In a chemical reaction, the law of conservation of mass states that matter cannot be created or destroyed. Therefore, the total mass of the reactants must equal the total mass of the products.
In the given reaction:
\[ \text{CaCO}_3 \rightarrow \text{CO}_2 + \text{CaO} \]
The scientist starts with 30 grams of calcium carbonate (CaCO3). According to the law of conservation of mass, the total mass of the products (CO2 and CaO) will also be 30 grams.
So, the answer is:
30 grams