posted by Jessica on .
Hi DrBob thank tou for your help so far its really inspiring I must say:)
I have another question though:P
Use the table to predict wheter a precipiatate forms if:
25.0 mL of 0.010 mol/L silver nitrate is mixed with 25.0 mL of 0.0050 mol/L potassium chloride
My teacher says the precipitate does form but I cant figure out how to arrive at this answer:
The table is a table of Ksp Values which I cannot send since it is in Microsoft Word
Has your teacher talked about the solubility quotient? That's how this is done.
AgCl ==> Ag+ + Cl^-
Ksp = (Ag^+)(Cl^-)
Summary: Calculate the concentrations of the AgNO3 and KCl after they are mixed and see if the (Ag^+)*Cl^-) equals, is less than, or is greater than Ksp. If it is less than Ksp, no ppt forms. If it is greater than Ksp, a ppt forms. If it is the same as Ksp (exactly equal to Ksp), the solution is saturated and the addition of one more Ag or Cl ion would ppt.
What you do is calculate mols AgNO3 from M x L = 0.01 M x 0.025 L = 0.00025 mols.
Then calculate mols KCl from M x L = 0.005 M x 0.025 L = 0.000125
After mixing the two solutions, the new concentration is (just before they react, if they will react)
(AgNO3) = 0.00025 mols/0.05 L (that's 25 mL + 25 mL = 50 mL = 0.05 L
) = 0.005 M.
(KCl) = 0.000125 M/0.05 L = 0.0025 M.
Now you have the concentrations of the two solutions just before they react but after they are mixed. Just multiply and see what the quotient is and compare with Ksp.
KQ = (At^+)(Cl^-) = 0.005*0.0025 = 1.25 x 10^-5. Now compare that with Ksp. I think your table will give you something like 1.7 x 10^-10 for Ksp of AgCl; therefore, the quotient is MUCH larger than Ksp. Since Ksp can't be exceeded, then a ppt MUST form.