[JK]

Hi guys, I'm working on some questions for a chemistry report and I honestly have no clue how to do it
Hopefully someone here can help

2a. In your notebook, write the dissociation equation for calcium hydroxide.
2b. Write the equilibrium constant expression (Ksp) for this equation.
2c. If the hydroxide ion concentration of a calcium hydroxide solution was found to be 0.004mol/L, what would be the concentration of the calcium ion?

I currently got
2A. Ca(OH)2(s) ⇌ Ca2+(aq) + 2OH-(aq)
2B. Ksp=6.5x10^-6

But no clue with 2C...help would be greatly appreciated!

[savy2020]

.
2b. Write the equilibrium constant expression (Ksp) for this equation.
...
2B. Ksp=6.5x10^-6

First write the expression for K_sp. {What you've written is the value of K_sp.}
After that 2C isn't a big deal.

[Borek]

2B. Ksp=6.5x10^-6

This is value of the Ksp, not expression for Ksp. You have to write expressions describing Ksp in terms of concentrations of ions present in the solution (reaction quotient).

But no clue with 2C...help would be greatly appreciated!

When 1 mole of Ca(OH)₂ dissolves - how many moles of Ca²⁺ are introduced into solution? How many moles of OH^-?

Edit: somehow missed savy's post. After reading his post I realized that question is a little bit ambiguous. Are you describing simple solution prepared by putting solid Ca(OH)₂ in distilled water (that was my assumption) or is it general question about any soolution being in equilibrium with solid Ca(OH)₂ and potentially containing ions (Ca²⁺ and/or OH^-) from other sources?

[savy2020]

Borek, I think it can't be that Ca(OH)₂ is dissolved in distilled water because the values don't agree with such assumption. So it means in some random equillibrium mixture containing Ca²⁺ and OH^- and may be others of which we don't need to think, we need to find the [Ca²⁺]- given [OH^-]

[Borek]

Borek, I think it can't be that Ca(OH)₂ is dissolved in distilled water because the values don't agree with such assumption. So it means in some random equillibrium mixture containing Ca²⁺ and OH^- and may be others of which we don't need to think, we need to find the [Ca²⁺]- given [OH^-]

You may be right, you may be wrong - honestly, I don't know. The longer I think about it the more it seems to me that wording of the question:

2c. If the hydroxide ion concentration of a calcium hydroxide solution was found to be 0.004mol/L, what would be the concentration of the calcium ion?

is ambiguous. It doesn't state if solution is saturated nor if it contains ions from other sources, so seems to me like both approaches (the one based on stoichiometry and the one based just on Ksp) can be valid. At least I can't point to one of them and state "this is wrong".

[savy2020]

It doesn't state if solution is saturated nor if it contains ions from other sources, so seems to me like both approaches (the one based on stoichiometry and the one based just on Ksp) can be valid. At least I can't point to one of them and state "this is wrong".

@ Borek
Fine the question is ambigious. But the stoichiometry one can't be correct since then the ionic product exceeds the solubility product which cannot be the case.
I can say that any answer less than what you get by K_sp method is valid. There is no unique concentration of OH^- atleast.

@igob8a
From where did you write the value of K_sp?
It seems to be incorrect. Please check here

[Borek]

Fine the question is ambigious. But the stoichiometry one can't be correct since then the ionic product exceeds the solubility product which cannot be the case.

Does it? I have to recheck my calculations then.

0.004²*0.002=3.2*10^-8

[savy2020]

[Does it? I have to recheck my calculations then.

0.004²*0.002=3.2*10^-8

Oh Sorry I've made the mistake in calculation of K_sp
Sorry I've bothered you unnecessarily

[Borek]

Don't worry, even if you were partially wrong, your point was valid