# How to calculate the Ksp

## Main Question or Discussion Point

My lab group and I are a bit confused. We are doing a lab where we need to calculate the Ksp of calcium hydroxide in pure water and then in different concentrations of calcium chloride and potassium chloride to analyze the diverse and common ion effect. I was absent one day but my lab partner told me that the teacher said Ksp for calcium hydroxide will remain constant for the solutions of potassium chloride and calcium chloride. She wants us to make a graph where we compare the molar solubilities of the different solutions, not the Ksp. My question is how on earth can something that tells us about solubility, Ksp, remain constant when more is forced to dissolve, KCl, and less is forced to dissolve, CaCl2????? Does this mean the molar solubility calculation is really as easy as just determining the calcium ion concentration in each solution and dividing it by the total volume? Gracias!

epenguin
Homework Helper
Gold Member
My lab group and I are a bit confused. We are doing a lab where we need to calculate the Ksp of calcium hydroxide in pure water and then in different concentrations of calcium chloride and potassium chloride to analyze the diverse and common ion effect. I was absent one day but my lab partner told me that the teacher said Ksp for calcium hydroxide will remain constant for the solutions of potassium chloride and calcium chloride. She wants us to make a graph where we compare the molar solubilities of the different solutions, not the Ksp. My question is how on earth can something that tells us about solubility, Ksp, remain constant when more is forced to dissolve, KCl, and less is forced to dissolve, CaCl2????? Does this mean the molar solubility calculation is really as easy as just determining the calcium ion concentration in each solution and dividing it by the total volume? Gracias!
Tell us what you exactly measure and how. Write the equation for the solubility constant What are you asked to make a graph of? If not told what seems a good idea?

Then if doing this does not already indicate to you the answers we have a chance of helping.

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It is called a constant for a reason, because it remains constant. It will change with temperature though, pretty much everything changes with temperature, as well as with solvents and such. The numbers may also look funny if you are working at very high concentrations/ionic strengths which require the use of activities instead of molarities in equilibrium concentrations.

Assuming you performed all trials under similar conditions then, yes, you do the fairly simple calculations using the common ion effect when necessary and calculate molar solubility based on how much of the CaOH went into solution.