Final answer:
The equilibrium association constant for the reaction A + B ⇌ A.B at 20 °C is 1.56 x 10⁵ M⁻¹.
Step-by-step explanation:
The equilibrium association constant (Ka) can be calculated using the equation Ka = kon/koff
Where
kon is the on rate
koff is the off rate.
In this case, the on rate is given as 1.0 x 10⁸ M⁻¹.s⁻¹ and the off rate is given as 640 s⁻¹.
Substituting the values into the equation,
Ka = (1.0 x 10⁸ M⁻¹.s⁻¹)/(640 s⁻¹) = 1.56 x 10⁵ M⁻¹.
Therefore, the equilibrium association constant for the reaction at 20 °C is 1.56 x 105 M⁻¹.