Titration Exam Qs

2020 Q23

A solution of citric acid, C₃H₅O(COOH)₃, was analysed by titration.

25.0 mL aliquots of the C₃H₅O(COOH)₃ solution were titrated against a standardised solution of 0.0250 M sodium hydroxide, NaOH. Phenolphthalein indicator was used and the average titre was found to be 24.0 mL.

Based on the titration, the concentration of C₃H₅O(COOH)₃ in the solution was

• 8.0 × 10⁻³ M

• 8.7 × 10⁻³ M

• 2.6 × 10⁻² M

• 7.2 × 10⁻² M

2020 Q24

A solution of citric acid, C₃H₅O(COOH)₃, was analysed by titration.

25.0 mL aliquots of the C₃H₅O(COOH)₃ solution were titrated against a standardised solution of 0.0250 M sodium hydroxide, NaOH. Phenolphthalein indicator was used and the average titre was found to be 24.0 mL.

Which one of the following would have resulted in a concentration that is higher than the actual concentration?

• The pipette was rinsed with NaOH solution.

• The pipette was rinsed with C₃H₅O(COOH)₃ solution.

• The conical flask was rinsed with NaOH solution.

• The conical flask was rinsed with C₃H₅O(COOH)₃ solution.

2019 Q29

The concentration of vitamin C in a filtered sample of grapefruit juice was determined by titrating the juice with

9.367 × 10⁻⁴ M iodine, I₂, solution using starch solution as an indicator. The molar mass of vitamin C is

176.0 g mol⁻¹. The reaction can be represented by the following equation.

C₆H₈O₆(aq) + I₂(aq) → C₆H₆O₆(aq) + 2H⁺(aq) + 2I⁻(aq)

The following method was used:

1. Weigh a clean 250 mL conical flask.

2. Use a 10 mL measuring cylinder to measure 5 mL of grapefruit juice into the conical flask and reweigh it.

3. Add 20 mL of deionised water to the conical flask.

4. Add a drop of starch solution to the conical flask.

5. Titrate the diluted grapefruit juice against the I2 solution.

Which one of the following errors would result in an underestimation of the concentration of vitamin C in grapefruit juice?

• 19 mL of deionised water was added to the conical flask.

• The concentration of the I₂ solution was actually 9.178 × 10⁻⁴ M.

• The initial volume of the I₂ solution in the burette was 1.50 mL, but it was read as 2.50 mL.

• The balance was faulty and the measured mass of grapefruit juice was lower than the actual mass.

2019 Q29

The concentration of vitamin C in a filtered sample of grapefruit juice was determined by titrating the juice with

9.367 × 10−4 M iodine, I2, solution using starch solution as an indicator. The molar mass of vitamin C is

176.0 g mol−1. The reaction can be represented by the following equation.

C6H8O6(aq) + I2(aq) → C6H6O6(aq) + 2H+(aq) + 2I−(aq)

The following method was used:

1. Weigh a clean 250 mL conical flask.

2. Use a 10 mL measuring cylinder to measure 5 mL of grapefruit juice into the conical flask and reweigh it.

3. Add 20 mL of deionised water to the conical flask.

4. Add a drop of starch solution to the conical flask.

5. Titrate the diluted grapefruit juice against the I2 solution.

If the measured mass of grapefruit juice was 4.90 g and the titre was 21.50 mL, what was the measured percentage mass/mass (% m/m) concentration of vitamin C in the grapefruit juice?

• 0.00987

• 0.0723

• 0.354

• 3.36

2019 NHT Q19

Which one of the following statements regarding titrations is correct?

• Phenolphthalein is an appropriate indicator to use for a titration of ethanol against acidified potassium permanganate.

• The most appropriate indicator for the titration of a weak acid with a strong base is methyl red.

• Dilute solutions of strong bases can be titrated more accurately than concentrated solutions.

• Distilled water should be used for the final rinse of all glassware that will be used in a titration.

2018 Q17

A clear, colourless liquid extract of the rhubarb plant was analysed for the concentration of oxalic acid, H2C2O4,

by direct titration with a recently standardised and acidified potassium permanganate solution, KMnO4(aq).

The balanced equation for this titration is shown below.

2MnO₄^–(aq) + 5C₂O₄ ^2–(aq) + 16H⁺(aq) → 2Mn²⁺(aq) +

purple colourless colourless

10CO₂(g) + 8H₂O(l)

The steps in the titration were as follows:

Step 1 – A 20.00 mL aliquot of the rhubarb extract was placed in a 200 mL conical flask.

Step 2 – The burette was filled with acidified 0.0200 M KMnO4 solution.

Step 3 – The acidified 0.0200 M KMnO₄ solution was titrated into the rhubarb extract in the conical flask. The

titration was considered to have reached the end point when the solution in the conical flask showed a

permanent change in colour to pink. The volume of the titre was recorded.

Step 4 – The titration was repeated until three concordant results were obtained. The average of the concordant titres was 21.7 mL.

The concentration of H₂C₂O₄ in the rhubarb extract is closest to

• 5.43 × 10^–2 M

• 5.00 × 10^–2 M

• 2.17 × 10^–2 M

• 7.40 × 10^–4 M

2018 Q17

A clear, colourless liquid extract of the rhubarb plant was analysed for the concentration of oxalic acid, H2C2O4,

by direct titration with a recently standardised and acidified potassium permanganate solution, KMnO4(aq).

The balanced equation for this titration is shown below.

2MnO₄^–(aq) + 5C₂O₄ ^2–(aq) + 16H⁺(aq) → 2Mn²⁺(aq) +

purple colourless colourless

10CO₂(g) + 8H₂O(l)

The steps in the titration were as follows:

Step 1 – A 20.00 mL aliquot of the rhubarb extract was placed in a 200 mL conical flask.

Step 2 – The burette was filled with acidified 0.0200 M KMnO4 solution.

Step 3 – The acidified 0.0200 M KMnO₄ solution was titrated into the rhubarb extract in the conical flask. The

titration was considered to have reached the end point when the solution in the conical flask showed a

permanent change in colour to pink. The volume of the titre was recorded.

Step 4 – The titration was repeated until three concordant results were obtained. The average of the concordant titres was 21.7 mL.

Which of the following rinses is least likely to affect the accuracy of the results?

• burette - rinsing with distilled water

• burette - rinsing with rhubarb extract

• pipette - rinsing with KMnO₄(aq)

• conical flask - rinsing with distilled water

2018 Q26

Organic acids, including vitamin C (ascorbic acid), are present in lemon juice. Since organic acids and vitamin C

are weak acids, they will undergo acid-base reactions. Only vitamin C, not the organic acids, will undergo a redox reaction with iodine, I₂.

Which one of the following methods would be most appropriate to determine the concentrations of the organic acids and the vitamin C in a sample of lemon juice?

• an acid-base titration with sodium hydroxide and phenolphthalein indicator

• an acid-base titration with ammonia and phenol red indicator, and a redox titration with iodine and permanganate ion indicator

• an acid-base titration with sodium hydroxide and methyl orange indicator, and a redox titration with iodine and a starch indicator

• an acid-base titration with potassium hydroxide and phenolphthalein indicator, and a redox titration with iodine

  and a starch indicator

2018 NHT Q9

Before water treatment authorities release water into the environment, the water is tested to ensure it is safe

and meets environmental standards.

The concentration of organic carbon is one indicator of water quality. In an experiment, a student determines

the concentration of organic carbon by conducting a redox titration between the organic carbon in a water

sample and standard acidified potassium permanganate solution, KMnO₄.

To accurately determine the concentration of organic carbon, an action the student should take is to

• collect samples before and after a storm.

• repeat the titration using a different standard solution.

• use a measuring cylinder to measure the volumes of water samples.

• rinse the burette with deionised water before filling it with the standard acidified KMnO₄ solution.

2015 Q1

Which one of the following graphs represents the pH change when a weak acid is added to a strong base?

• A

• B

• C

• D