when to place equilibrium constant in front concentration for Kc expression?

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Smooth Operater

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May 22, 2004
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for rxn C(s) + CO(g) --> 2 CO(g), what is the concentration of CO at equilibirum if the CO2 concentration is 0.5 M?

I am kinda confused about writing expression for equilibrium constant. How come in some questions, I need to place the stiochimetric coefficient in front the concentration like:

Kc = 2[CO]^2/[CO2]

and there are times that I don't need to place the coefficient to solve a problem correctly (such as this question) like:

Kc = [CO]^2/[CO2]

How some experts can clear it up. THANKS!
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