If equal volumes of $0.05\,\text{M}$ ammonium hydroxide solution and $0.001\,\text{M}$ ammonium chloride solution are mixed, what will be the $\text{OH}^-$ ion concentration of the resulting solution? ($K_b$ for $\text{NH}_4\text{OH} = 1.8 \times 10^{-5}$)