Year 12 Chemistry Chapter 14

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) + 10CO₂(g) + 8H₂O(l)

purple colourless colourless

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?

• Item: burette

  Rinse solution: distilled water

• Item: burette

  Rinse solution: rhubarb extract

• Item: pipette

  Rinse solution: KMnO₄(aq)

• Item: conical flask

  Rinse solution: distilled water

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

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.

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

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

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.

A 10.00 mL aliquot of methanoic acid was pipetted into a 150 mL conical flask and titrated with a standardised 0.150 M sodium hydroxide solution. The titration was repeated several times. An average titre of 13.4 mL was required to reach the end point.

The concentration of methanoic acid is closest to

• 0.100 M

• 0.200 M

• 0.300 M

• 0.400 M

A 10.00 mL aliquot of methanoic acid was pipetted into a 150 mL conical flask and titrated with a standardised 0.150 M sodium hydroxide solution. The titration was repeated several times. An average titre of 13.4 mL was required to reach the end point.

The experiment was repeated but all of the glassware used had been rinsed with water and left wet. A pipette

was used to take the aliquot, a burette was used to dispense the sodium hydroxide solution and a flask was

used for the titration reaction.

Which of these pieces of glassware being left wet with water would affect the calculated concentration of

methanoic acid?

• pipette only

• pipette and burette only

• burette and flask only

• pipette, burette and flask

A 10.00 mL aliquot of methanoic acid was pipetted into a 150 mL conical flask and titrated with a standardised 0.150 M sodium hydroxide solution. The titration was repeated several times. An average titre of 13.4 mL was required to reach the end point.

Which of the following is an example of a systematic error which could occur during this

titration experiment?

• The sodium hydroxide solution was standardised three times and values of 0.151 M, 0.150 M and

  0.149 M were determined.

• The student misread the burette during the second titration.

• The pipette dispensed aliquots of 9.95 ± 0.03 mL.

• A 200 mL conical flask was used in place of the 150 mL flask.

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) + 10CO₂(g) + 8H₂O(l)

purple colourless colourless

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