How do I determine the equilibrium concentration given Kc and the concentrations of component gases?

Question

I feel my method for solving it is correct and I've checked my calculations yet I can't get the right answer.

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Nitrogen and oxygen gases may react to form nitrogen monoxide. At 1500 °C, $K_c$ equals 1.0E-5.

$\ce{N2_{(g)} + O2_{(g)} <=> 2 NO_{(g)}}$

If $2.75 \times 10^{-2}$ mol $N_2$ and $2.75 \times 10^{-2}$ mol $O_2$ are sealed in a $0.886 L$ flask at $1500 °C$, what is the concentration of $\ce{NO_{(g)}}$ when equilibrium is established?

Notes: $K_c$ = equilibrium constant

a) 1.38E-7 M
b) 4.35E-5 M
c) 3.85E-5 M
d) 8.70E-5 M
e) 2.30E+4 M

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I set up the problem with $(2x)^2$ as a numerator and $(.03104 - x)^2$ as the denominator

I took square root of both sides and did algebra and got $4.9*10^{-5}$

For the record the answer is $8.7 \times 10^{-5}$

Answer 1

For a reaction in the form $\ce{ \mathit{a}\,A + \mathit{b}\,B <=> \mathit{c}\,C}$, the quilibrium constant $K_c$ is given by

$$K_c = \frac{[C]^c}{[A]^a[B]^b}$$

You might want to check yours.