[lechatelier]

A quick question about titration that's been bugging me.

A 25ml sample solution contains Cl- ions and we have to determine the concentration of Cl- ions.

A standard solution of 0.1 M AgNO₃ is used to supply the Ag+ ions

Also 0.1 M Na₂CrO₄ is added as indicator.

Ag+ from the AgNO3 can form two possible componds:

Ag⁺ + Cl^- --> AgCl ...(Ksp 1.8 x 10^-10)

2Ag⁺+CrO₄^2- --> Ag₂CrO₄.(Ksp = 1.1 x 10^-12)

My question is that since the Ksp of the indicator AgCrO₄ is lower, why doesn't it precipitate first???

I mean the AgCl has a higher Ksp and therefore more soluble than AgCrO₄

Please tell me what I'm missing. Thankyou!

[Borek]

Calculate concentration of Ag⁺ in saturated solutions of AgCl and Ag₂CrO₄.

[lechatelier]

Calculate concentration of Ag⁺ in saturated solutions of AgCl and Ag₂CrO₄.

Hi Borek,
Thanks. You're right when I calculate the quantity of Ag reuired to precipitate either AgCl or Ag2CrO4, it's LOWER for AgCl. But I'm still not sure why the AgCrO4 will not precipitate first even though it has the lower Ksp

It's like the questions "which compound will precipitate first" and it's always the one with lower Ksp. So that's why I'm puzzled by the above case

[Borek]

It's like the questions "which compound will precipitate first" and it's always the one with lower Ksp.

You have already calculated that's not the case yet you repeat the same incorrect assumption.

You can compare K_sp directly only for salts of identical type - Me_aX_b with identical a and b values. When a and b are different, direct comparison of K_sp is not enough.

[lechatelier]

You can compare K_sp directly only for salts of identical type - Me_aX_b with identical a and b values. When a and b are different, direct comparison of K_sp is not enough.

I get that ksp of Agcl = [Ag][Cl]
And Ksp of Ag2CrO4 = [Ag]^2[CrO4]

My question is whether Ag will always be attracted to Cl- first regardless of concentration of Cl and CrO4 in the solution. What if the concentration of CrO4 is very high, will Ag still be attracted to Cl to form AgCl? I don't understand why Ag is attracted to Cl- as opposed to CrO4. Any help is appreciated

[AWK]

My question is whether Ag will always be attracted to Cl- first regardless of concentration of Cl and CrO4 in the solution.

Try calculation - what concentration of chromate and chloride is needed that both salts start precipitation together for the same concentration of Ag⁺. Take rather small concentration of Ag⁺, eg 0.00001 M.

[lechatelier]

My question is whether Ag will always be attracted to Cl- first regardless of concentration of Cl and CrO4 in the solution.

Try calculation - what concentration of chromate and chloride is needed that both salts start precipitation together for the same concentration of Ag⁺. Take rather small concentration of Ag⁺, eg 0.00001 M.

Thanks! I wouldn't know how to go about doing that
But if it is possible to figure out the specific conentration of chloride and chromate so they precipitate together. If we take this one step further, can we figure out the concentration of chloride and chromate so that chlorate and Ag precipitates first??

[Borek]

If we take this one step further, can we figure out the concentration of chloride and chromate so that chlorate and Ag precipitates first??

Chlorate can't precipitate first, as it is not present in the solution 

But I guess you meant chromate - and the answer is yes. Try to calculate concentration of Ag⁺ in saturated chloride solution, calculate concentration of chromate that would be in the equilibrium with this concentration of silver - anything larger and chromate precipitates first.

[lechatelier]

Thanks for the answers. I had asked my teacher the same and just received the reply. According to him, Silver chromate will never precipitate first. As long as there are Cl- ions in the solution, Ag has greater attraction for Cl- and AgCl will always precipitate first regardless of concentration of Cl or chromate ions.

[Borek]

That's obviously wrong - if the solution is 10^-10M in chlorides and 1M in chromate, silver chromate will precipitate first.