In titration with Ag, why AgCl and not Ag2CrO4 precipitates first although...?

Question

Ksp for AgCl (10-10) is greater than Ksp Ag2CrO4 (10-12)? (Mohr titration)

In another words - if silver chromate is less soluble than silver chloride why it is chloride to ppt first, when titrated with silver and chromate as inicator?

Answer 1

The answer to your question is that silver chromate is more soluble than silver chloride.

You cannot just look at the Ksp, see the value of one is less than the other and conclude that the smaller value is what will ppt out.

The way to approach the problem is to actually calulate the solubility based on the Ksp (for that temp).

For example:

Ksp Ag2CrO4 = [ag]^2 [CrO4] = 1.1 * (10^-12),
let x be the molarity of CrO4, we know then that the molarity of the silver would be twice that value. Therefore
[2x]^2[x] = 1.1*10^(-12), solve for x
4x^3 x = 1.1*10^(-12)
x = 6.51*10(-5) moles of CrO4-, and therefore is the same as the number of moles of Ag2CrO4 that is in solution at the saturation point.

So, how many grams/liter is this?
6.51*10^(-5) mole Ag2CrO4/liter * 331.73 g/mole = 2.15*10^(-2) g
that is about 20 mg in a liter.

What about AgCL solubilty?
Ksp AgCl = 1.6 * 10^(-10), that works out to be 1.26*10^-5 Molar
1.26*10^-5 moles/liter * 143.32g/mole = 1.81 *10^-3 g, that is almost 2 mg in a liter.

Therefor e AgCrO4 is less soluble than AgCl

Answer 2

Ag2CrO4 is more soluble than AgCl so AgCl ppts out first

Answer 3

Ksp Of Silver Chloride