C=mdx100/ms
31,5= mdx100/500----> md=157,5gHNO3
n=m/M
n= 157,5g/63g/mol=2,5molHNO3
HNO3+NaOH----> NaNO3 + H2O=====>
2,5mol > ...2,5mol
2Na + 2H2O---> 2NaOH+ H2...
2,5mol..<...............2,5mol
n,Na= 2,5mol; daca se cere masa,
mNa=2,5molx23g/mol=.....................