Question

In the reaction $A+B\iff C+D$, the initial concentration of reactants is 1 mole / litre each. The equilibrium concentration of each product is 0.9 $moles.li{t}^{-1}$. Its $K_c$ is :

• A. 0.9
• B. 0.81
• C. 81
• D. 8.1

Answer 1

The correct option is C 81

The equilibrium reaction is $A+B\iff C+D$.
The initial concentrations of A,B,C and are 1 M,1 M,0 M and 0 M respectively.
At equilibrium, the concentrations of A,B,C and D are respectively $(1-0.9=0.1M),(1-0.9=0.1M),0.9Mand0.9M$ respectively.
The expression for the equilibrium constant is $K_c=\frac{\left[C\right]\left[D\right]}{\left[A\right]\left[B\right]}$.
Substitute values in the above expression.
$K_c=\frac{0.9\times 0.9}{0.1\times 0.1}=81$.