chem edexcel a level

ly2021

i need help im stuck on this question
When solid lead(II) sulfate is added to aqueous sodium iodide, an equilibrium
is established.
PbSO4(s) + 2I
–
(aq)  PbI2(s) + SO4
2–(aq)
The expression for the equilibrium constant, Kc , for this reaction is
Kc =
[SO (aq)]
[ (aq)]2
4
2−
−
I
In an experiment, Kc may be determined by adding excess lead(II) sulfate to
25.0cm3
of 0.100moldm–3 sodium iodide.
The volume remains constant at 25.0cm3
.
The mixture is left to reach equilibrium at room temperature.
Ice-cold water is added to freeze the position of equilibrium and the mixture is
then titrated with standard silver nitrate solution.
The whole mixture requires 12.20cm3
of 0.0500moldm–3 silver nitrate solution to
react with the aqueous iodide ions at equilibrium.
Ag+
(aq) + I–
(aq) → AgI(s)
Calculate the equilibrium concentrations of the sulfate ions and the iodide ions,
and hence the value of Kc at room temperature.
Give your answer to an appropriate number of significant figures and include
units for Kc , if any.

i have got to the step where you divide the value by 2 to make it equal.

but after that the mark scheme divides each value separately by 25 and multiplies by a thousand i have no clue why...
its the 2022 paper 1 edexcel the very last question

Reply 1

UtterlyUseless69

Division by 25, followed by multiplication by 1000 is the same as dividing the whole thing by (25/1000).

25/1000 represents the volume of the solution in the equilibrium mixture in dm^3. What I presume has been divided by this quantity is the number of moles as moles divided by volume gives concentration.

My attempt at a full solution

(edited 1 year ago)

Reply 2

ly2021

how come its multiplied by 1000

Reply 3

UtterlyUseless69

Original post by ly2021

how come its multiplied by 1000

Let me re-phrase the above.

If you divide something by a fraction like 25/1000 (i.e the volume in dm^3), you times that something by the bottom of the fraction (in this case, 1000) and then divide by the top of the fraction (in this case, 25).