The graph shows the volume of a gaseous product formed during two trials of a reaction. A different concentration of reactant was used during each trial, whereas the other factors were kept constant.

Which of the following statements explains which trial has a lower concentration of the reactant?

A) Trial X, because the final volume of product formed is lower than Trial Y.
B) Trial X, because this reaction was initially fast and later stopped completely.
C) Trial Y, because the reaction was initially slow and later stopped completely.
D) Trial Y, because the volume of product formed per unit time is lower than Trial X.

Trial Y, because the volume of product formed per unit time is lower than Trial X.

Explanation:

The rate of reaction depends on the amount of reactants present. Now remember that the rate of reaction is either measured as the rate of disappearance of reactants or as the rate of appearance of products.

In this case, we consider the rate of reaction as the rate of appearance of products per unit time. Reading off from the graph, it is quite easy to observe that the volume of product formed per unit time for trial Y is lower than that of Trial X. Since we have earlier agreed that the rate of reaction depends on the initial concentration of reactants. It follows that the reason for the lower volume of product formed per unit time for trial Y is because of a lower initial concentration of the reactants in trial Y. Hence the answer.