Question

The K_sp of Ag₂SO₄ is 1.4 x 10^-5. Woud you have expected to obtain precipitate of Ag₂SO₄ in any of the exeriments that you did while determining how you were going to test for chloride ion (Add 6M HNO₃ to unknown sample. Test with blue litmus paper, if red, continue. If blue litmus paper stays blue, add more acid. Add a few drops of 0.02M AgNO₃. If white precipitate forms, Cl^- is present.)? Justify your anwer with a calculation.

Answer 1

Ag₂SO₄(s) ---- > 2Ag⁺(aq) + SO₄^2-(aq),

Ksp = 1.4 x 10^-5

if We added 1 one drop (1 mL) of 0.02M AgNO3.

then moles of Ag⁺(aq) added = 0.02 x 1.0 x 10^-3

                                              = 2.0 x 10^-5 mol

If the final volume of the solution after addition of sample solution, HNO3 and AgNO3 is 10 mL,

the concentration of [Ag⁺(aq)] = 2.0 x 10^-5 mol / 10 x 10^-3 L = 2.0 x 10^-3 M

Also the sulphate ion concentration would be of the same order.

Hence ionic product,

IP = [Ag⁺]²x[SO₄^2-] < Ksp (1.4 x 10^-5)

However for precipitation to occur IP (ionic product) should be greater than Ksp.

Hence precipitation will not occur.