a,
$Ba(OH)_2+H_2SO_4\to BaSO_4+2H_2O$
b,
$n_{Ba(OH)_2}=0,5.0,1=0,05(mol)$
$n_{H_2SO_4}=0,5.0,2=0,1(mol)$
$\Rightarrow n_{BaSO_4}=0,05(mol)$
$\to m_{\downarrow}=0,05.233=11,65g$
c,
$n_{H_2SO_4\text{dư}}=0,1-0,05=0,05(mol)$
$\Rightarrow n_{H^+}=0,05.2=0,1(mol)$
$V_{dd}=0,5+0,5=1l\Rightarrow [H^+]=0,1M$
$\to pH=-\lg[H^+]=1$