Which of the following could best be used to measure the reaction rate?

The reaction is:
$\text{CH₃COOH}_{\text{(aq)}} + \text{NaHCO₃}_{\text{(s)}} \rightarrow \text{NaCH₃COO}_{\text{(aq)}} + \text{CO₂}_{\text{(g)}} + \text{H₂O}_{\text{(l)}}$

To measure the reaction rate, we need to monitor a change in a measurable property over time.

• A. Volume of H₂O: Water is produced, but the reaction occurs in an aqueous solution, making it difficult to accurately measure the small change in water volume against the large volume of solvent.
• B. Volume of CO₂: Carbon dioxide is produced as a gas. The volume of gas produced can be easily and accurately measured over time using a gas syringe or by collecting it over water. This provides a clear and direct measure of the reaction progress.
• C. Mass of CH₃COOH: Acetic acid is a reactant in an aqueous solution. Measuring its mass directly as it is consumed would be challenging.
• D. Surface area of NaHCO₃: Sodium bicarbonate is a solid reactant. While its surface area changes, accurately measuring this change during the reaction is impractical.

Therefore, measuring the volume of carbon dioxide gas produced is the most suitable method.

The best option is $\boxed{\text{B}}$.