Final answer:
The concentration of reactant at the start of the reaction depends on the initial concentrations of the reactants.
Step-by-step explanation:
The concentration of reactant at the start of the reaction depends on the specific reaction and its reactants. It is typically determined based on the initial concentrations of the reactants before the reaction begins.
For example, if we have a reaction A + B -> C, and the initial concentration of A is 0.010 M and the initial concentration of B is 0.020 M, then the concentration of reactant at the start of the reaction would be 0.010 M for A and 0.020 M for B.
It is important to note that the concentration of reactant at the start of the reaction is an initial condition and can change over time as the reaction progresses.