a/ \(m_{H_2SO_4}=490.10\%=49\left(g\right)\Rightarrow n_{H_2SO_4}=\dfrac{49}{98}=0,5\left(mol\right)\)
PTHH: 2NaOH + H2SO4 → Na2SO4 + 2H2O
Mol: 1 0,5 0,5
\(m_{ddNaOH}=\dfrac{1.40.100}{20}=200\left(g\right)\)
b/ \(m_{Na_2SO_4}=0,5.142=71\left(g\right)\)