[flubug]

Here it is:

Calcium ion triggers blood clotting, so when blood is donated, the receiving bag contains sodium oxalate solution to precipitate the Ca2+ and prevent clotting.  A 104 mL sample of blood conatins 9.7x10^-5 g Ca2+/mL.  A technologist treats the sample with 100.0 mL of 0.1550 M Na2C204.  Calculate [Ca2+] after the treatment.  Ksp of CaC204*H20 = 2.3x10^-9.

First I found the new molarities of Ca2+ and C204-2 to be 0.0012 M and 0.076 M, respectively.

Now the part I don't understand.  Somebody told me that because in the reaction Ca2+ and C204-2 are in 1:1 mole ratios that you subtract the concentration of Ca2+ from C204-2 to find the C204-2 left over.  Then you put that new concentration in the Ksp equation to find the new concentration of Ca2+.

I believe the answer is 3.1x10^-8 M.  Is this correct?

[Borek]

Now the part I don't understand.  Somebody told me that because in the reaction Ca2+ and C204-2 are in 1:1 mole ratios that you subtract the concentration of Ca2+ from C204-2 to find the C204-2 left over.  Then you put that new concentration in the Ksp equation to find the new concentration of Ca2+.

That's only approximation - but it is correct in many cases. It will be not correct if the salt in question has relatively high solubility product and.or the concentrations are low.

To be precise you should assume that x moles of CaC₂O₄ precipitated, use this x to calculate amounts of ions left in the solution, put it all into Ksp definition and solve for x.

I believe the answer is 3.1x10^-8 M.  Is this correct?

Seems OK.