The rate of a gas-evolving reaction can be investigated by continuously monitoring the mass loss of a reaction mixture over time:
Na2SO3(s)+2HNO3(aq)→2NaNO3(aq)+H2O(l)+SO2(g) \text{Na}_2\text{SO}_3(\text{s}) + 2\text{HNO}_3(\text{aq}) \rightarrow 2\text{NaNO}_3(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{SO}_2(\text{g}) Na2SO3(s)+2HNO3(aq)→2NaNO3(aq)+H2O(l)+SO2(g)In an experiment, a chemist adds a large excess of coarse sodium sulfite grains to 75 cm375\text{ cm}^375 cm3 of 0.40 mol dm−30.40\text{ mol dm}^{-3}0.40 mol dm−3 nitric acid in a conical flask. The flask is placed on a digital balance, and a loose cotton wool plug is inserted into its neck. The mass loss due to escaping sulfur dioxide is recorded at regular intervals.
Explain why a loose plug of cotton wool is inserted into the neck of the flask, rather than:
Explain why using a large excess of the coarse sodium sulfite grains ensures that the reaction rate is only dependent on the changing concentration of the nitric acid as the reaction progresses.
Let mtm_tmt be the mass loss of the flask at time ttt due to the evolution of SO2\text{SO}_2SO2, and let mtotalm_{\text{total}}mtotal be the total mass loss when the reaction is complete. Explain why the quantity (mtotal−mt)(m_{\text{total}} - m_t)(mtotal−mt) is directly proportional to the concentration of nitric acid (HNO3\text{HNO}_3HNO3) remaining in the flask at time ttt.
