Question

Three units of solid calcium carbonate react with two molecules of aqueous phosphoric acid to form one unit of solid calcium phosphate and three molecules of aqueous carbonic acid

Answer 1

$3 CaCO_{3(s)} + 2H_3PO_{4(aq)} \to 1 Ca_3(PO_4)_{2(aq)} + 3CO_2{(g)} + 3H_2O_{(l)}$

However, according to the question aqueous carbonic acid is formed not carbon dioxide and water, also solid not aqueous calcium phosphate so the reaction becomes,

$3 CaCO_{3(s)} + 2H_3PO_{4(aq)} \to Ca_3(PO_4)_{2(s)} + 3H_2CO_3{(aq)}$