Find the pH after diluting barium hydroxide

Answer: C ($$12.30$$)

Barium hydroxide dissociates completely according to $$\mathrm{Ba(OH)_{2}(aq) }$$$$\mathrm{{}\rightarrow Ba^{2+}(aq) }$$$$\mathrm{{}+ 2OH^{-}(aq)}$$. The amount of hydroxide ions is $$2(0.200\ \mathrm{mol/L})$$$${}(0.0150\ \mathrm{L})$$$${}=0.00600\ \mathrm{mol}$$. After dilution, $$[\mathrm{OH^{-}}]$$$${}=\frac{0.00600\ \mathrm{mol}}{0.300\ \mathrm{L}}$$$${}=0.0200\ \mathrm{M}$$. Thus, $$\mathrm{pOH}$$$${}=-\log(0.0200)$$$${}=1.70$$ and $$\mathrm{pH}$$$${}=14.00-1.70$$$${}=12.30$$.