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Acid rain contains a mixture of weak acids, but the primary acid to consider is sulfuric acid, H2SO4. When acid rain comes into contact with limestone, which is composed of calcium carbonate (CaCO3), a reaction occurs where H2SO4 reacts with CaCO3 to form CaSO4, CO2, and H2O. The balanced chemical equation for this reaction is: \[H_{2}SO_{4} + CaCO_{3} \rightarrow CaSO_{4} + CO_{2} + H_{2}O\]

A layer of calcium sulfate on the surface of a limestone sculpture would help slow down the effects of acid rain. Calcium sulfate (CaSO4) is less reactive with acid rain compared to calcium carbonate (CaCO3) because it is already combined with the sulfuric acid component of the acid rain. Therefore, a surface layer of CaSO4 would not react easily with additional H2SO4 from acid rain, and it would provide a protective barrier between the acid rain and the CaCO3 beneath. In conclusion: a) The balanced chemical equation for the attack of acid rain on limestone is: \[H_{2}SO_{4} + CaCO_{3} \rightarrow CaSO_{4} + CO_{2} + H_{2}O\] b) Treating a limestone sculpture with a layer of calcium sulfate would help slow down the effects of acid rain since CaSO4 is less reactive with H2SO4.