Chemistry
posted by MEG .
Quantitative anaylsis of cl ion is often performed by a titration with silver nitrate, using sodium chromate as an indicator. As standardized AgNo3 is addded, both white AgCl and red Ag2CrO4 precipitate, but so long as some cl remains, the Ag2CrO4 dissovles as the mixgture is stirred, When the red color is permanent, the equivalence point has been reached.
a) calculate the equilibrium constant for the reaction:
2AgCl +CrO4=Ag2CrO4 +2Cl
b)if 36.00cm^3 of 0.2200 m NaCl is mixed with 36.00cm^3 of 0.2200 m AgNO3, what is the concentration of Ag remaining in the solution?

For a), use Hess' Law.
AgCl(s) ==> Ag^+ + Cl^
Ag2CrO4(s) ==> 2Ag^+ + CrO4^2
b)Since these are equal amounts of NaCl and AgNO3, this will be a saturated solution of AgCl(s) and the solubility is governed by the Ksp for AgCl.
AgCl ==> Ag^+ + Cl^
Ksp = (Ag^+)(Cl^)
Set up an ICE chart and solve for Ag^+/
Post your work if you get stuck. 
i don't understand how and why you use Hess's law to find the equilbiruim constant

I will number the equations.
1) AgCl > Ag^+ + Cl^ Ksp = ??
2)Ag2CrO4 ==> 2Ag^+ + CrO4^2 Ksp = ??
Technically, I may not have named it right. Perhaps I should have said in a Hess' Law process.
Multilply equation 1 by 2 (which will make Ksp for AgCl squared.
Reverse equation 2 (which will mean the reciprocal of Ksp for Ag2CrO4. Then add the resulting two equations to obtain the equation the questions asks for, then Keq (what the problem wants) will be K^2(AgCl)*(1/K(Ag2CrO4). 
i got part a right but i tried part b and i got an answer of 1.34 x 10^5 but it was wrong