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FB 1 is a solution made by dissolving an unknown mass of a mixture of ethanedioic acid, (COOH)₂, and sodium ethanedioate, (COONa)₂. You will carry out two titrations to find the percentage by mass of ethanedioic acid in the mixture.
Titration 1
In aqueous solution both ethanedioic acid and sodium ethanedioate release all their ethanedioate ions, (COO⁻)₂. These ions react with manganate(VII) ions as shown.
2MnO₄⁻(aq) + 16H⁺(aq) + 5(COO⁻)₂(aq) → 10CO₂(g) + 2Mn²⁺(aq) + 8H₂O(l)
FB 1 is an aqueous solution of the mixture containing ethanedioic acid and sodium ethanedioate.
FB 2 is 0.0200 mol dm⁻³ potassium manganate(VII), KMnO₄.
FB 3 is 1.00 mol dm⁻³ sulfuric acid, H₂SO₄.
(a) Method
- Fill a burette with FB 2.
- Pipette 25.0 cm³ of FB 1 into a conical flask.
- Use the measuring cylinder to add 30 cm³ of FB 3 to the same conical flask.
- Place the conical flask on the tripod and gauze and heat until the solution is at a temperature of approximately 70 °C.
- Carefully remove the flask from the tripod and place it under the burette, ready for the titration.
- Add FB 2 from the burette, slowly at first, until a permanent pale pink colour is formed. If the reaction mixture turns brown, reheat it to about 70 °C. If the brown colour disappears, continue with the titration. If the brown colour remains, discard the contents of the flask and begin a new titration.
- Perform a rough titration and record your burette readings in the space below.
The rough titre is ............................. cm³.
- Carry out as many accurate titrations as you think necessary to obtain consistent results.
- Make sure any recorded results show the precision of your practical work.
- Record in a suitable form below all of your burette readings and the volume of FB 2 added in each accurate titration.
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(b) From your accurate titration results, obtain a suitable value for the volume of FB 2 to be used in your calculations.
Show clearly how you obtained this value.
25.0 cm³ of FB 1 required ............................. cm³ of FB 2.
(c) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) Calculate the number of moles of manganate(VII) ions in the volume of FB 2 calculated in (b).
moles of MnO₄⁻ = ............................. mol
(ii) Calculate the total number of moles of ethanedioate ions present in 25.0 cm³ of FB 1.
total moles of (COO⁻)₂ = ............................. mol
Titration 2
Ethanedioic acid reacts with aqueous sodium hydroxide. In this reaction both the H⁺ ions formed by the acid molecule react.
(d) Complete the equation showing the reaction between ethanedioic acid and sodium hydroxide including state symbols.
........(COOH)₂(aq) + ........NaOH(aq) → ............................. + .............................
FB 4 is 0.0400 mol dm⁻³ sodium hydroxide, NaOH.
thymol blue indicator
(e) Method
- Fill the second burette with FB 4.
- Pipette 25.0 cm³ of FB 1 into a conical flask.
- Add about 10 drops of thymol blue indicator.
- Add FB 4 from the burette until the end-point has been reached.
- Perform a rough titration and record your burette readings in the space below.
The rough titre is ............................. cm³.
- Carry out as many accurate titrations as you think necessary to obtain consistent results.
- Make sure any recorded results show the precision of your practical work.
- Record in a suitable form below all of your burette readings and the volume of FB 4 added in each accurate titration.
(f) Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(i) From your accurate titration results, obtain a suitable value for the volume of FB 4 to be used in your calculations.
25.0 cm³ of FB 1 required ............................. cm³ of FB 4.
(ii) Calculate the number of moles of sodium hydroxide in the volume of FB 4 calculated in (i).
moles of NaOH = ............................. mol
(iii) Use your equation from (d) to calculate the number of moles of ethanedioic acid present in 25.0 cm³ of FB 1.
moles of (COOH)₂ = ............................. mol
(g) (i) Use your answers to (c)(ii) and (f)(iii) to calculate the number of moles of sodium ethanedioate, (COONa)₂, present in 25.0 cm³ of FB 1.
moles of (COONa)₂ = ............................. mol
(ii) Calculate the mass of sodium ethanedioate present in 25.0 cm³ of FB 1.
mass of (COONa)₂ = ............................. g
(iii) Use your answer to (f)(iii) to calculate the mass of ethanedioic acid present in 25.0 cm³ of FB 1.
mass of (COOH)₂ = ............................. g
(iv) Calculate the percentage by mass of ethanedioic acid in the solid mixture used to prepare FB 1.
percentage by mass of (COOH)₂ = ............................. %
(h) A student checked the formula of ethanedioic acid on the internet and found it to be (COOH)₂.2H₂O. This differs from the formula (COOH)₂ that you used in your calculations.
The FB 1 you used was made from (COOH)₂.2H₂O and sodium ethanedioate.
State and explain the effect this knowledge has on;
(i) the volume of FB 4 needed for reaction in (e),
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(ii) the calculated percentage by mass of (COOH)₂ in the solid mixture used to prepare FB 1.
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(i) Another student suggested that the investigation could be improved by making the titrations more accurate. He said that the concentrations of FB 2 and FB 4 should be reduced.
State and explain whether or not this suggestion would make the titrations more accurate.
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549
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507
FB 5, FB 6 and FB 7 are aqueous solutions that each have an ion containing one of the metals from those listed in the Qualitative Analysis Notes.
(a) Carry out the following tests and record your observations.
[Table]
(i) To a 1 cm depth of FB 5 in a test-tube add a 1 cm depth of aqueous sodium hydroxide, then add several drops of hydrogen peroxide.
(ii) To a 1 cm depth of FB 6 in a test-tube add aqueous sodium hydroxide.
(iii) To a 1 cm depth of FB 6 in a test-tube add several drops of hydrogen peroxide and then add aqueous sodium hydroxide.
(iv) To a 1 cm depth of FB 6 in a test-tube add a 1 cm depth of dilute sulfuric acid and then add a few drops of FB 7.
(v) To a 1 cm depth of FB 6 in a test-tube add a 1 cm depth of FB 7.
(vi) To a 1 cm depth of aqueous potassium iodide in a test-tube add a few drops of FB 7, then add a few drops of aqueous starch.
(b) Identify the metal present in FB 5, FB 6 and FB 7.
FB 5 contains ..................................................
FB 6 contains ..................................................
FB 7 contains ..................................................
(c) What do your observations in (a)(vi) tell you about what has happened to the iodide ions on addition of FB 7 to KI(aq)? You may give your answer in the form of an equation.
(d) (i) FB 8 is a solid sample of the compound present in aqueous solution FB 7. Heat all of FB 8 in a hard-glass test-tube gently for about 10s and then strongly for about 20s.
observations ..................................................
(ii) Leave the test-tube and contents to cool completely.
To the cooled test-tube add a 1 cm depth of aqueous sodium hydroxide. Observe the appearance of the contents of the test-tube.
Appearance ..................................................
(e) FB 6 contains one of the anions \( Cl^- \), \( Br^- \), \( I^- \), \( SO_4^{2-} \) or \( SO_3^{2-} \).
(i) Construct a table to show reagents you would use to identify which anion is present in FB 6. Include in your table space to record your observations and deductions.
(ii) Carry out your tests on FB 6 until you have identified the anion. Record your observations and deductions in your table.
anion in FB 6 = ..................................................
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