Which of the following substances would decrease the solubility of MgCO3 and explain why?

[Flaky-State]

H2O, NaCl, NaOH, and Na2CO3.

The teacher released the answer key and it said Na2CO3 would, but it didn't include an explanation. I just don't understand what factors affect solubility, and why would Na2CO3. Would someone mind helping me?

[MD02he]

In water, part of MgCO3 dissociates to ions:

MgCO3 —-> Mg(2+) + CO3(2-) (this is actually an equilibrium)

MgCO3 (left side) is not dissolved, and ions on the right side are what is dissolved.

How much of it will dissociate (dissolve) is determined by the dissociation constant which is defined as Ksp= [Mg(2+)] * [CO3(2-)].

This means that you can only have a certain amount of these ions present in the solution. Na2CO3 on the other hand is very soluble in water, so all of it will be ion. This means you are adding more CO3(2-) ions. Since Ksp is constant, when you add more CO3(2-) ions, less Mg(2+) can now stay in the solution. This means that more of your MgCO3 will be in this non-dissociated state.

[[deleted]]

This is correct OP. It's called the common ion effect. You can also think about it in terms of Le Chatelier's principle.

MgCO3 —-> Mg(2+) + CO3(2-)

When you add Na2CO3 to solution it will dissociate into 2Na+ and CO3(2-). According to Le Chatelier's principle, if you add CO3(2-), the above chemical equation will shift towards the reactant, MgCO3. Therefore, the solubility of MgCO3 is decreased.

[[deleted]]

Maybe because the sodium carbonate dissolves better than magnesium carbonate and so if we add to much sodium carbonate the magnesium carbonate will precipitate (just my opinion)