CK-12-Chemistry Intermediate

19.3. Solubility Equilibrium http://www.ck12.org

3. Use the molar mass to convert from molar solubility to solubility in g/L.

The Ks pvalue of calcium carbonate is 4.5× 10 −^9. We begin by setting up an ICE table showing the dissociation of
CaCO 3 into calcium ions and carbonate ions. The variable s will be used to represent the molar solubility of CaCO 3.
In this case, each formula unit of CaCO 3 yields one Ca^2 +ion and one CO 32 −ion. Therefore, the equilibrium
concentrations of each ion are equal to s.

TABLE19.6:ICE table

CaCO 3 (s)⇀↽ Ca^2 +(aq)+ CO^23 −(aq)

Initial (M) 0.00 0.00

Change (M) +s +s

Equilibrium (M) s s

The Ks pexpression can be written in terms of s and then used to solve for s.

Ksp= [Ca^2 +][CO^23 −] = (s)(s) =s^2

s=

√

Ks p=

√

4. 5 × 10 −^9 = 6. 7 × 10 −^5 M

The concentration of each of the ions at equilibrium is 6.7× 10 −^5 M. We can now use the molar mass to convert
from molar solubility to solubility in g/L.

6. 7 × 10 −^5 mol

1 L

×

100 .09 g

(^1) mol
= 6. 7 × 10 −^3 g/L
So the maximum amount of calcium carbonate that is capable of dissolving in 1 liter of water at 25°C is 6.7× 10 −^3
grams. Note that in the case above, the 1:1 ratio of the ions upon dissociation led to the Ks pbeing equal to s^2. This
is referred to as a formula of the type AB, where A is the cation and B is the anion. Now let’s consider a formula of
the type AB 2 , such as Fe(OH) 2. In this case, the setup of the ICE table would look like the following:
TABLE19.7:ICE table
Fe(OH) 2 (s)⇀↽ Fe^2 +(aq)+ 2OH−(aq)
Initial (M) 0.00 0.00
Change (M) +s +2s
Equilibrium (M) s 2s
Ksp= [Fe^2 +][OH−]^2 = (s)(2s)^2 =4s^3
s=
3

√

Ksp

4

=

3

√

7. 9 × 10 −^16

4

= 5. 8 × 10 −^6 M

The table below (Table19.8) shows the relationship between Ks pand molar solubility based on the formula.

TABLE19.8:Ksps and Molar Solubility

Compound

Type

Example Ks pExpression Cation Anion Ks pin Terms of

s

AB CuS [Cu^2 +][S^2 −] s s s^2

AB 2 or A 2 B Ag 2 CrO 4 [Ag+]^2 [CrO 42 −] 2s s 4s^3

AB 3 or A 3 B Al(OH) 3 [Al^3 +][OH−]^3 s 3s 27s^4

A 2 B 3 or A 3 B 2 Ba 3 (PO 4 ) 2 [Ba^2 +]^3 [PO 43 −]^2 3s 2s 108s^5