puritatea = masa pura/ masa impura*100 => masa pura = puritatea*masa impura/100 = 10*100/100=10g NaNO3
2NaNO3 = 2NaNO2 + O2
MNaNO3=ANa+AN+3AO=23+14+3*16=85g/mol
2*85g NaNO3...........................1 mol O2
10g NaNO3..............................x moli O2
x=10/2*85=0,0588 moli O2