# Calculating Molar Solubility of CaCO3: A Puzzling Problem

• Chemistry
• samy4408

#### samy4408

Homework Statement
Relevant Equations
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hello , i am solving a problem about solubility and the solution seems weird to me , the problem is the following :
we are asked to calculate the molar solubility of (CaCO3) with (Ks = 5*10^-9) in water and in a solution of (NaCO3) with a concentration of 0,1M, knowing that (NaCO3) is totally soluble
i found that the molar solubility in water was s=7,07*10^-5M
the problem is in the second question , the solubility in a solution of (NaCO3) , because [CO3-]>>s
(CaCO3) should not dissolve at all .
that don't match the correction , can anyone tell me what is wrong ?
thanks !

There is no such thing as "should not dissolve at all". It always dissolves till the Ksp is satisfied.

7.07E-5 M is a very low concentration. That's 0.0000707 M !
Are you familiar with ICE tables and how you use them to solve these kinds of problems?

There is no such thing as "should not dissolve at all". It always dissolves till the Ksp is satisfied.
take this dissolution reaction :
AB => A+ + B-
if we have a large concentration of B- in the water such that [B-] >> Ksp , the dissolution of each molecule of AB will increase the concentration of [B-] (which means that we are going to move away from the equilibrium)
, and we don't have free A+ in the solution so i don't understand how the reaction can shift toward the left

take this dissolution reaction :
AB => A+ + B-
if we have a large concentration of B- in the water such that [B-] >> Ksp , the dissolution of each molecule of AB will increase the concentration of [B-] (which means that we are going to move away from the equilibrium)
, and we don't have free A+ in the solution so i don't understand how the reaction can shift toward the left

Molar solubility will be the concentration of A^+, so it is just

$$[A^+]=\frac{K_{sp}}{[B^-]}$$

You need to take into account all sources of B^- though, as @Mayhem already suggested ICE table is your friend here.

"we are asked to calculate the molar solubility of (CaCO3) with (Ks = 5*10^-9) in water and in a solution of (NaCO3) with a concentration of 0,1M, knowing that (NaCO3) is totally soluble
i found that the molar solubility in water was s=7,07*10^-5M
the problem is in the second question , the solubility in a solution of (NaCO3) , because [CO3-]>>s
"

Don't know whether we can help you. At least get the formulae in the question right.

Last edited:
Let x be the Ca ion conc. Then the carbonate ion conc will be 0.1+x

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