\(n_{Na_2SO_3}=\frac{31,5}{126}=0,25\left(mol\right)\)
a)
PTHH: \(Na_2SO_3+2HCl\rightarrow2NaCl+SO_2+H_2O\)
________0,25-------->0,5------------------->0,25________(mol)
\(V_{SO_2}=0,25.22,4=5,6\left(l\right)\)
b) \(C_{M\left(ddHCl\right)}=\frac{0,5}{0,8}=0,625M\)