What is the $$\left[ {O{H^ - }} \right]$$  in the final solution prepared by mixing $$20.0$$ $$mL$$  of $$0.050\,M\,HCl$$    with $$30.0\,mL$$   of $$0.10\,M\,Ba{\left( {OH} \right)_2}?$$

Solution :
Number of milliequivalents of $$HCl$$
$$ = 20 \times 0.050 \times 1 = 1$$
Number of milliequivalents of $$Ba{\left( {OH} \right)_2}$$
$$ = 2 \times 30 \times 0.10 = 6$$
$$\eqalign{ & \left[ {O{H^ - }} \right]\,{\text{of final solution}} \cr & {\text{Milliequivalents of}}\,Ba{\left( {OH} \right)_2} \cr & = \frac{{ - \,{\text{milliequivalents of}}\,\,HCl}}{{{\text{Total volume}}}} \cr & = \frac{{6 - 1}}{{50}} \cr & = 0.1\,M \cr} $$