[hannah1414]

how do you work out the pseudo-order rate of a reaction?

[Jesse2024]

I'll assume you are talking about pseudo first order rates here.  Basically when you have a second order rate equation v = k[A][ B], it can be hard to determine the rate since you have to monitor both concentrations at the same time. 
To get around this, keep either A or B constant and you get the following expressions:

v = k[A][ B] = k'[A]
k' = k[ B]₀

Here B is being kept constant as denoted by [ B]₀

By plotting k' vs. [ B] for multiple reactions, k can be found via the slope.