At a particular temperature a 2.0 liter flask contains 2.0 mol of H2S,
0.40 mol of H2, and 0.80 mol of S2. Calculate the equilibrium constant
for the reaction:
2H2 (g) + S2 (g) <---> 2H2S (g)

As described in the site below, all you need to do is divide the
concentration of the right-hand side of the equation  (since there's
only one term on the right-hand side) by the product of the left-hand
side, but in the balanced equation you *square* each term that has a
'2' beside it.

K = [2]^2 / ( [.4]^2 * [.8] )

K = 4/(.16 * .8)

K = 31.25


http://dbhs.wvusd.k12.ca.us/Equilibrium/Calc-K-from-equilib-conc.html

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"Calculate the equilibrium constant"