Measuring reaction rate via sulfur precipitate formation

Steps:

• Identify products: Reaction yields sulfur solid (S(s)), forming a cloudy suspension in solution.
• Select observable change: Sulfur obscures visibility, enabling timing of a mark's disappearance.
• Control variables: Vary concentration or temperature while timing the endpoint.
• Calculate rate: Rate as inverse of time for fixed visibility loss (e.g., cross vanishes).

Why C is correct:

• C uses the precipitate's opacity to track product formation over time, directly linking to rate definition as change in concentration per unit time.

Why the others are wrong:

• A: Gas volume measurement tracks SO₂ but requires complex setup, not ideal for thiosulfate focus.
• B: Mass loss is negligible due to minimal water evaporation or gas escape.
• D: pH change is indirect, as H⁺ consumption occurs but isn't easily quantifiable for rate.

Final answer: C