Answer
$$\begin{aligned} K_{\mathrm{I}} &=2.1\ \mu \mathrm{M} \end{aligned}$$
Work Step by Step
Use Equation $13-8,$ letting $[\mathrm{L}]=1 \mu \mathrm{M}$ and $[\mathrm{B}]=\left[\mathrm{I}_{50}\right]=2.5 \mu \mathrm{M}$
$$\begin{aligned} K_{\mathrm{I}} &=\frac{\left[\mathrm{I}_{50}\right]}{\left(1+\frac{[\mathrm{L}]}{K_{\mathrm{L}}}\right)} \\ \\ &=\frac{\left(2.5 \times 10^{-6}\right)}{\left(1+\frac{\left(1 \times 10^{-6}\right)}{\left(5 \times 10^{-6}\right)}\right)} \\ \\ &=\frac{2.5 \times 10^{-6}}{1.2}=2.1\ \mu \mathrm{M} \end{aligned}$$
