What is the equation for calculating the mean rate of a reaction using the quantity of reactant?

The mean rate of a reaction is defined as the change in the amount of a reactant or product over a specific time period. When calculating the mean rate using the quantity of reactant, the correct approach is to measure how much of the reactant has been consumed over a given duration.

In this case, the equation states that the mean rate of reaction equals the quantity of reactant used divided by the time taken. This formula directly reflects the relationship between the consumption of reactants and the time it takes for the reaction to occur. As reactants are used up during the reaction, this calculation provides a straightforward quantification of the reaction's speed or rate.

Other choices present variations that may not align with the typical focus on reactants. For example, measuring the total mass lost or the volume of gas produced pertains more to specific conditions of a reaction rather than a general mean rate from the standpoint of reactants consumed. Additionally, while forming products is crucial in reactions, the focus in this context is on the reactant itself, making the provided equation the most accurate representation for determining the mean rate of a reaction based on the quantity of reactant used.