[hellowurld]

If I have a reversible equation that looks like this:

2HCl(g) <-> H₂(g) + Cl₂(g)

I want to find the equilibrium constant of all parties involved.  I know the concentration of the CL₂ and the H₂ but not the HCl.  Do I need to know the concentration of HCl, or do I assume it is zero?  I thought it would have to be something greater than zero because it is what is being dissoluted.  If so, how do I figure that out?

[Borek]

You need all concentrations. Sometimes some of them can be calculated from reaction stoichiometry, if you know how much reactants were at the beginning.