8 The liquid is allowed to drain out without blowing. The tip of the pipette is touched to the glass side of the flask.One more drop is pulled out by surface tension.
9 It is convenient to prepare three flasks at once.
10 Prepare the buretteThe potassium permanganate to be standardised is transferred to a clean, dry beaker.
11 Pour a little of the permanganate solution into a burette. Make sure the tap is turned off before you start adding liquid!
12 Tilt and roll the burette to coat it with permanganate solution.
13 Discard this liquid.Rinse the permanganate from the sink with tap water.
14 Fill the burette with fresh permanganate solution. Make sure there are no air bubbles trapped in the tip of the burette.Finally, remove the funnel from the top of the burette, otherwise it may drip.
15 The titrationPermanganate is so strongly coloured that you cannot read the bottom of the meniscus. For this compound we read the level at the top.What is your reading?2.9 mL
16 10 mL of dilute sulfuric acid is added. If no acid is added, what would you expect to happen?The MnO4- will be reduced to brown MnO2 instead of colourless Mn2+.
17 While swirling the flask, slowly add permanganate solution While swirling the flask, slowly add permanganate solution. At first the purple colour disappears.The endpoint is reached with the first drop of purple that does not turn colourless.
18 Take a note of the burette reading. 13.2 mLSo this first titre was13.2 – 2.9 mL = 10.3 mL
19 Record the burette reading just before the next titration. 13.7 mL
20 This time we can run the liquid in quickly until we’ve added about 9 This time we can run the liquid in quickly until we’ve added about 9.5 mL.
21 Add the permanganate one drop at a time until the final drop which does not turn colourless.
22 Record the burette reading. 24.05 mLThe second titre is13.65 – 24.1 = mL
23 If you’re not certain you had reached the endpoint, add another drop. This is definitely overshot.
24 Calculate the concentration of the standard Fe2+ solution M(FeSO4.7H2O) = g mol m(FeSO4.7H2O) = gV(FeSO4.7H2O) = LLeast accurate data is 4 sig fig,so answer is quoted to 4 sig fig.
25 Calculate the amount of Fe2+ solution used in each titration c(Fe2+) = mol L V(Fe2+) = 20.0 mL= 20.0 10-3 LThe least accurate data is accurate to 3 sig fig,but we will be using this number in the next calculation, so to avoid rounding errors, record it to 4 sig fig– but do not clear the calculator between calculations.
26 Write the equation for the redox reaction occurring in the titration Purple MnO4- is reduced to colourless Mn2+:MnO H e- → Mn H2OTherefore Fe2+ has been oxidised. It will form Fe3+:Fe2+ → Fe e-Overall ionic equation:MnO H Fe2+ → Mn H2O + 5Fe3+
27 Use the equation to find the amount of MnO4- which reacted MnO H Fe2+ → Mn H2O + 5Fe3+n(Fe2+) = 10-4 mol, n(MnO4-) = ?Unknown on top
28 Calculate the concentration of the MnO4- solution First runSecond runInitial reading2.9 mL13.65 mLFinal reading13.2 mL24.1 mLTitre10.3 mL10.35 mLAverage titre10.33 mLn(MnO4-) = 10-4 mol V(MnO4-) = mL= 10-3 LNote: although we are using 4 sig fig here, the final answer will only be quoted to 3 sig fig, since that is the accuracy of the pipette.
29 n(MnO4-) = 1.475 10-4 mol V(MnO4-) = 10.33 mL Don’t forget to change mL into litres.Final answer is quoted to 3 sig fig.The concentration of the potassium permanganate solution is mol L-1.
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