The answer is A) only
A swimming pool was sufficiently alkaline so that the carbon dioxide absorbed from the air produced a solution
in the pool that was
If the pool originally contained
RIght from the get-go, you can predict which precipitate will form by inspecting the concentrations of the ions and the solubility product constants.
In your case, the only plausible candidates are
In order for a precipitate to form, you need the product of the concentrations of the ions that form that precipitate to exceed the value of the solubility product constant,
So, for instance, in the case of calcium carbonate, you'd get
By definition, the solubility product constant will be
You can use the initial concentration of calcium cations to determine the minimum concentration of carbonate ions that will allow for calcium carbonate to precipitate out of solution.
This is the minimum amount of carbonate ions that are needed in order to precipitate the existing calcium cations. Since you have
After the calcium carbonate precipitates, the concentration of carbonate ions decreases. This happens because of the carbonate ions that are now part of the precipitate.
Now take a look at iron (II) carbonate,
Once again, the minimum concentration of carbonate ions needed to precipitate iron (II) carbonate from solution is
Looks like you have more carbonate ions present in solution than you'd need
This will leave you with
Finally, take a look at magnesium carbonate,
To precipitate magnesium carbonate, you'd need
This time, the amount of carbonate ions present in solution is not enough to precipitate magnesium carbonate.
So there you have it. Your solution will contain two precipitates,