QCE Vault / Chemistry Acids, bases and titration practice QCE Chemistry · Original practice questions with worked solutions
Browse all questions Chemical equilibrium practice Acids, bases and titration practice Original practice exam Practise pH, weak-acid equilibria and titration reasoning. Write the balanced reaction first, then separate mole calculations from equilibrium calculations.
Key ideas For the concentration model used in these questions, p H = − log 10 [ H 3 O + ] \mathrm{pH}=-\log_{10}[\mathrm{H_3O^+}] pH = − log 10 [ H 3 O + ] . State any complete-dissociation assumptions. Use the balanced equation to relate reacting amounts at equivalence. The acid and base amounts are equal only for a 1:1 reaction.
Choose an indicator whose transition range lies within the steep pH change around the equivalence point.
Worked example A 25.0 m L 25.0\,\mathrm{mL} 25.0 mL sample of 0.100 m o l L − 1 0.100\,\mathrm{mol\,L^{-1}} 0.100 mol L − 1 HCl contains 0.00250 m o l 0.00250\,\mathrm{mol} 0.00250 mol . It requires 12.5 m L 12.5\,\mathrm{mL} 12.5 mL of 0.200 m o l L − 1 0.200\,\mathrm{mol\,L^{-1}} 0.200 mol L − 1 NaOH at equivalence. A common mistake The equivalence point is not necessarily pH 7. Its pH depends on the acid, base and their conjugates.
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Q6 · Practice question Simple familiar 1 mark
QUESTION 6 At a particular temperature,
K w = 2.5 × 10 − 14 . A neutral solution at this temperature has a pH closest to
WORKED SOLUTION
Answer A 1 mark A
Neutral means [ H + ] = [ O H − ] = K w p H = 1 2 p K w = 6.80
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabus Q39 · Practice question Simple familiar 4 marks
QUESTION 39 (4 marks) A cleaning solution contains
4.0 × 10 − 3 m o l L − 1 sodium hydroxide at
25 ∘ C .
a) Calculate its pOH and pH. [2 marks]
b) Calculate its pH after 10.0 mL is diluted to 250.0 mL. [2 marks]
WORKED SOLUTION
Practice marking scheme 4 marks a) pOH = 2.40; pH = 11.60. b) pH = 10.20.
Initially p O H = − log ( 0.0040 ) = 2.398 p H = 14 − p O H [ O H − ] = 0.0040 ( 10 / 250 ) = 1.6 × 10 − 4
Calculates initial pOH = 2.40. [1 marks] Calculates initial pH = 11.60. [1 marks] Calculates diluted [OH⁻] = 1.6 × 10⁻⁴ mol L⁻¹. [1 marks] Calculates diluted pH = 10.20. [1 marks] Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabus How many marks did you earn? Compare your working with the guide above.
Q40 · Practice question Complex familiar 4 marks
QUESTION 40 (4 marks) A 0.080 mol L⁻¹ solution of a weak monoprotic acid has pH 3.00 at
25 ∘ C .
a) Calculate the hydrogen-ion concentration and percentage ionisation. [2 marks]
b) Calculate the acid dissociation constant without assuming the initial acid concentration equals its equilibrium concentration. [2 marks]
WORKED SOLUTION
Practice marking scheme 4 marks [ H + ] = 1.00 × 10 − 3 m o l L − 1 ; ionisation 1.25%;
K a = 1.27 × 10 − 5 .
Let x = 10 − 3 100 x / 0.080 = 1.25 % [ H A ] = 0.079 K a = x 2 / ( 0.080 − x )
Converts pH to [H⁺]. [1 marks] Calculates 1.25% ionisation. [1 marks] Uses equilibrium acid concentration 0.079 mol L⁻¹. [1 marks] Calculates Ka = 1.27 × 10⁻⁵. [1 marks] Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
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All acids, bases and titration practice questions 24 original questions · Page 1 of 1
Q6 · Original practice · 1 mark At a particular temperature, K w = 2.5 × 10 − 14 K_w=2.5\times10^{-14} K w = 2.5 × 1 0 − 14 . A neutral solution at this temperature has a pH closest to Acids, bases and titration Q7 · Original practice · 1 mark Which species can both accept and donate a proton in aqueous solution?
Acids, bases and titration Q8 · Original practice · 1 mark A monoprotic acid has p K a = 4.20 pK_a=4.20 p K a = 4.20 . At pH 5.20, the ratio of conjugate base to undissociated acid is Acids, bases and titration Q9 · Original practice · 1 mark A weak monoprotic acid is titrated with sodium hydroxide at 25 ∘ C 25^\circ\mathrm C 2 5 ∘ C . Which statement about the equivalence point is correct? Acids, bases and titration Q10 · Original practice · 1 mark An indicator has K a = 1.0 × 10 − 8 K_a=1.0\times10^{-8} K a = 1.0 × 1 0 − 8 . Its approximate colour-change range is Acids, bases and titration Q39 · Original practice · 4 marks A cleaning solution contains 4.0 × 10 − 3 m o l L − 1 4.0\times10^{-3}\ \mathrm{mol\,L^{-1}} 4.0 × 1 0 − 3 mol L − 1 sodium hydroxide at 25 ∘ C 25^\circ\mathrm C 2 5 ∘ C . Acids, bases and titration Q40 · Original practice · 4 marks A 0.080 mol L⁻¹ solution of a weak monoprotic acid has pH 3.00 at 25 ∘ C 25^\circ\mathrm C 2 5 ∘ C . Acids, bases and titration Q41 · Original practice · 5 marks Methanoic acid has K a = 1.8 × 10 − 4 K_a=1.8\times10^{-4} K a = 1.8 × 1 0 − 4 . A student prepares a 0.020 mol L⁻¹ solution. Acids, bases and titration Q42 · Original practice · 5 marks A weak base B has K b = 4.0 × 10 − 5 K_b=4.0\times10^{-5} K b = 4.0 × 1 0 − 5 at 25 ∘ C 25^\circ\mathrm C 2 5 ∘ C . Acids, bases and titration Q43 · Original practice · 4 marks An aqueous mixture contains appreciable ethanoic acid and sodium ethanoate.
Acids, bases and titration Q44 · Original practice · 6 marks The curve shows titration of 25.0 mL of a weak monoprotic acid with 0.100 mol L⁻¹ NaOH at 25 ∘ C 25^\circ\mathrm C 2 5 ∘ C . The equivalence volume is 20.0 mL and the pH at half-equivalence is 5.00. Acids, bases and titration Q45 · Original practice · 5 marks A solid sample of anhydrous sodium carbonate of mass 1.325 g is dissolved and made up to 250.0 mL. A 25.00 mL aliquot requires 24.60 mL HCl for complete neutralisation. Use M ( N a 2 C O 3 ) = 106.0 g m o l − 1 M(\mathrm{Na_2CO_3})=106.0\ \mathrm{g\,mol^{-1}} M ( N a 2 C O 3 ) = 106.0 g mo l − 1 . Acids, bases and titration Q46 · Original practice · 5 marks A vinegar sample is diluted by transferring 10.00 mL to a 100.0 mL volumetric flask. A 20.00 mL aliquot of the dilution requires 16.40 mL of 0.1000 mol L⁻¹ NaOH. Treat all acidity as ethanoic acid; its molar mass is 60.05 g mol⁻¹.
Acids, bases and titration Q47 · Original practice · 5 marks The idealised conductometric curve shows 25.0 mL of hydrochloric acid titrated with 0.0500 mol L⁻¹ NaOH. Both branches intersect at 20.0 mL.
Acids, bases and titration Q48 · Original practice · 4 marks A student titrates a weak acid with strong base. Its steep pH change spans pH 7.5–10.5. Indicator X has p K a = 4.0 pK_a=4.0 p K a = 4.0 and indicator Y has p K a = 9.0 pK_a=9.0 p K a = 9.0 . Acids, bases and titration Q85 · Original practice · 5 marks A 20.00 mL sample of an unknown weak monoprotic acid requires 15.00 mL of 0.1200 mol L⁻¹ NaOH. The untreated acid has pH 2.80.
Acids, bases and titration Q103 · Original practice · 1 mark Two weak monoprotic acids are compared at the same temperature. Assume that their measured hydrogen ions come from acid ionisation. Which conclusion is supported by the data?
Acids, bases and titration Q104 · Original practice · 1 mark An indicator has equilibrium H I n ( a q ) ⇌ H + ( a q ) + I n − ( a q ) \mathrm{HIn(aq)\rightleftharpoons H^+(aq)+In^-(aq)} HIn ( aq ) ⇌ H + ( aq ) + I n − ( aq ) and p K a = 6.0 pK_a=6.0 p K a = 6.0 . HIn is yellow and In⁻ is blue. What is the expected dominant colour at pH 8.0? Acids, bases and titration Q127 · Original practice · 5 marks The table gives measured pH values for pure water at two temperatures. Use the self-ionisation equilibrium to analyse the observations.
Acids, bases and titration Q128 · Original practice · 6 marks The curve shows 25.0 mL of a weak monobasic base B titrated with 0.100 mol L⁻¹ HCl at 25 °C. The equivalence volume is 20.0 mL. At half-equivalence, pH = 9.20.
Acids, bases and titration Q129 · Original practice · 5 marks The idealised conductometric curve shows a weak monoprotic acid titrated with NaOH. It has been corrected for dilution. A 30.0 mL acid sample has equivalence volume 15.0 mL when the NaOH concentration is 0.0800 mol L⁻¹.
Acids, bases and titration Q130 · Original practice · 4 marks At 25 °C, 20.0 mL of 0.0600 mol L⁻¹ HCl is mixed with 15.0 mL of 0.0500 mol L⁻¹ Ba(OH)₂. Assume complete dissociation and additive volumes.
Acids, bases and titration Q131 · Original practice · 4 marks Solution X is 0.00500 mol L⁻¹ HCl with pH 2.30. Solution Y is 0.0500 mol L⁻¹ of a weak monoprotic acid with pH 3.30. Both are at the same temperature.
Acids, bases and titration Q132 · Original practice · 6 marks A 5.105 g sample of a pure monoprotic acid standard (molar mass 204.2 g mol⁻¹) is dissolved and made up to 250.0 mL. Separate 25.00 mL aliquots are titrated with NaOH. The titres are shown. A 25.00 mL aliquot of another monoprotic acid requires 18.80 mL of the same NaOH.
Acids, bases and titration