Amount of substance: AQA A-Level Chemistry past-paper questions

05.1 Calculate the relative molecular mass of H₂A

Relative molecular mass of H₂A

05.2 The uncertainty in the use of the burette in each titration is ±0.15 cm³

Calculate the percentage uncertainty in the use of the burette in titration 1.

Percentage uncertainty

05.3 Suggest one way to reduce the percentage uncertainty in each titration without changing the apparatus.

Justify your answer.

Suggestion

Justification

08.1 Use the results to determine an experimental value for the M_r of Y.

The gas constant, R = 8.31 J K⁻¹ mol⁻¹

Show your working.

M_r

08.2 The percentage uncertainty in the volume of Y in the gas syringe is 0.95%

The percentage uncertainty in the temperature is 0.12%

The uncertainty in each balance reading is ± 0.0005 g

Assume that the uncertainty in the value for pressure is negligible.

Calculate the percentage uncertainty in the use of the balance in this experiment.

Use your answer and the percentage uncertainties for volume and temperature to calculate a range of values for the M_r of Y.

(If you were unable to calculate an experimental value for the M_r of Y in

Question 08.1, assume it is 141. This is not the correct answer.)

Percentage uncertainty for balance

M_r range

08.3 A teacher suggests placing a seal over the needle of the hypodermic syringe after collecting the sample of liquid Y, and then replacing the seal after injecting the liquid Y

into the gas syringe.

Give a reason why the teacher’s suggestion will improve the experiment.

08.4 A mixture of methane and propane has a volume of 40 cm³

This mixture needs exactly 170 cm³ of oxygen for complete combustion.

The equations for these reactions are

CH₄ + 2O₂ → CO₂ + 2H₂O

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Calculate the volume

• of methane in the original mixture
• of propane in the original mixture.

All volumes are measured at the same temperature and pressure.

Volume of methane cm³

Volume of propane cm³

(a) State why the atomic radius of calcium is greater than the atomic radius of magnesium.

(b) Magnesium reacts with steam.

Give an equation, including state symbols, for this reaction.

(c) Similar-sized pieces of barium and magnesium are added to separate 100 cm³ samples of dilute sulfuric acid. In each case the sulfuric acid is in excess.

The barium reacts quickly at first. After a few minutes the reaction stops, even though there is still some unreacted barium in the flask.

The magnesium reacts more slowly than the barium, but the reaction continues until all the magnesium has reacted.

Explain why

• the barium initially reacts more quickly than the magnesium
• the barium reaction stops before all the barium has reacted.

(d) A metal nitrate X(NO₃)₂ completely decomposes when heated.

2 X(NO₃)₂(s) → 2 XO(s) + 4 NO₂(g) + O₂(g)

A 0.832 g sample of X(NO₃)₂ decomposes on heating to produce a total of 348 cm³ of gas at 298 K and 100 kPa

Deduce the identity of metal X.

The ideal gas constant, R = 8.31 J K⁻¹ mol⁻¹

Identity of metal X

(e) Sodium reacts with aluminium and hydrogen to form solid NaAlH₄

Give an equation for this reaction.

Suggest why NaAlH₄ has a high melting point.

Equation

Suggestion

(f) Give the equation for the reaction between H₃PO₄ and an excess of NaOH

Lithium is an important metal used in cells to power mobile phones.

(g) In a lithium cell, a lithium cobalt oxide electrode and a lithium electrode are used.

Give the equation for the reaction that occurs at the positive electrode.

(h) Commercial electrochemical cells can be rechargeable or non-rechargeable.

State why lithium cells can be recharged.

(a) Suggest why the solution is kept until no more solid needs to be added.

(b) Suggest why it is important to remove the undissolved strontium hydroxide before the titration.

(c) After the filtration, the solution is stored in a stoppered flask.

Suggest a reason for stoppering the flask.

(d) The diagrams in the figure below show the part of a pipette with the graduation line.

Which diagram identifies the pipette that is correctly filled?

Tick (✓) one box.

(e) Solubility can be quoted as ‘g of solute per 100 cm³ of solution’.

The table below shows the results of the titrations between strontium hydroxide and hydrochloric acid. These can be used to determine the solubility of strontium hydroxide.

Give the equation for the reaction between strontium hydroxide and hydrochloric acid.

Use the results in the above table to calculate the mean titre.

Use the mean titre to calculate the solubility of strontium hydroxide, in g per 100 cm³ of solution, at 20 °C

Equation

Mean titre cm³

Solubility of strontium hydroxide g per 100 cm³ solution

(a) Magnesium reacts with hydrochloric acid.

Mg(s) + 2 HCl(aq) → MgCl₂(aq) + H₂(g)

0.400 g of magnesium is added to 20.0 cm³ of 1.50 mol dm⁻³ hydrochloric acid.

Identify the limiting reagent.

Justify your answer.

Calculate the volume, in m³, of hydrogen produced at 101 kPa and 15 °C

The gas constant, R = 8.31 J K⁻¹ mol⁻¹

Limiting Reagent

Justification

Volume of hydrogen m³

(b) Propane (C₃H₈) undergoes complete combustion in a plentiful supply of oxygen.

Give an equation for the complete combustion of propane.

Use this equation to calculate the minimum volume, in cm³, of oxygen gas needed for the complete combustion of 50 cm³ of propane gas.

Assume that the volumes of both gases are measured at the same temperature and pressure.

Equation for combustion

Volume of oxygen gas cm³

(a) Each reading from the balance has an uncertainty of ±0.005 g

Calculate the percentage uncertainty in using the balance in this experiment.

Percentage uncertainty

(b) Calculate the amount, in moles, of hydrochloric acid that was added to the tablets in Step 3.

Give your answer to an appropriate precision.

Amount of hydrochloric acid mol

(c) Use your answer to part (b) and the information given to calculate the percentage by mass of magnesium oxide in the tablets.

Percentage by mass of MgO

(a) Calculate the volume, in cm³, of propanone in the gas syringe.

The gas constant, R = 8.31 J K⁻¹ mol⁻¹

Volume of propanone cm³

(b) The gas syringe is then cooled to 75 °C, without changing the pressure.

Calculate the decrease in volume.

(If you were unable to calculate the volume in part (a), you should use the volume 89 cm³. This is not the correct answer.)

Decrease in volume cm³

(c) The total uncertainty in using the balance to measure the mass of propanone in part (a) is ±0.001 g

Calculate the uncertainty that this causes in the volume, in cm³, of propanone calculated in part (a).

(If you were unable to calculate the volume in part (a), you should use the volume 89 cm³. This is not the correct answer.)

Uncertainty cm³

(d) A 600 cm³ sample of propanone is mixed with 2800 cm³ of oxygen in a container at 60 °C and 100 kPa. The mixture is ignited.

When the reaction is complete, the remaining mixture of gases is cooled to 60 °C at 100 kPa

CH₃COCH₃(g) + 4 O₂(g) → 3 CO₂(g) + 3 H₂O(l)

Calculate the total volume of the remaining gas mixture.

Volume cm³

(a) Calculate the mass, in g, of solid barium hydroxide (M_r = 171.3) needed to prepare 250 cm³ of 0.100 mol dm⁻³ barium hydroxide solution.

Mass g

(b) The mass of barium hydroxide from part (a) is dissolved in a beaker containing 150 cm³ of distilled water.

Describe how this solution is used to make 250 cm³ of the

0.100 mol dm⁻³ barium hydroxide solution.

(c) Before the first titration, the 25 cm³ pipette is rinsed with a small volume of the 0.100 mol dm⁻³ barium hydroxide solution.

State why it is good practice to rinse the pipette in this way.

(d) Hydrochloric acid is added to the burette using a funnel.

State why it is good practice to remove the funnel from the burette before the titration.

(e) In a different experiment, 0.952 g of solid barium hydroxide is used to make 250 cm³ of standard barium hydroxide solution.

25.0 cm³ of this barium hydroxide solution reacts with exactly

24.50 cm³ of hydrochloric acid.

Calculate the concentration of the hydrochloric acid.

Concentration mol dm⁻³

(f) The uncertainty in the 25.0 cm³ of solution from the pipette is ±0.05 cm³

The total uncertainty in the 24.50 cm³ of solution from the burette is ±0.15 cm³

Calculate the total percentage error in using the pipette and burette.

Percentage error

(a) Suggest three improvements to this method.

1

2

3

(b) A different student uses the correct method to prepare

250 cm³ of sodium carbonate solution in a volumetric flask.

The uncertainty for the volumetric flask is ±0.20 cm³

Calculate the percentage uncertainty in the volume of this sodium carbonate solution.

Percentage uncertainty

(a) A suitable target titre for the titration is 30.0 cm³ of 0.025 mol dm⁻³ Br₂(aq).

Justify why a much smaller target titre would not be appropriate.

Calculate the amount, in moles, of bromine in the target titre.

Justification

Amount of bromine mol

(b) Calculate a suitable mass of olive oil to transfer to the volumetric flask using your answer to part (a) and the structure of Y.

Assume that the olive oil contains 85% of Y by mass.

(If you were unable to calculate the amount of bromine in the target titre, you should assume it is 6.25 ×10⁻⁴ mol. This is not the correct amount.)

Mass of olive oil g

The olive oil solution can be prepared using this method.

• Place a weighing bottle on a balance and record the mass, in g, to 2 decimal places.
• Add olive oil to the weighing bottle until a suitable mass has been added.
• Record the mass of the weighing bottle and olive oil.
• Pour the olive oil into a 250 cm³ volumetric flask.
• Add organic solvent to the volumetric flask until it is made up to the mark.
• Place a stopper in the flask and invert the flask several times.

(c) Suggest an extra step to ensure that the mass of olive oil in the solution is recorded accurately.

Justify your suggestion.

Extra step

Justification

(d) State the reason for inverting the flask several times.

(e) A sample of the olive oil was dissolved in methanol and placed in a mass spectrometer. The sample was ionised using electrospray ionisation.

Each molecule gained a hydrogen ion (H⁺) during ionisation.

The spectrum showed a peak for an ion with m/z = 345 formed from an impurity in the olive oil.

The ion with m/z = 345 was formed from a compound with the empirical formula C₅H₁₀O

Deduce the molecular formula of this compound.

Show your working.

Molecular formula

(a) Give an ionic equation for the reaction between potassium manganate(VII) and acidified hydrogen peroxide.

Calculate the concentration, in mol dm⁻³, of hydrogen peroxide in the original hair bleach solution.

(If you were unable to write an equation for the reaction you may assume that the mole ratio of potassium manganate(VII) to hydrogen peroxide is 3:4

This is not the correct mole ratio.)

Concentration mol dm⁻³

(b) State why an indicator is not added in this titration.

(c) Give the oxidation state of oxygen in hydrogen peroxide.

(d) Hydrogen peroxide decomposes to form water and oxygen.

Give an equation for this reaction.

Calculate the amount, in moles, of hydrogen peroxide that would be needed to produce 185 cm³ of oxygen gas at 100 kPa and 298 K

The gas constant, R = 8.31 J K⁻¹ mol⁻¹

Equation

Amount mol

(e) Hydrazine (N₂H₄) is used as a rocket fuel that is oxidised by hydrogen peroxide. The equation for this reaction in the gas phase is

The enthalpy change for this reaction, ΔH = –789 kJ mol⁻¹

The table below shows some mean bond enthalpy values.

Define the term mean bond enthalpy.

Use the equation and the data in the table above to calculate a value for the O–O bond enthalpy in hydrogen peroxide.

Definition

Bond enthalpy kJ mol⁻¹

(a) TNT (C₇H₅N₃O₆) is an explosive because it can decompose very quickly and exothermically to form a large volume of gas. An equation for this decomposition is

2 C₇H₅N₃O₆(s) → 3 N₂(g) + 5 H₂(g) + 12 CO(g) + 2 C(s)

Calculate the volume of gas, in m³, measured at 1250 °C and 101 000 Pa, produced by the decomposition of 1.00 kg of TNT (M_r = 227.0).

The gas constant, R = 8.31 J mol⁻¹ K⁻¹

Volume of gas m³

(b) Alkenes have the general formula C_nH_2n

When alkenes undergo complete combustion, 1.0 mol of C_nH_2n reacts with 3n/2 mol of oxygen.

Calculate the volume of oxygen needed for the complete combustion of 200 cm³ of but-1-ene.

The volumes of all gases are measured at the same temperature and pressure.

Volume of oxygen cm³

(c) Alkanes have the general formula C_nH_2n+2

Alkanes undergo complete combustion in a plentiful supply of oxygen.

C_nH_2n+2 + x O₂ → n CO₂ + (n+1) H₂O

Determine x in terms of n

x

(a) A student makes a solution of citric acid by dissolving some solid citric acid in water.

Describe a method to add an accurately known mass of solid to a beaker to make a solution.

(b) The student dissolves 0.834 g of citric acid in water and makes the solution up to 500 cm³

Calculate the concentration, in mol dm⁻³, of citric acid in this solution.

Concentration mol dm⁻³

(c) The student uses this method to complete a titration.

• Rinse a burette with distilled water.
• Fill the burette with sodium hydroxide solution.
• Use a measuring cylinder to transfer 25 cm³ of the citric acid solution into a conical flask.
• Add 5 cm³ of indicator.
• Slowly add the sodium hydroxide solution from the burette into the conical flask.
• Add the sodium hydroxide solution dropwise near the end point until the indicator just changes colour.
• Repeat the titration to get concordant results.

The method used by the student includes three practical steps that will lead to an inaccurate final result.

For each of these three steps

• identify the mistake
• explain why it is a mistake
• suggest how the mistake can be overcome.

(d) The table below shows the student’s burette readings after the mistakes in the practical procedure have been corrected.

Complete the table.

Use the data in the table above to calculate the mean titre.

Mean titre cm³

(e) The total uncertainty in the use of the burette is ±0.15 cm³

Calculate the percentage uncertainty in the use of the burette in Run 1.

Percentage uncertainty

(a) In the first experiment, liquid Y is injected into a sealed flask under vacuum. The liquid vaporises in the flask.

The table below shows data for this experiment.

Calculate the relative molecular mass of Y.

Show your working.

The gas constant, R = 8.31 J K⁻¹ mol⁻¹

Relative molecular mass of Y

(b) In the second experiment, another flask is used for a combustion reaction.

Method

• Remove all the air from the flask.
• Add 0.0010 mol of 2,2,4-trimethylpentane (C₈H₁₈) to the flask.
• Add 0.0200 mol of oxygen to the flask.
• Spark the mixture to ensure complete combustion.
• Cool the mixture to the original temperature.

The equation is

C₈H₁₈(g) + 12½O₂(g) → 8 CO₂(g) + 9 H₂O(l)

Calculate the amount, in moles, of gas in the flask after the reaction.

Amount of gas mol

(a) Complete the equation by drawing the structure of the other product of this reaction in the box.

Name the type of compound shown by the formula RCOOK

Give one use for this type of compound.

Type of compound

Use

(b) The triester in coconut oil has a relative molecular mass, M_r = 638.0

In the equation shown at the start of this question, R represents an alkyl group that can be written as CH₃(CH₂)_n

Deduce the value of n in CH₃(CH₂)_n

Show your working.

n

(c) A 1.450 g sample of coconut oil is heated with 0.421 g of KOH in aqueous ethanol until all of the triester is hydrolysed.

The mixture is cooled.

The remaining KOH is neutralised by exactly 15.65 cm³ of 0.100 mol dm⁻³ HCl

Calculate the percentage by mass of the triester (M_r = 638.0) in the coconut oil.

Percentage by mass

(d) Suggest why aqueous ethanol is a suitable solvent when heating the coconut oil with KOH.

Give a safety precaution used when heating the mixture.

Justify your choice.

Reason

Safety precaution

Justification

(a) Use the results in the table above to calculate the mean titre value.

Use the mean titre to calculate the percentage by mass of sodium ethanoate in the original sample.

Mean titre value cm³

Percentage by mass

(b) The student rinses the burette with deionised water before filling with sodium hydroxide solution.

State and explain the effect, if any, that this rinsing will have on the value of the titre.

(a) Give an expression for the equilibrium constant (K_c) for this reaction.

K_c

(b) A mixture of sulfur dioxide and oxygen is allowed to reach equilibrium in a container of volume 1800 cm³ at temperature T.

At equilibrium, the mixture contains 0.176 mol of sulfur dioxide and 0.461 mol of sulfur trioxide.

At temperature T the equilibrium constant, K_c = 15.0 mol⁻¹ dm³

Calculate the amount, in moles, of oxygen at equilibrium.

Amount of oxygen mol

(c) At a different temperature, a mixture contains

0.025 mol of sulfur dioxide

0.049 mol of oxygen

0.034 mol of sulfur trioxide.

The total pressure of the mixture in a 3500 cm³ reaction vessel is 255 kPa

Use the data to calculate the temperature, in °C, of the mixture.

The ideal gas constant, R = 8.31 J K⁻¹ mol⁻¹

Temperature °C

(a) Ethanedioic acid reacts with propane-1,3-diol (HOCH₂CH₂CH₂OH) to form a polyester.

Draw the repeating unit of this polyester.

(b) Explain why polyesters are biodegradable but polyalkenes are not biodegradable.

(c) Sodium ethanedioate is used to find the concentration of solutions of potassium manganate(VII) by titration. The equation for this reaction is

2 MnO₄⁻ + 16 H⁺ + 5 C₂O₄²⁻ → 2 Mn²⁺ + 8 H₂O + 10 CO₂

A standard solution is made by dissolving 162 mg of Na₂C₂O₄ (M_r = 134.0) in water and making up to 250 cm³ in a volumetric flask.

25.0 cm³ of this solution and an excess of sulfuric acid are added to a conical flask.

The mixture is warmed and titrated with potassium manganate(VII) solution.

The titration is repeated until concordant results are obtained.

The mean titre is 23.85 cm³

Calculate the concentration, in mol dm⁻³, of the potassium manganate(VII) solution.

Concentration mol dm⁻³

(d) Figure 1 shows the 25.0 cm³ pipette used to measure the sodium ethanedioate solution.

Figure 1

On Figure 1, draw the meniscus of the solution when the pipette is ready to transfer 25.0 cm³ of the sodium ethanedioate solution.

(e) Potassium manganate(VII) is oxidising and harmful.

Sodium ethanedioate is toxic.

Suggest safety precautions, other than eye protection, that should be taken when:

• filling the burette with potassium manganate(VII) solution
• dissolving the solid sodium ethanedioate in water.

Filling the burette

Dissolving the solid

(f) State the colour change seen at the end point of each titration.

(g) Figure 2 shows the burette containing potassium manganate(VII) solution.

Figure 2

Give two practical steps needed before recording the initial burette reading.

1

2

(h) When Na₂C₂O₄(aq) is added to a solution containing [Fe(H₂O)₆]³⁺ ions, a reaction occurs in which all six water ligands are replaced by ethanedioate ions.

Explain why the replacement of the water ligands by ethanedioate ions is favourable. In your answer refer to:

• the enthalpy and entropy changes for the reaction
• how the enthalpy and entropy changes influence the free-energy change for the reaction.

(a) Draw the skeletal formula of ethanedioic acid.

(b) Ethanedioic acid is formed by the oxidation of ethane-1,2-diol (HOCH₂CH₂OH).

State suitable reagent(s) and a condition for this reaction.

Reagent(s)

Condition

(c) Ethanedioic acid reacts with an excess of sodium hydroxide to form sodium ethanedioate.

H₂C₂O₄ (aq) + 2 NaOH (aq) → Na₂C₂O₄ (aq) + 2 H₂O (I)

A student mixes 10.0 cm³ of 0.400 mol dm⁻³ ethanedioic acid with 50.0 cm³ of 0.200 mol dm⁻³ sodium hydroxide.

Show that the sodium hydroxide is in excess.

Calculate the mass, in mg, of sodium ethanedioate that can be formed in this reaction.

Mass of sodium ethanedioate mg

(a) A 1.50 g sample of H₃Y·xH₂O contains 0.913 g of oxygen by mass.

The sample burns completely in air to form 1.89 g of CO₂ and 0.643 g of H₂O

Show that the empirical formula of citric acid is C₃H₅O₄

(b) A 3.00 g sample of H₃Y·xH₂O (M_r = 210.0) is heated to constant mass. The anhydrous H₃Y that remains has a mass of 2.74 g

Show, using these data, that the value of x = 1

The figure shows the structure of H₃Y

(c) Complete this IUPAC name for H₃Y

propane-1, 2, 3-tricarboxylic acid

(d) State the number of peaks you would expect in the ¹³C NMR spectrum for H₃Y

(a) Calculate the mass of sodium hydroxide used to make the original solution.

Mass of sodium hydroxide g

(b) The student uses a funnel to fill the burette with sulfuric acid before starting the titration. After filling, the student forgets to remove the funnel from the top of the burette.

Suggest why this might affect the titre volume recorded.

(c) State one advantage of using a conical flask rather than a beaker for the titration.

06.1 Use the results to calculate the M_r of H₂A

M_r of H₂A

06.2 The uncertainty in using the pipette in this experiment is ±0.06 cm³

Calculate the percentage uncertainty in using the pipette.

% uncertainty

06.3 Before adding the solution from the burette in the rough titration, there was an air bubble below the tap.

At the end of this titration the air bubble was not there.

Explain why this air bubble increases the final burette reading of the rough titration.

06.4 During the titration the student washed the inside of the conical flask with some distilled water.

Suggest why this washing does not give an incorrect result.

02.1 The student rinsed the burette before filling it with the sodium hydroxide solution.

State why the student should use sodium hydroxide solution rather than water for the final rinse of the burette.

02.2 The student carried out several titrations. The results are shown in Table 2.

Complete Table 2 to show the titre in each titration.

Table 2

02.3 Calculate the mean titre using the concordant results.

Give your answer to the appropriate number of significant figures.

Mean titre cm³

02.4 The total uncertainty when using the burette is ±0.15 cm³. This is the combination of uncertainties in the start reading, final reading and the determination of the end point.

Use your answer to Question 02.3 to calculate the percentage uncertainty for the use of the burette in this experiment.

Percentage uncertainty %

02.5 Use your answer to Question 02.3 to find the mass, in mg, of citric acid dissolved in

250 cm³ of the solution.

The relative molecular mass (M_r) of citric acid is 192.0

Mass mg

02.6 Calculate the percentage purity of this sample of citric acid.

Percentage purity %

10.1 A student added 627 mg of hydrated sodium carbonate (Na₂CO₃.xH₂O) to 200 cm³ of

0.250 mol dm⁻³ hydrochloric acid in a beaker and stirred the mixture.

After the reaction was complete, the resulting solution was transferred to a volumetric flask, made up to 250 cm³ with deionised water and mixed thoroughly.

Several 25.0cm³ portions of the resulting solution were titrated with 0.150 mol dm⁻³

aqueoussodium hydroxide. The mean titre was 26.60 cm³ of aqueoussodium hydroxide.

Calculate the value of x in Na₂CO₃.xH₂O

Show your working.

Give your answer as an integer.

Value of x

06.1 Balance the equations for the two stages in this process.

..…CuFeS₂ + ..…O₂ + ..…SiO₂ → ..…Cu₂S + ..…Cu₂O + ..…SO₂ + ..…FeSiO₃

……Cu₂S + ……Cu₂O → ……Cu + ……SO₂

06.2 Suggest two reasons why the sulfur dioxide by-product of this process is removed from the exhaust gases.

Reason 1

Reason 2

06.3 A passenger jet contains 4050 kg of copper wiring.

A rock sample contains 1.25% CuFeS₂ by mass.

Calculate the mass, in tonnes, of rock needed to produce enough copper wire for a passenger jet. (1 tonne = 1000 kg)

Mass of rock tonnes

06.4 Copper can also be produced by the reaction of carbon with copper(II) oxide according to the equation

2CuO + C → 2Cu + CO₂

Calculate the percentage atom economy for the production of copper by this process.

Give your answer to the appropriate number of significant figures.

Percentage atom economy

02.1 Calculate the relative molecular mass of X.

Show your working.

Give your answer to the appropriate number of significant figures.

The gas constant, R = 8.31 J K⁻¹mol⁻¹

Relative molecular mass

02.2 Analysis of a different hydrocarbon Y shows that it contains 83.7% by mass of carbon.

Calculate the empirical formula of Y.

Use this empirical formula and the relative molecular mass of Y (M_r = 86.0) to calculate the molecular formula of Y.

Empirical formula

Molecular formula

05.1 The student found that 25.0 cm³ of the ethanedioic acid solution reacted completely with 25.30 cm³ of 0.500 mol dm⁻³ sodium hydroxide solution.

Calculate the mass, in mg, of the acid in 25.0 cm³ of this solution.

Mass of acid mg

05.2 The student used a wash bottle containing deionised water when approaching the end-point to rinse the inside of the conical flask.

Explain why this improved the accuracy of the titration.

05.3 Give the meaning of the term concordant titres.

11.1 A white solid is a mixture of sodium ethanedioate (Na₂C₂O₄), ethanedioic acid dihydrate (H₂C₂O₄.2H₂O) and an inert solid. A volumetric flask contained

1.90 g of this solid mixture in 250 cm³ of aqueous solution.

Two different titrations were carried out using this solution.

In the first titration 25.0 cm³ of the solution were added to an excess of sulfuric acid in a conical flask. The flask and contents were heated to 60 °C and then titrated with a 0.0200 mol dm⁻³ solution of potassium manganate(VII). When

26.50 cm³ of potassium manganate(VII) had been added the solution changed colour.

The equation for this reaction is

2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 8H₂O + 10CO₂

In the second titration 25.0 cm³ of the solution were titrated with a

0.100 mol dm⁻³solution of sodium hydroxide using phenolphthalein as an indicator. The indicator changed colour after the addition of 10.45 cm³ of sodium hydroxide solution.

The equation for this reaction is

H₂C₂O₄ + 2OH⁻ → C₂O₄²⁻ + 2H₂O

Calculate the percentage by mass of sodium ethanedioate in the white solid.

Give your answer to the appropriate number of significant figures.

Show your working.

Percentage by mass of sodium ethanedioate %

11.2 Ethanedioate ions react with aqueous iron(III) ions in a ligand substitution reaction.

Write an equation for this reaction.

Suggest why the value of the enthalpy change for this reaction is close to zero.

11.3 Draw the displayed formula of the iron complex produced in the reaction in

Question 11.2

Indicate the value of the O—Fe—O bond angle.

State the type of isomerism shown by the iron complex.

Bond angle

Type of isomerism

11.4 Ethanedioate ions are poisonous because they react with iron ions in the body.

Ethanedioate ions are present in foods such as broccoli and spinach.

Suggest one reason why people who eat these foods do not suffer from poisoning.