Hi Keira!

This question is mean, because it doesn't really have anything to do with the passage OTHER than the fact that it deals with calcium carbonate. 🤨

So, none of the reactions given in the passage have anything to do with the question. 😫

Solubility product, or Ksp, is a number that we use to represent how much a SPARINGLY SOLUBLE (or barely soluble) solid dissolves in water. Notice that the Ksp value for CaCO3 is on the order of 10^-9? That means that the equilibrium lies very far to the LEFT, or very much towards the reactants side. So CaCO3 reeeeally doesn't dissolve in water. Instead, it prefers to exist as a solid!

Ksp reactions ALWAYS look like this:

Solid ---> Aqueous ions

In this case, it will be

CaCO3(s) --> Ca2+(aq) + CO3 2-(aq)

Then, Ksp = [Ca2+][CO3 2-]

Note that we don't include solids or liquids in any K expressions!

Then, since Ksp = 4.9 x 10^-9, and we can solve for the conc of each ion.

Due to the 1:1 stoic coefficients in the Ksp reaction, we can do this:

Ksp = [Ca2+][CO3 2-] = x²

x² = 4.9 x 10^-9

x = sqrt(4.9 x 10^-9) = sqrt(49 x 10^-10) = sqrt(49) x sqrt(10^-10) = 7 x 10^-5

Note that we can split the square roots ONLY because the values are MULTIPLYING. You cannot split square roots if the terms are ADDING or SUBTRACTING.

The best answer is C.