Chemical equilibrium and stoichiometry In reactions that go until the end the stoichiometric number indicates the exact amounts of reactants and products you will get. However, this is not the case in equilibrium reactions and you will get a different amounts (concentrations) than what exhibits the stoichiometric numbers.
$$\ce{Na + Cl -&gt; NaCl}$$
You will get in the end 1 mole of NaCl from 1 mole of Na and one mole of Cl.
$$\ce{2SO3&lt;=&gt;2SO2 + O2}$$
In equilibrium you will not get 1 molar of $\ce{O2}$ and 2 molar of $\ce{SO2}$ from 2 molar of $\ce{SO3}$. 
Then what do the stoichiometric numbers indicate in equilibrium?
Let us take the example of your equation:
$$\ce{2SO3&lt;=&gt;2SO2 + O2}$$

The stoichiometric coefficients represent two facts:


If 2 mole $\ce{SO3}$ reacts, it will give 2 mole $\ce{SO2}$ and 1 mole $\ce{O2}$ but one needs to remember here that all of the 2 moles will not react. But the number of moles that do dissociate, give products in the above ratio i.e. if initially have 2 moles of $\ce{SO3}$ and degree of dissociation is 50%, then we will have 1 mole of $\ce{SO3}$ reacting to give 1 mole of $\ce{SO2}$ and half mole of $\ce{O2}$
The stoichiometric coefficients also provide the powers to which the concentrations of each compound will be raised to in the expression for equilibrium constant $$K_\mathrm c = \frac{[\ce{SO2}]^2[\ce{O2}]}{[\ce{SO3}]^2}$$