2020 O Level Chem P1 Q17 — Rate of Reaction / Effect of Concentration Solution

Answer: B. Two features change when the acid concentration is increased from 1.0 to 2.0 mol/dm³ with the same 0.5 g of magnesium. First, the higher concentration means more frequent collisions, so the reaction is faster and the curve is steeper at the start. Second, and just as important, the magnesium is the limiting reactant in both runs because the acid is in excess each time, so the same number of moles of hydrogen is produced.

This means the new graph must rise more steeply but level off at the same final volume of hydrogen as graph 1.

Graph B shows exactly that: a steeper initial slope reaching the same plateau. Graph A wrongly gives a higher final volume, graph C a lower final volume, and graph D is slower than graph 1. The answer is B.