The equilibrium constant, K, of a reaction at a particular temperature is determined by the concentrations or pressures of the reactants and products at equilibrium.
For a gaseous reaction with the general form
$$aA + bB \rightleftharpoons cC + dD$$
The K$_c$ and K$_p$ expressions are given by
$$K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$$
$$K_p = \frac{(P_c)^c(P_d)^d}{(P_a)^a(P_b)^b}$$
The subscript c or p indicates whether K is expressed in terms of concentrations or pressures. Equilibrium-constant expressions do not include a term for any pure solids or liquids that may be involved since their composition does not change throughout the reaction. The standard state of a pure substance is the pure substance itself, and although the quantity may change the sample remain pure. The constant value is incorporated into the value of K, and does not need to be accounted for separately.
The following reaction was performed in a sealed vessel at 719 °C:
$$H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$$
Initially, only H$_2$ and I$_2$ were present at concentrations of [H$_2$] = 3.15M and [I$_2$] = 2.85M. The equilibrium concentration of I$_2$ is 0.0200 M.
What is the equilibrium constant, K$_c$, for the reaction at this temperature?
Express your answer numerically.
K$_c$ = 542.34
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