The reaction you're describing is a neutralization reaction between acetic acid (C2H4O2, which is often represented as CH3COOH) and sodium bicarbonate (NaHCO3). The products of this reaction include sodium acetate (NaC2H3O2), water (H2O), and carbon dioxide (CO2).
So the balanced equation would be:
\[ \text{C}_2\text{H}_4\text{O}_2 + \text{NaHCO}_3 \rightarrow \text{NaC}_2\text{H}_3\text{O}_2 + \text{H}_2\text{O} + \text{CO}_2 \]
Thus, the missing product is \(\text{CO}_2\).