Question

In a chemical equilibrium $A+B\rightleftharpoons C+D$ when one mole of each of the two reactants are mixed 0.6 mole each of the products are formed. The equilibrium constant calculated is:

• A. 1
• B. 0.3
• C. 2.25
• D. 4/9

Answer 1

Answer: (C)

2.25

The equilibrium reaction is $A+B\iff C+D$.
The expression for the equilibrium constant is $K_c=\frac{\left[C\right]\left[D\right]}{\left[A\right]\left[B\right]}$.
Initially, 1 mole each of A and B are present.
At equilibrium 0.6 mole each of C and D are present.
The amount of A and B unreacted is $1-0.6=0.4$ moles each.

	A	B	C	D
Initially	1	1	0	0
At eqm	0.4	0.4	0.6	0.6

Substitute values in the equilibrium constant expression.
$K_c=\frac{\left[C\right]\left[D\right]}{\left[A\right]\left[B\right]}=\frac{0.6\times 0.6}{0.4\times 0.4}=2.25$.