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Finding the Value of Kc

To calculate the equilibrium constant, the concentrations of the reactants and products at equilibrium must be known. For example for the following equilibrium:H2(g) + I2(g) ⇌ 2HI (g)the equilibrium concentrations are[H2(g)] = 8.00 x 10-4 moldm-3
[I2(g)] = 1.20 x 10-4 moldm-3
[HI(g)] = 2.28 x 10-3 moldm-3
Kc=     [HI(g)]2     
[H2(g)][I2(g)]
therefore:
Kc=      (2.28 x 10-3)2             = 54.2
(1.20 x 10-4)(8.00 x 10-4)

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Specification: AQA Chemistry (7404), AS-level, 3.1.6.2 Equilibrium constant Kc for homogeneous systems. Topic: Equilibria.

Key terms in this lesson

Equilibrium constant
A value, Kc, that expresses the relative concentrations of products and reactants at equilibrium for a reaction.

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