Burette readings (initials and final) must be given to two decimal places. Volume of pipette used must also be recorded but no account experimental procedure is required. Al calculations must be done in your answer book. A solution containing \(0.05\mathrm{moldm}^3\) \(\mathrm{H_2SO_4}\). B is a solution containing \(1.4\mathrm{g}\) per \(250\mathrm{cm}^3\) .XOH
(a) Put A into the burette and titrate it against \(20.0\mathrm{cm}^3\) or \(25.0\mathrm{cm}^3\) portions of B using methyl orange indicator. Repeat the titration to obtain consistent titres. Tabulate your results and calculate the average volume of A used. The equation for the reaction involved in the titration is; \(\mathrm{H_2SO_{4(aq)} + 2XOH_{(aq)} \to X_2SO_{4(aq)} + 2H_2O_{(l)}}\)
(b) From your results and the information provided above, calculate the;
(i) concentration of B in \(\mathrm{moldm}^{-3}\)
(ii) molar mass XOH
(iii) relative atomic mass of X. [H = 1.00; O = 16.0 S =32.0]
(a) Titration results
Rough
1
2
Final reading (cm3)
25.20
25.00
50.00
Initial reading (cm3)
0.00
0.00
25.00
Volume of A used (cm3)
25.20
25.00
25.00
Average volume of A used = (25.00 + 25.00) / 2 = 25.00 cm3.
(a)i) How would an aqueous solution of iron (II) tetraoxosulphate (VI) be converted into an aqueous solution of magnesium tetraoxosulphate (VI)?
(ii) Write a balanced equation for the reaction in (a)(i) above
(b)(i) Why are some compounds recrystallized after preparation?
(ii) Outline the steps in recrystallization
(C)(i) Name two gases that can cause color changes in an acidified solution of potassium heptaoxodichromate (VI)
(ii) State the color change expected in (c)i) above
(a)(i) Add excess magnesium (turnings/powder) to the aqueous iron(II) tetraoxosulphate(VI) solution and warm. Magnesium is more reactive than iron, so it displaces iron from the solution; the grey iron is deposited while magnesium tetraoxosulphate(VI) forms in solution. Filter off the excess magnesium and the displaced iron to obtain the \(MgSO_4\) solution.
(a)(ii) \[Mg + FeSO_4 \to MgSO_4 + Fe\]
(b)(i) Compounds are recrystallised after preparation in order to purify them, that is, to remove soluble impurities and obtain pure, well-formed crystals.
(b)(ii) Steps in recrystallisation
Dissolve the impure solid in the minimum volume of a hot suitable solvent to make a hot saturated solution.
Filter the hot solution to remove insoluble impurities.
Allow the filtrate to cool so that pure crystals separate out while soluble impurities remain in solution.
Filter off the crystals, wash with a little cold solvent, and dry them.
(c)(i) Two gases that change the colour of acidified potassium heptaoxodichromate(VI): sulphur(IV) oxide, \(SO_2\), and hydrogen sulphide, \(H_2S\) (both are reducing gases).
(c)(ii) The colour changes from orange to green (the orange \(Cr_2O_7^{2-}\) is reduced to green \(Cr^{3+}\)).
(a)(i) Add excess magnesium (turnings/powder) to the aqueous iron(II) tetraoxosulphate(VI) solution and warm. Magnesium is more reactive than iron, so it displaces iron from the solution; the grey iron is deposited while magnesium tetraoxosulphate(VI) forms in solution. Filter off the excess magnesium and the displaced iron to obtain the \(MgSO_4\) solution.
(a)(ii) \[Mg + FeSO_4 \to MgSO_4 + Fe\]
(b)(i) Compounds are recrystallised after preparation in order to purify them, that is, to remove soluble impurities and obtain pure, well-formed crystals.
(b)(ii) Steps in recrystallisation
Dissolve the impure solid in the minimum volume of a hot suitable solvent to make a hot saturated solution.
Filter the hot solution to remove insoluble impurities.
Allow the filtrate to cool so that pure crystals separate out while soluble impurities remain in solution.
Filter off the crystals, wash with a little cold solvent, and dry them.
(c)(i) Two gases that change the colour of acidified potassium heptaoxodichromate(VI): sulphur(IV) oxide, \(SO_2\), and hydrogen sulphide, \(H_2S\) (both are reducing gases).
(c)(ii) The colour changes from orange to green (the orange \(Cr_2O_7^{2-}\) is reduced to green \(Cr^{3+}\)).
Credit will be given for strict adherence to the instructions, for observations precisely recorded, and for accurate inferences. AIl tests. observations and inferences must be clearly entered in your answer book, in ink, at the time they are made.
C is a double salt. Carry out the following exercises on C. Record your observations and identify any gas(es) evolved. State the conclusion drawn from the result of each test.
a) Put all of C into a test tube. Add about 5cm\(^3\) of distilled water, stir and test with litmus paper. Divide the Solution into two portions
(b) To the first portion, add sodium hydroxide solution in drops and then in excess. Heat the resulting mixture and keep it for minutes.
(c) To the second portion, add few drops of Bacl\(_{2(aq)}\) followed by excess dilute hydrochloric acid
Test
Observation
Inference
(a) All of C + about 5 cm3 distilled water, stir, test with litmus
C dissolves completely to give a light green solution; the solution turns blue litmus paper red
C is a soluble salt; the solution is acidic
(b) First portion + NaOH(aq) dropwise then in excess, then heat and keep for a few minutes
A green gelatinous precipitate forms which is insoluble in excess NaOH; on heating a colourless, pungent, choking gas is evolved which turns damp red litmus paper blue and gives dense white fumes with conc. HCl; the precipitate slowly turns brown at the upper part of the tube
Fe2+ present (green Fe(OH)2); NH4+ present (ammonia gas from the ammonium ion); the Fe2+ is oxidized to Fe3+ in air, giving the brown colour
(c) Second portion + a few drops of BaCl2(aq), then excess dilute HCl
A white precipitate forms which does not dissolve in excess dilute HCl
SO42- present (white BaSO4, insoluble in acid)
Conclusion: C contains Fe2+, NH4+ and SO42- ions, so C is the double salt ammonium iron(II) tetraoxosulphate(VI), \((NH_4)_2SO_4 \cdot FeSO_4 \cdot 6H_2O\).
(a) All of C + about 5 cm3 distilled water, stir, test with litmus
C dissolves completely to give a light green solution; the solution turns blue litmus paper red
C is a soluble salt; the solution is acidic
(b) First portion + NaOH(aq) dropwise then in excess, then heat and keep for a few minutes
A green gelatinous precipitate forms which is insoluble in excess NaOH; on heating a colourless, pungent, choking gas is evolved which turns damp red litmus paper blue and gives dense white fumes with conc. HCl; the precipitate slowly turns brown at the upper part of the tube
Fe2+ present (green Fe(OH)2); NH4+ present (ammonia gas from the ammonium ion); the Fe2+ is oxidized to Fe3+ in air, giving the brown colour
(c) Second portion + a few drops of BaCl2(aq), then excess dilute HCl
A white precipitate forms which does not dissolve in excess dilute HCl
SO42- present (white BaSO4, insoluble in acid)
Conclusion: C contains Fe2+, NH4+ and SO42- ions, so C is the double salt ammonium iron(II) tetraoxosulphate(VI), \((NH_4)_2SO_4 \cdot FeSO_4 \cdot 6H_2O\).
