in the middle of the curve is exploited for buffer preparation.

Weak base / strong acid ( $NH_3$ + HCl). Equivalence pH < 7 because the conjugate acid ( $NH_4^+$ ) hydrolyses water to give $H_3O^+$ .

Weak acid / weak base. No sharp transition. No indicator gives a reliable end point. Not used quantitatively in HSC.

Indicator selection rule

Choose an indicator whose colour change range straddles the equivalence pH. If the equivalence pH is 8.7 (weak acid / strong base), phenolphthalein (8.3-10.0) brackets it; methyl orange (3.1-4.4) would change far too early.

Common HSC indicators

Indicator	pH range	Use
Methyl orange	3.1 to 4.4	Weak base / strong acid
Methyl red	4.2 to 6.3	Weak base / strong acid
Bromothymol blue	6.0 to 7.6	Strong acid / strong base
Phenolphthalein	8.3 to 10.0	Weak acid / strong base, also strong/strong

Worked example

A 20.0 mL sample of HCl of unknown concentration was titrated against 0.0500 mol/L NaOH. The average titre was 18.75 mL. Calculate the HCl concentration and identify a suitable indicator.

Step 1: Balanced equation.

HCl + NaOH \rightarrow NaCl + H_2O

1:1 stoichiometry.

Step 2: Moles of NaOH used.

n(NaOH) = 0.0500 \times 0.01875 = 9.375 \times 10^{-4} \text{ mol}

Step 3: Moles of HCl.

n(HCl) = 9.375 \times 10^{-4} \text{ mol}

Step 4: Concentration.

c(HCl) = \frac{9.375 \times 10^{-4}}{0.0200} = 0.0469 \text{ mol/L}

Step 5: Indicator. This is a strong acid / strong base titration. The equivalence pH is 7, and the steep transition allows several indicators. Bromothymol blue (pH 6.0 to 7.6) is the most precise choice because its range straddles pH 7. Phenolphthalein also works because the curve is steep enough that the colour change occurs within 1 mL of equivalence.

Common traps

Confusing equivalence and end point. Equivalence is stoichiometric; end point is indicator-dependent. Always say which one you mean.

Choosing methyl orange for a weak acid / strong base titration. This is the classic mark-losing error. The end point comes far too early (around pH 4), giving a falsely low titre.

Forgetting the stoichiometric ratio for diprotic species. $H_2SO_4 + 2NaOH$ means moles of NaOH equals 2 times moles of acid.

Reporting too many sig figs. Burette readings give 3 to 4 sig figs typically. Match your answer.

Rinsing the conical flask with the analyte solution. Rinse only with distilled water. Rinsing with the analyte changes the moles of analyte present.

In one sentence

In an acid-base titration, the equivalence point is when stoichiometrically equivalent moles of acid and base have reacted, the end point is when the indicator changes colour, and a suitable indicator is one whose colour change range straddles the equivalence pH (phenolphthalein for weak acid / strong base, methyl orange for weak base / strong acid, bromothymol blue for strong / strong).

Past exam questions, worked

Real questions from past NESA papers on this dot point, with our answer explainer.

2021 HSC5 marksA 25.0 mL sample of vinegar (containing ethanoic acid, CH₃COOH) was titrated against 0.100 mol/L NaOH. The mean titre was 22.40 mL. Calculate the concentration of ethanoic acid in the vinegar and identify a suitable indicator for the titration. Justify your indicator choice.

Show worked answer →

A 5 mark answer needs the stoichiometry, the concentration calculation, the indicator name, and a justification linked to the equivalence pH.

Equation.

CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O

One mole of acid reacts with one mole of base.

Moles of NaOH used.

n(NaOH) = 0.100 \times 0.02240 = 2.24 \times 10^{-3} \text{ mol}

Moles of acid in the 25.0 mL sample. From 1:1 stoichiometry, $n(CH_3COOH) = 2.24 \times 10^{-3}$ mol.

Concentration of ethanoic acid.

c(CH_3COOH) = \frac{2.24 \times 10^{-3}}{0.0250} = 0.0896 \text{ mol/L}

Indicator. This is a weak acid / strong base titration, so the equivalence pH is above 7 (the conjugate base $CH_3COO^-$ is basic). The salt $CH_3COONa$ hydrolyses, giving an equivalence pH around 8.7.

Phenolphthalein (colour change pH 8.3 to 10.0) is appropriate because its colour change range straddles the equivalence pH. Methyl orange (pH 3.1 to 4.4) would change colour too early and give a falsely low titre.

Markers reward (1) the balanced equation, (2) correct stoichiometric calculation to 3 sig figs, (3) identifying the curve type, (4) naming an indicator with its pH range, (5) explicit justification linking range to equivalence pH.

2018 HSC3 marksDistinguish between the terms equivalence point and end point in an acid-base titration.

Show worked answer →

The equivalence point is the point in a titration at which stoichiometrically equivalent amounts of acid and base have been added. For a 1:1 reaction, moles of acid added equals moles of base. The equivalence point is a property of the chemistry of the titration.

The end point is the point at which the indicator changes colour. It depends on the indicator chosen, not on the chemistry alone.

A well-chosen indicator has its colour change range straddling the equivalence pH, so the end point coincides closely with the equivalence point. A poorly chosen indicator can make the end point occur before or after the equivalence point, giving an inaccurate titre.

Markers reward (1) a clear definition of each, (2) the contrast that equivalence is stoichiometric and end point is indicator-dependent, (3) the connection (a good indicator makes them approximately coincide).