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Reaction 11.2, p.297. Measurement of an equilibrium constant Kc.

Method 1.

Required apparatus:

3 100cm3 conical flask                                                   titrating apparatus

2 pipettes (20cm3 and 5cm3) with safety-filler            flask stopper

2 beakers (100cm3) and 1 200 cm3.

30 cm3 of 0.10M AgNO3                                30cm3 of 0.10M FeSO4

100 cm3 of 0.020M KCNS


1.       Using pipettes transfer 25cm3 of AgNO3 and FeSO4 into conical flask. Adjust the pipettes so that at the beginning take a little bit more, raise the pipette out of the solution, holding it in the level of my eyes drop the quantity until it's exact.

2.       Stopper the flask (so that no liquid can vaporise) and shake it for the reactants to mix.

3.       Label and leave for overnight

4.       Transfer (using the pipette as described in 1) 10cm3 of the solution to conical flask (carefully, do not disturb the Ag precipitate).

5.       Set up titrating apparatus (wash with KCNS). The end is marked with red colour.

6.       Titrate the sample

7.       Repeat last 3 steps 2 times.


Titration number

Initial burette reading

Final burette reading

Amount of potassium thiocyanate used













Average amount



I measured the equilibrium for the reaction Ag+(aq)+Fe3+(aq)Ag(s)+Fe3+(aq)

The equilibrium constant is calculated Kc=

Note: Ag(s) does not appear in equation, because its solid, thus its mass does not affect its concentration.

When titrating, first KCNS(aq)+ AgNO3(aq)AgCNS(s)+KNO3(aq)

When all Ag+ ions are used up the CNS- ion reacts with Fe3+ to give red colour:


1.question. It is necessary to leave the flask closed overnight to let the reactants achieve their equilibrium states. If the flask would be open then some of the reactants might evaporate or react with dust from air.

When I mixed the things I effectively diluted them (both concentrations became 0.05mol dm-3).

Based on the equation above the concentration of [Ag+(aq)]eq is

There was x0.02=1.78x10-4mol KCNS used to neutralise 10 cm3 of the reaction, so the concentration of Ag+ was 100x1.78x10-4=1.78x10-2mol dm3.

Since [Fe2+(aq)]initial=[Ag+(aq)]initial it follows that


So [Fe2+(aq)]eq=1.78x10-2mol dm3

and [Fe3+(aq)]eq=[Fe2+(aq)]initial _ [Fe2+(aq)]eq=0.05-0.0178=0.0322 mol dm3.

Substituting values into Kc=


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