Precipitation of salts

For a salt (sparingly soluble) when dissolved in water : AB <—> A+ + B at equilibrium (saturation) Ksp= [A+] [B]

When, we mix ions or if there be two or more ions in water, we define reaction coefficient (Q), called as a ionic − product (IP) in this case, giving the products of ions in water (ions of soluble salts and other common ions).

I.P. is product of ionic concentration due to ions already present in water or from a salt. I.P. may be and may not be equal to Ksp. To illustrate it more clearly , consider a case when 500 ml of an 0.005 M solution of AgNO3 is added to 500 ml of 0.001 M solution KCl. Now in solution (mixture), there are Ag+, NO3 , K+ and Cl ions. The concentration of [Ag+] = [NO3] = 0.005 = 0.0025 M (equal volumes are mixed) and [K+] = [Cl] = 0.001/2 = 0.0005 M as equal volumes of two solutions are mixed.

Now, we know that Ag+ will react with Cl ions to form AgCl since:

AgCl <–> Ag+ + Cl is a reversible reaction with a higher tendency towards left (solidifying or precipitating).

Now, question is, whether AgCl will be formed or not (precipitation of Ag+ and Cl as AgCl) and if it formed, how much of it will be formed ? For this we define some rules .

1. If ionic Product (IP) > Ksp ; precipitation takes place till I.P. equals Ksp

2. If Ionic Product < Ksp ; a precipitate will not be formed and the solution will be unsaturated

3. If Ionic Product = Ksp ; a precipitate will not form an the solution is satuated in that salt.

(or we can say that solution is at a critical stage, when precipitation just begins, but actually has not occurred yet in real sense).

In present case,

I.P. = [Ag+] [Cl] = (0.0025) (0.0005) = 1.25 × 10−6

(Only for the salt which is sparing soluble not for KNO3)

Ionic Product > Ksp in this case (Ksp AgCl = 1.56 × 10–10)

Which mean precipitation takes place.

Exercise 2:(a) The solubility of CaF2 in water at 20°C is 15.6 mg per dm3. Calculate the solubility product of CaF2.
b) The solubility of BaSO4 in water is 2.3 × 10−4 gm/100 mL. Calculate the percentage loss in weight when 0.2 gm of BaSO4 is washed with (a) 1lt of water (b) 1lt of 0.01 NH2SO4.

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