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Question 12 :  Used for MWD Revision Given the half-reaction

Question 12 :  Used for MWD Revision Given the half-reaction NO 3 - ( aq ) + 2H + ( aq ) + 2e - ==> NO 2 - ( aq ) + H 2 O (l) give the ionic equation for potassium manganate (VII) oxidising nitrate(III) to nitrate(V)

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Question 12 :  Used for MWD Revision Given the half-reaction

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  1. Question 12:  Used for MWD Revision Given the half-reaction NO3-(aq) + 2H+(aq) + 2e- ==> NO2-(aq) + H2O(l) • give the ionic equation for potassium manganate(VII) oxidising nitrate(III) to nitrate(V) • 24.2 cm3 of sodium nitrate(III) [sodium nitrite] solution, added from a burette, were needed to discharge the colour of 25 cm3 of an acidified 0.025 mol dm-3 KMnO4 solution. • What was the concentration of the nitrate(III)  solution in grammes of anhydrous salt per dm3?

  2. Question 12:  (a) 2MnO4-(aq) + 6H+(aq)  + 5NO2-(aq) ==> Mn2+(aq) + 5NO3-(aq) +  3H2O(l) (b) mol KMnO4 in titration = 0.025 x 25 / 1000 = 0.000625, mol ratio MnO4-:NO2- is 2:5 or 1:2.5, so mol NO2- in titration = 0.000625 x 2.5 = 0.0015625 in 24.2 cm3, scaling up to 1 dm3 gives a molar concentration of NaNO2 of  0.0015625 x 1000 / 24.2 = 0.0646 mol dm-3 Mr(NaNO2) = 69, so in terms of mass concentration = 0.0646 x 69 = 4.46 g dm-3

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