The OH^- concentration in a mixture of 5.0 mL of 0.0504 M NH_4 Cl and 2 mL of 0.0210 M NH_3 solution is x × 10^-6 M. The value of x is \ \ \ \.…

The $\displaystyle \mathrm{OH}^{-}$ concentration in a mixture of $\displaystyle 5.0$ mL of $\displaystyle 0.0504 \mathrm{M} \mathrm{NH}_4 \mathrm{Cl}$ and $\displaystyle 2$ mL of $\displaystyle 0.0210 \mathrm{M} \mathrm{NH}_3$ solution is $\displaystyle x \times 10^{-6} \mathrm{M}$. The value of $\displaystyle x$ is \_\_\_\_. (Nearest integer) [Given $\displaystyle \mathrm{K}_{\mathrm{w}}=1 \times 10^{-14}$ and $\displaystyle \mathrm{K}_{\mathrm{b}}=1.8 \times 10^{-5}$ ]