can somebody tell me if the following questions come from the ocr endorsed book, posted above:
[INDENT]33. Vanadium can exist in a number of different oxidation states. One compound of vanadium is ammonium vanadate(V) and this contains the ion VO3–. This can be reduced to V2+ in several steps, using zinc metal and aqueous sulphuric acid.
(a) 25.0 cm3 of 0.100 mol dm–3 ammonium vanadate(V) is completely reduced to V2+(aq) using zinc and aqueous sulphuric acid. The resulting solution is titrated with 0.0500 mol dm–3 MnO4–(aq) and 30.0 cm3 is required to oxidise the V2+(aq) back to VO3–(aq).
The half equation for acidified MnO4– acting as an oxidising agent is shown below.
MnO4– + 8H+ + 5e– ----> Mn2+ + 4H2O
Show that the vanadium has changed oxidation state from +2 to +5 in this titration.
[4]
(b) Suggest an equation for the oxidation of V2+(aq) to VO3–(aq) by MnO4–(aq) under acid conditions.
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[2]
[Total 6 marks]
33. (a) Moles V2+ = 25.0 × 0.100 / 1000 = 0.0025 mols 1
Moles MnO4– = 30.0 × 0.0500 / 1000 = 0.00150 mols 1
1 mole of MnO4- changes its Oxidation State by 5 to change
the Oxidation State of 1.67 moles of V2+ 1
Oxidation State of V2+ changes by 5 / 1.67 = 3 1
(b) 3MnO4– + 5V2+ + 3H2O → 3Mn2+ + 5VO3– + 6H+
(1 mark for correct species, 1 mark for balanced) 2
[6][/INDENT]
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