Reaction rate depends on H+ ion concentration from acid dissociation

Steps:

• Identify acids: HCl is strong (fully dissociates), butanoic acid is weak (partially dissociates).
• Compare concentrations: P has 0.50 mol/dm³ HCl (all H+ ions), Q has 0.10 mol/dm³ butanoic acid (fewer H+ ions).
• Reaction: Mg + 2H+ → Mg²+ + H₂ (fizzing from H₂ gas); rate increases with [H+].
• Observation: Faster fizzing in P due to higher [H+] from full dissociation despite lower volume.

Why B is correct:

• Strong acids like HCl fully dissociate, providing more H+ ions per mole than weak acids like butanoic acid, increasing reaction rate with Mg.

Why the others are wrong:

• A: Both acids react with Mg to form salts (MgCl₂ and magnesium butanoate).
• C: HCl in P is stronger than butanoic acid in Q.
• D: Particles (ions) are more concentrated in P (0.50 mol/dm³ vs. 0.10 mol/dm³).

Final answer: B