M,Na= 23g/mol
M,H2O=16g/mol
M,NaOH=40g/mol
2mol........2mol.........2mol sau, in grame
2Na + 2H2O----> 2NaOH + H2
46g.........36g.............80g
m,NaOH= 80g=md
c=mdx100/ms---> ms= 80x100/20g=400g
dar, ms= md+m,apa----> m,apa= 400-80=> 360g
dar, din reactie, rezulta 36g apa. deci mai trebuie adaugat: m=360-36g apa