Saturday, August 30, 2008

Relationship between Solubility and Solubility product

Relationship between solubility (S) and solubility product (Ksp)

Consider MqAr a sparingly soluble salt.
q = Number of cations (Mr+) and
r = Number of anions (Aq-)

That is we have in dissolved state

MqAr ↔ qMr+ + r Aq-


Ksp = [Mr+]q [Aq-]r

If solubility is S, according to the definition of solubility product

We have
[Mr+]q = q.S mol/dm³
[Aq-]r = r.S mol/dm³

Hence Ksp = [q.S] q [r.S] r

= Sq+r. qq.rr

For example for the salt, calcium Phophate, Ca3(PO4)2

Ca3(PO4)2 ↔ 3Caaq2+ + 2PO4(aq)3-

Ksp = [Ca2+] 3 [PO43-]2

= S3+2.33.22
= 108S5

Past JEE Question

For a sparingly soluble salt ApBq, the relationship of its solubility product (Ksp) with its solubility (s) is

a. Ksp = sp+q.pp.qq
b. Ksp = sp+q.pq.qp
c. Ksp = spq.pp.qq
d. Ksp = spq.(pq)p+q)


Answer: a

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