3/
a,
Na2O+ H2O -> 2NaOH
nNa2O= 0,25 mol => nNaOH= 0,5 mol
CM NaOH= $\frac{0,5}{0,5}$= 1M
b,
2NaOH+ H2SO4 -> Na2SO4+ 2H2O
=> nH2SO4= 0,25 mol
=> mdd H2SO4= $\frac{0,25.98.100}{20}$= 122,5g
=> V dd H2SO4= $\frac{122,5}{1,4}$= 87,5ml
4/
a, 2Fe(OH)3 $\buildrel{{t^o}}\over\longrightarrow$ Fe2O3+ 3H2O
b, H2SO4+ 2NaOH -> Na2SO4+ 2H2O
c, Zn(OH)2+ H2SO4 -> ZnSO4+ 2H2O
d, NaOH+ HCl -> NaCl+ H2O
e, 2NaOH+ CO2 -> Na2CO3+ H2O