[Fmeub]

This is a question from an exam paper whose memo I don't have.

Solubility product of Ag₂CrO₄ is 1.1 x 10^-11 at 25 degrees Celsius and pressure 1 bar. Calculate the silver ion equilibrium concentration at 25 degrees Celsius and pressure of 3 bars after placing 20g of siver chromate in a beaker with:
a) 100 mL pure water
b) 100 mL of 0.005 M potassium chromate.

The answers I got is:
a) 1.77 x 10^-5 M
b) 4.70 x 10^-6 M

In b) the assumption was made that [CrO₄^2-] = 0.005 M.

Are my answer incorrect and should I have taken the pressure into account? It doesn't seem to make sense to incorporate the pressure when one is not working with a gas.               

[Arkcon]

For solids and liquids, pressure has a minimal effect and is usually ignored.  You can find a formula for pressure dependence at wikipedia {here}.  Seems like a pretty obscure point to put on an exam, if the prof hasn't made a point of teaching it.

[Fmeub]

 Thank you! 
Does your calculations agree with my answers?

[Borek]

Show your work, either I have a dumb moment or you are twice wrong.

[Fmeub]

I have reconcidered it. Does a concentration of 2.80 x 10^-5 M seem a better answer for a)  ?

For b) I'm not sure. This is what I've done:

From the stoichiometric ratio, [CrO₄^2-] = 0.5[Ag⁺].

In the solution then
[CrO₄^2-] = 0.5 [Ag⁺] + 0.005 M (from potassium dichromate)

Assume the dissociation of potassium dichromate surpresses the solubility of silver dichromate, so that [CrO₄] = 0.005.
In other words 0.5[Ag⁺] << 0.005.

 [Ag⁺]² (0.005) = 1.1 x 10^-11
Therefore  [Ag⁺] = 4.69 x 10^-5 M
i.e. 4.69 x 10^-6 M in 100 mL.

OR should the final answer be 4.69 x 10^-4 M in 100 mL. Should I multiply or divide by ten? I'm confused.

Another thing that I'm not sure about is whether I should use the the "20g" given or not. With the exam paper there wasn't any periodic table or molar mass indicated, and it's not as if I memorize elements' molar mass to four decimal places.

[Borek]

2.8x10^-4M and 4.69x10^-5M. Equilbrium concentration is intensive property and doesn't depend on the sample volume!

[Fmeub]

My heartfelt gratitude for reminding me...