During the complete combustion of butane (C4H10), the products are carbon dioxide (CO2) and water (H2O). The balanced chemical equation for the combustion of butane is:
\[ 2C_4H_{10} + 13O_2 \rightarrow 8CO_2 + 10H_2O \]
Therefore, in the reaction:
\[ 2C_4H_{10} + 13O_2 \rightarrow 8 \underline{CO_2} + 10H_2O \]
The blank should be filled with CO2.