A white solid is added to dilute hydrochloric acid, and a gas comes out that turns lime water milky…

Step 1. (a) The gas is carbon dioxide, so the solid is a carbonate (or a hydrogencarbonate).

Step 2. (b) $\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$.

Step 3. (c) $\dfrac{2.5\ \text{g}}{100\ \text{g/mol}} = 0.025\ \text{mol}$ of carbonate gives $0.025\ \text{mol}$ of $\text{CO}_2$.