[Verela]

Refer to the following reaction:
          X₂ + 2Y 2XY

The reaction occurs in a series of steps:
          X₂   2X        slow
          X + Y   XY    fast

What is the rate expression for this reaction?
A. rate = k[XY]
B. rate = k[X₂][Y]²
C. rate = k[X₂]
D. rate = k[2X]

The answer is C to this question but I just don't understand how they get that answer. I guess I'm unsure of what a rate expression is maybe. Also I'm not exactly sure how I'm supposed to show my work on this one, so hopefully someone will answer.  

[Schrödinger]

When you are confronted with a multistep mechanism, always look for the slowest step. That is what decides the rate of the reaction. That's why slowest step is also rightly called 'rate determining step'.

The rate law for the slowest step is the rate law for the entire reaction, writing which may sometimes involve a little bit of basic algebra.