This problem is providing information about possible causes whereby mussel shells are being eroded due to the acidity in the ocean. In such a way, it claims that more acidic oceans dissolve calcium carbonate in a faster way and produce hydrogen carbonate ions, and thus, a feasible explanation is required as well as a hypothesis according to the following choices:
a. Lower CO₂; this reduces the H₂CO₃ and increases the pH.
b. Add CO₃²⁻: this will add base and increase its concentration.
c. Add Ca²⁺: this will increase the precipitation rate of calcium carbonate (correct choice).
At first instance, we should recall the equilibrium equations that take place when acidic oceans dissolve calcium carbonate in a faster way:
[tex]CaCO_3(s)\rightleftharpoons Ca^{2+}(aq)+CO_3^{2-}(aq)\\\\CO_3^{2-}(aq)+H^+(aq)\rightleftharpoons HCO_3^-(aq)[/tex]
[tex]CO_2(g)+H_2O(l)\rightleftharpoons H_2CO_3(aq)[/tex]
Where we can see that the first choice is thoroughly discarded as the addition of CO₂ actually increases the ionizable carbonic acid (acidity). Moreover, the addition of CO₃²⁻ may also lead to the formation of more protons-releasing carbonic acid which also contributes to the acidity of the ocean.
Thereby, the correct condition that, for sure, contributes to the preservation of mussel shells will be the addition of Ca²⁺ and the hypothesis will be that it shifts the equilibrium towards the formation of more CaCO₃, the active compound in these shells.
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