Solubility and Gibbs free energy Qs

[LeilaNami]

Homework Statement

There are a problems I wasn't sure about on my test. Trying to get answers for ones I had to guess on.

1. What is the maximum [Mg] that can be achieved in an aqueous solution of Mg(OH)₂ with a pH=12.50? Ksp (Mg(OH)₂)=5.6e-12

2. Determine the max solubility of CaF₂ that can dissolve in a 0.10 M HF solution. Given Ksp (CaF₂)=1.5e-10 and Ka (HF)= 3.5e-4

3. Consider the reaction below:
N₂(g) + 3H₂(g) -> 2NH₃
The standard free energy change for this reaction is ΔG⁰ = -33.3 kJ at 25^oC. What is the reaction mixture consisting of 0.50 atm N₂, 0.50 atm H₂ and 10.0 atm NH₃?

4. For the following, determine Kp at 25C.
NH₄HS (s) <--> H₂S (g) + NH₃ (g)
ΔH⁰=83.47 kJ/mol and ΔG⁰=17.5 kJ/mol

Homework Equations

Ksp = [products]ⁿ
Ka = [products]ⁿ/[reactants]^m
ΔG⁰=-RTlnK
Kp=Kc(RT)^Δn(gas)

The Attempt at a Solution

1. So I have no idea how to do this one with a certain pH and don't have a clue even where to start. I know how to do this without a given pH however.

2. I think this uses the common ion effect.
HF + H₂O <--> F^- + H₃O
Using the Ka given I found that [F]= 5.74e-3 M
Since this is a common ion, it pushes the reaction back towards CaF₂ and so technically, the amount dissolved is the same as [Ca].
CaF₂ -> Ca + F₂
So Ksp = [2S]²
1.5e-10 = [Ca][(5.74e-3)+2S]²
[Ca]=? I'm not getting one of the answer choices

3.Kp=P(NH₃)²/[P(N₂) x (P(H₂))³]
Kp= 1.6e3
1.6e3 = Kc[(0.0821)(298)]^-2
Kc=.958
ΔG = -33.3 + (8.314e-3)(298)ln(.958) = ? Not getting one of the answer choices

4. Not getting anywhere close to one of the answers. Pretty much tried to solve it the same as #3 but in reverse I guess.

 

[Borek]

1. If pH is given you can calculate concentration of [OH^-], rest should be obvious - this is a common ion effect.

2. What are possible answers? My bet is that you can ignore F^- from the CaF₂ dissolution, I tried with and without and the difference was around 0.2%.

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