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QUESTION

# Quest (1) determine the ksp for magnesium hydroxide Mg(OH)2 where the molar solubility of Mg(OH)2 is 1.4X10^-4 M? quest (2) the Ksp for Zn(OH)2 is 5.0x10^-17 .determine the molar solubility of Zn(OH)2 in a buffer solution with a pH of 11.5?

Question 1: ##K_(sp)= 1.1 xx10^(-11)##

Question 2: ##s= 4.9 xx10^(-12)M##

Quest (1) determine the for magnesium hydroxide ##Mg(OH)_2## where the molar solubility of ##Mg(OH)_2## is ##1.4xx10^(-4) M##?

We will use ICE table to solve this question.

The dissolution of ##Mg(OH)_2## can be written as follows:

##" " " " " " " " " " "## ##Mg(OH)_2(s) -> Mg^(2+)(aq) + 2OH^(-)(aq)## ##" " " " " " ## Initial: ## " " " " " " " " " " " " 0 M " " " " " " 0M## ##" " " " " ## Change: ## - sM " " " " " " " " " +sM " " " " " +2s M## ##" " " ## Equilibrium: ## " " " " " " " " " " " " s M " " " " " " " " " 2s M##

The expression of ##K_(sp)## can be written as follows:

##K_(sp) = [Mg^(2+)(aq)] *[OH^(-)(aq)]^2##

## => K_(sp) = s*(2s)^2 = 4s^3= 4xx(1.4xx10^-4)^3##

##=> K_(sp)= 1.1 xx10^(-11)##

Quest (2) The ##K_(sp)## for ##Zn(OH)_2## is ##5.0xx10^(-17)## . Determine the molar solubility of ##Zn(OH)_2## in a buffer solution with a ##pH " of " 11.5##?

We will use ICE table to solve this question.

##pH = 11.5 => [H^+] = 10^(-11.5)##

##=> [OH^-] =10^(-2.5) = 3.2xx10^(-3) M##

The dissolution of ##Zn(OH)_2## can be written as follows:

##" " " " " " " " " " "## ##Zn(OH)_2(s) -> Zn^(2+)(aq) + 2OH^(-)(aq)## ##" " " " " " ## Initial: ## " " " " " " " " " " " " 0 M " " " " " 3.2xx10^(-3) M## ##" " " " " ## Change: ## - sM " " " " " " "" +sM " " " " " +2s M## ##" " " ## Equilibrium: ## " " " " " " " " " " " " s M " " " " " (3.2xx10^(-3) + 2s) M##

The expression of ##K_(sp)## can be written as follows:

##K_(sp) = [Zn^(2+)(aq)] *[OH^(-)(aq)]^2##

##5.0xx10^(-17) =s *(3.2xx10^(-3) + 2s)^2##

Solve for ##s=4.9xx10^(-12)M##

Here is a video that explains the solving of Q2 (start at minute 5:14):