Calcium carbonate is an ionic salt that is slightly soluble in water according to the following equation:
CaCO3(s) ? Ca^2+(aq) + CO3^2-(aq)
The carbonate ions (CO3^2-) further interact with water molecules forming hydrogen carbonate ions and hydroxide ions according to the following equilibrium equation:
CO3^2-(aq) + H2O(l) ? HCO3^-(aq) + OH^-(aq)
When acid rains falls, whatever acids are dissolved in it contribute the H^+ ions that react with hydroxide ions (OH^-) forming water molecules according to the following equation:
H^+(aq) + OH^-(aq) → H2O(l)
The above equation tends to decrease the amount of available hydroxide ions making the above equilibrium to shift to the right, trying to replace the missing hydroxide ions. This in turn, uses up the available carbonate ions (from calcium carbonate present). As long as there is a steady supply of extra carbonate ions, the concentration of hydroxide ions will remain fairly constant in spite of addition of acid rain. That in turn will determine the concentration of hydrogen ions, and the pH of the water.
Answer: If Lake X has a greater parts-per-million amount of calcium carbonate than Lake Z, it will be able to withstand a greater amount of acid rain than Lake Z, because it will have a greater supply of carbonate ions.