The graph shows how the rate of a reaction varies with the concentration of one of the reactants - Scottish Highers Chemistry - Question 5 - 2022

Given that rate is defined by the change in concentration over time, we can use the formula:

ext{Rate} = rac{ ext{Change in Concentration}}{ ext{Change in Time}}

For a reaction time of 10 s, we need to find the rate that corresponds to this time.

From the graph, if we set the reaction rate (1/s) at a certain point, for instance, at 0.25, the concentration can be deduced to be 0.25 mol L^-1 corresponding to the plotted points. To find the concentration when the reaction time is 10 s, we refer back to the plotted line, indicating that the rate of reaction at this concentration sustains a consistent value.

Based on the analysis and using the visual representation, the closest matching concentration when the reaction time was 10 s is C 0.25 mol L^-1.