One way to obtain a pseudo-first order reaction is to use a large excess of one of the reactants ([B]>>[A]) that works in the previous example) so that, as the reaction progresses, only a small amount of the reactant is consumed, and its concentration can be considered to stay constant. By collecting k' for many reactions with different (but excess) concentrations of [B], a plot of k' versus [B] gives k (the regular second order rate constant) as the slope.