Presentation on theme: "Standardising potassium permanganate solution with iron(II) sulfate."— Presentation transcript:
Standardising potassium permanganate solution with iron(II) sulfate
Prepare a standard solution of iron(II) sulfate g of FeSO 4.7H 2 O is weighed in a dry beaker.
The powder is dissolved in water and transferred to a L volumetric flask, then made up to volume with distilled water and mixed well.
A pipette filler is used to half- fill a pipette with the solution. Rolling the wheel controls the level of liquid in the pipette. Prepare the pipette
The pipette is rolled and tilted to coat the inside with the Fe 2+ solution. Then this liquid is discarded.
The rinsed pipette is filled with the Fe 2+ solution. The liquid level is adjusted so that the bottom of the meniscus is touching the line on the pipette. Measure out the Fe 2+
The solution is transferred to a conical flask.
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.
It is convenient to prepare three flasks at once.
The potassium permanganate to be standardised is transferred to a clean, dry beaker. Prepare the burette
Pour a little of the permanganate solution into a burette. Make sure the tap is turned off before you start adding liquid!
Tilt and roll the burette to coat it with permanganate solution.
Discard this liquid. Rinse the permanganate from the sink with tap water.
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.
Permanganate 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 The titration
10 mL of dilute sulfuric acid is added. If no acid is added, what would you expect to happen? The MnO 4 - will be reduced to brown MnO 2 instead of colourless Mn 2+.
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.
Take a note of the burette reading mL So this first titre was 13.2 – 2.9 mL = 10.3 mL
Record the burette reading just before the next titration mL
This time we can run the liquid in quickly until we’ve added about 9.5 mL.
Add the permanganate one drop at a time until the final drop which does not turn colourless.
Record the burette reading mL The second titre is – 24.1 = mL
If you’re not certain you had reached the endpoint, add another drop. This is definitely overshot.
Calculate the concentration of the standard Fe 2+ solution M(FeSO 4.7H 2 O) = g mol -1 m(FeSO 4.7H 2 O) = g V(FeSO 4.7H 2 O) = L Least accurate data is 4 sig fig, so answer is quoted to 4 sig fig.
Calculate the amount of Fe 2+ solution used in each titration c(Fe 2+ ) = mol L -1 V(Fe 2+ ) = 20.0 mL = 20.0 L The 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.
Write the equation for the redox reaction occurring in the titration Purple MnO 4 - is reduced to colourless Mn 2+ : MnO H + + 5e - → Mn H 2 O Therefore Fe 2+ has been oxidised. It will form Fe 3+ : Fe 2+ → Fe 3+ + e - Overall ionic equation: MnO H Fe 2+ → Mn H 2 O + 5Fe 3+
n(Fe 2+ ) = mol, n(MnO 4 - ) = ? Use the equation to find the amount of MnO 4 - which reacted Unknown on top
Calculate the concentration of the MnO 4 - solution First runSecond run Initial reading2.9 mL13.65 mL Final reading13.2 mL24.1 mL Titre10.3 mL10.35 mL Average titre mL n(MnO 4 - ) = mol V(MnO 4 - ) = mL = L Note: 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.
n(MnO 4 - ) = mol V(MnO 4 - ) = mL = L Final answer is quoted to 3 sig fig. Don’t forget to change mL into litres. The concentration of the potassium permanganate solution is mol L -1.