Final answer:
The relative rate of change of reactant A in the reaction A + 3B -> 2C, with a given rate of 0.420 M/s, is ∆[A]/∆t = 0.210 M/s. This accounts for the stoichiometric ratio presented in the reaction equation.
Step-by-step explanation:
The rate of the reaction A + 3B -> 2C is 0.420 M/s. To find the relative rate of change of the reactant A, represented as ∆[A]/∆t, we need to consider the stoichiometry of the reaction. Since one mole of A is consumed for every two moles of C produced, the rate of disappearance of A is half the given reaction rate, because the stoichiometric coefficients must be considered.
Therefore, ∆[A]/∆t = 0.5 × (rate of reaction) = 0.5 × 0.420 M/s = 0.210 M/s. This is the average rate of disappearance of A with respect to time.