Difference between products and reactants? Equilibrium

[Ned Taylor]

Homework Statement

Given the reaction, at equilibrium, it was found that [C] = 0.20 mol/L, [D] = 0.55 mol/L, [A] = 0.30 mol/L and [ B] = 0.48 mol/L. Calculate the value of Keq.
Reaction: C + D <--> A + B

Homework Equations

K=products/reactants

The Attempt at a Solution

[/B]
If we assume that C and D are the reactants then the value of Keq would be:
K=A+B/C+D since K= products/reactants

But if we assume that A+B are the reactants than Keq would be;
K=C+D/A+B

Since these give two difference answers( K=1.3 for the first method, and K=.7638) how does one tell what are the products and reactants. Thanks to anyone who can help me with this!
Ned

 

[Borek]

You are given reaction and it is clear which way it is intended to be treated.

You are right it is not always obvious, but writing down the equation you are following will always make the situation unambiguous.

 

[epenguin]

Commonly said there are no stupid questions. But if there were any this would be it!

You need to look up the definition of an equilibrium constant. And even without thermodynamics you can understand enough of its rationale and then you would not have written K=C+D/A+B