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2018 O Level Chem P1 Q19 — Speed of Reaction / Rate Graphs Solution

Speed of Reaction / Rate Graphs 2018 O Level Pure Chemistry · Paper 1 · Q19

Question

1 mol of X reacts with 1 mol of Y to produce a gas. A student investigates the rate of this reaction by measuring the volume of gas produced at timed intervals. He plots his results on a graph. The table gives the volumes and concentrations of X and Y that he uses to produce line R. The student varies both volume and concentration of X and Y over a series of experiments. The results of one of these experiments is plotted on the same graph. Which row shows the volumes and concentrations that the student used to obtain line U? Question diagram

Question diagram

Question diagram (continued)

A
B
C
D

Correct answer: B

Video Solution

Explanation

Answer: B (X: 100 cm³ at 1.0 mol/dm³; Y: 50 cm³ at 2.0 mol/dm³). The final volume of gas (the plateau) is proportional to the moles of limiting reactant, while a higher concentration gives a steeper initial gradient.

Line R uses \(n(\text{X}) = n(\text{Y}) = 0.100\) mol. Curve U plateaus at the same height as R, so U uses the same moles (0.100 mol), but U rises more steeply, so its reactants are more concentrated.

Option B gives \(n(\text{X}) = 0.100 \times 1.0 = 0.100\) mol and \(n(\text{Y}) = 0.050 \times 2.0 = 0.100\) mol (same plateau as R), with Y at 2.0 mol/dm³ giving the steeper start. Options A gives too little gas; C is not more concentrated; D doubles the moles, so its plateau would be twice as high. Answer B.

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