Which equilibrium equation to use

[Leah Savage 2F]

In the example Lavelle did in class, he used the equilibrium equation for the step 1 (K1) and subbed this in for the intermediate in the second step. How do you know to use the first equilibrium concentration for the first step? Is it because this is the one with reactants from the problem? Is this always a rule?

[Cristina Sarmiento 1E]

If a fast step is before a slow step, we can assume the fast step is in equilibrium. I believe Lavelle called it pseudo equilibrium.
We substitute equilibrium constant K to get rid of any intermediates because intermediates cannot be in the rate law.

[Anne 2L]

I just wanted to elaborate on Cristina's answer. The concept behind pseudo equilibrium is that having the fast step before the slow step results in a build up of products for the fast step. As a result, the reaction begins to reverse and create reactants, creating a fake equilibrium. As a result, we can use K (equilibrium constant) for the fast step (Step 1) because technically the reaction is in equilibrium.