Question

What is the molar solubility of barium hydroxide in water? Would you characterize barium hydroxide as being "soluble"? Why? 3. 4. What is the molar solubility of barium hydroxide in a pH 9.0 buffer?

Answer 1

3. Molar solubility is of any compound is the number of moles that can be dissolved per liter of solution until it is saturated . It can be calculated if substance's solubility product constant (K_sp) and stoichiometry are known. The units are mol/L (written as M). Ksp is an equilibrium constant for equilibrium between a solids and its ions in a solution. The higher the value of Ksp, the more soluble the compound.

Ba(OH)2 ionises in water as  

Ba(OH)2 $\rightleftharpoons$ Ba²⁺ + 2HO^-

Its Ksp value is 2.55 x 10^-4 at 25 ^oC , So Ksp = [Ba²⁺] [ HO^-]²

According to the balanced equation, for every one mole of Ba²⁺ formed, 2 moles of HO^- are formed. Let's assign the variable S for the solubility of the Ba²⁺

[Ba²⁺] = S [ HO^-]² = 2S So, Ksp = S x (2S)² = 4S³ = 2.55 x 10^-4

Molar solubility of Ba(OH)2 is S = 0.04 mole/L

Its molar solubility is very low. So, It can not be said as "Soluble".

4. We know

pH + pOH = 14
pOH = 14-9 =5 (given pH =9)
pOH = -log[OH-]
[OH-] = 10-pOH= 10-5.....Eq 1

Now   [Ba+2] = S and [OH-]= 2S, So molar solubility at pH = 9 is 
S = 10-5/2 = 0.5 x 10-5  mole/L