$ $解$:$设100.0 g废液中${\mathrm {Ba}(\mathrm {OH)}_{2}}$的质量为$x。$
$ {\mathrm {H}_{2}\mathrm {SO}_{4} + \mathrm {Ba}(\mathrm {OH)}_{2} = \mathrm {BaSO}_{4}↓+ 2\mathrm {H}_{2}\mathrm {O}}$
171 233
$x$ 2.33 g
$ \frac {171}{233}=\frac {x}{2.33 \mathrm {g}},$解得$x=1.71 \mathrm {g}$
$ $废液中${\mathrm {Ba}(\mathrm {OH)}_{2}}$的质量分数为$\frac {1.71 \mathrm {g}}{100.0 \mathrm {g}}×100\%=1.71\%$
$ $答$:$废液中${\mathrm {Ba}(\mathrm {OH)}_{2}}$的质量分数为$1.71\%。$