\(\begin{array}{l}
a)\\
2NaOH + C{O_2} \to N{a_2}C{O_3} + {H_2}O\\
 =  > nNaOH = 0,25 \times 2 = 0,5\,mol\\
nN{a_2}C{O_3} = nC{O_2} = 0,25\,mol\\
x = \frac{{0,5}}{{0,2}} = 2,5l\\
CMN{a_2}C{O_3} = \frac{{0,25}}{{2,5}} = 0,1M\\
b)\\
NaOH + C{O_2} \to NaHC{O_3}\\
nNaOH = nNaHC{O_3} = nC{O_2} = 0,25mol\\
x = \frac{{0,25}}{{0,2}} = 1,25l\\
CMNaHC{O_3} = \frac{{0,25}}{{1,25}} = 0,2M
\end{array}\)