Redox Calculations #2
3
Redox calculations 1. A piece of limestone weighing 0.1965 g was allowed to react with an excess of HCl. The calcium in it was precipitated as CaC 2 O 4 . The precipitate was dissolved in H 2 SO 4 and the ethanedioate ions in the solution needed 35.6 cm³ of a 0.0200 M solution of potassium permanganate for titration. Calculate the percentage of CaCO 3 in the limestone. 2. 25.0 cm³ of a sodium sulphite solution require 45.0 cm³ of a 0.0200 M potassium permanganate solution for oxidation. What is the concentration of the sodium sulphite solution. 3. What volume of acidified potassium permanganate solution of concentration 0.0200 M is decolorized by 100 cm³ of hydrogen peroxide of concentration 0.0100 M? 4. Manganese(II) sulphate is oxidized to manganese(IV) oxide by potassium permanganate in acid solution. A flocculant is added to settle the solid manganese(IV) oxide so that it does not obscure the colour of the potassium permanganate. If 25.0 cm³ of manganese(II) sulphate solution require 22.5 cm³ of 0.0200M potassium permanganate solution, what is the concentration of manganese(II) sulphate. 5. 2.83 g of a sample of haematite iron ore [iron (III) oxide, Fe 2 O 3 ] were dissolved in concentrated hydrochloric acid and the solution diluted to 250 cm 3 . 25.0 cm 3 of this solution was reduced with tin(II) chloride (which is oxidised to Sn 4+ in the process) to form a solution of iron(II) ions. This solution of iron(II) ions required 26.4 cm 3 of a 0.0200 mol dm –3 potassium dichromate(VI) solution for complete oxidation back to iron(III) ions. Calculate the percentage of iron(III) oxide in the ore. 6. 10.0 g of iron(II) ammonium sulphate crystals were made up to 250 cm 3 of acidified aqueous solution. 25.0 cm 3 of this solution required 21.25 cm 3 of 0.0200 mol dm –3 potassium dichromate(VI) for oxidation. Calculate x in the formula FeSO 4 . (NH 4 ) 2 S0 4 .xH 2 O 7. Given the half–reaction NO 3 – (aq) + 2H + (aq) + 2e – ==> NO 2 – (aq) + H 2 O (l) (a) give the ionic equation for potassium manganate(VII) oxidising nitrate(III) to nitrate(V) (b) 24.2 cm 3 of sodium nitrate(III) [sodium nitrite] solution, added from a burette, were needed to discharge the colour of 25.0 cm 3 of an acidified 0.0250 mol dm –3 KMnO 4 solution.
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Transcript of Redox Calculations #2
Redox calculations
1. A piece of limestone weighing 0.1965 g was allowed to react with an excess of HCl. The calcium in it was precipitated as CaC2O4. The precipitate was dissolved in H2SO4 and the ethanedioate ions in the solution needed 35.6 cm³ of a 0.0200 M solution of potassium permanganate for titration. Calculate the percentage of CaCO3 in the limestone.
2. 25.0 cm³ of a sodium sulphite solution require 45.0 cm³ of a 0.0200 M potassium permanganate solution for oxidation. What is the concentration of the sodium sulphite solution.
3. What volume of acidified potassium permanganate solution of concentration 0.0200 M is decolorized by 100 cm³ of hydrogen peroxide of concentration 0.0100 M?
4. Manganese(II) sulphate is oxidized to manganese(IV) oxide by potassium permanganate in acid solution. A flocculant is added to settle the solid manganese(IV) oxide so that it does not obscure the colour of the potassium permanganate. If 25.0 cm³ of manganese(II) sulphate solution require 22.5 cm³ of 0.0200M potassium permanganate solution, what is the concentration of manganese(II) sulphate.
5. 2.83 g of a sample of haematite iron ore [iron (III) oxide, Fe2O3] were dissolved in concentrated hydrochloric acid and the solution diluted to 250 cm3. 25.0 cm3 of this solution was reduced with tin(II) chloride (which is oxidised to Sn4+ in the process) to form a solution of iron(II) ions. This solution of iron(II) ions required 26.4 cm3 of a 0.0200 mol dm–3 potassium dichromate(VI) solution for complete oxidation back to iron(III) ions. Calculate the percentage of iron(III) oxide in the ore.
6. 10.0 g of iron(II) ammonium sulphate crystals were made up to 250 cm3 of acidified aqueous solution. 25.0 cm3 of this solution required 21.25 cm3 of 0.0200 mol dm–3 potassium dichromate(VI) for oxidation. Calculate x in the formula FeSO4.(NH4)2S04.xH2O
7. Given the half–reaction NO3–(aq) + 2H+
(aq) + 2e– ==> NO2–(aq) + H2O(l)
(a) give the ionic equation for potassium manganate(VII) oxidising nitrate(III) to nitrate(V) (b) 24.2 cm3 of sodium nitrate(III) [sodium nitrite] solution, added from a burette, were needed to discharge the colour of 25.0 cm3 of an acidified 0.0250 mol dm–3 KMnO4 solution. What was the concentration of the nitrate(III) solution in grammes of anhydrous salt per dm3?
8. 1.01g of an impure sample of potassium dichromate(VI), K2Cr2O7, was dissolved in dil. sulphuric acid and made up to250 cm3 in a calibrated volumetric flask. A 25.0 cm3 aliquot of this solution pipetted into a conical flask and excess potassium iodide solution and starch indicator were added. The liberated iodine was titrated with 0.100 mol dm–3 sodium thiosulphate and the starch turned colourless after 20.0 cm3 was added. Calculate the total mass of potassium dichromate(VI) in the original sample and hence its % purity.
9. Given the half–cell reaction IO3–(aq) + 6H+
(aq) + 5e– ==> 1/2I2(aq) + 3H2O(l)
(a) Deduce the redox equation for iodate(V) ions oxidising iodide ions. (b) What volume of 0.0120 mol dm–3 iodate(V) solution reacts with 20.0 cm3 of 0.100 mol dm–3 iodide solution? (c) 25.0 cm3 of the potassium iodate solution were added to about 15 cm3 of a 15% solution of potassium iodide (ensures excess iodide ion). On acidification, the liberated iodine needed 24.1 cm3 of 0.0500 mol dm–3 sodium thiosulphate solution to titrate it. (i) Calculate the concentration of potassium iodate(V) in g dm–3
(ii) What indicator is used for this titration and what is the colour change at the end–point?10. Ammonium iron(II) sulphate crystals have the following formula: (NH4)2.FeSO4.nH20. In an
experiment to determine n, 8.492 g of the salt were dissolved and made up to 250 cm³ of solution with distilled water and dilute sulphuric acid. A 25.0 cm³ portion of the solution was further acidified and titrated against potassium manganate(VII) solution of concentration 0.0150 mol dm-
3. A volume of 22.5 cm³ was required. Find the value of n. [10]
11.1.01g of an impure sample of potassium dichromate(VI), K2Cr2O7, was dissolved in dil. sulphuric acid and made up to250 cm3 in a calibrated volumetric flask. A 25.0 cm3 aliquot of this solution pipetted into a conical flask and excess potassium iodide solution and starch indicator were added. The liberated iodine was titrated with 0.100 mol dm–3 sodium thiosulphate and the starch turned colourless after 20.0 cm3 was added. Calculate the total mass of potassium dichromate(VI) in the original sample and hence its % purity. [10]
