-calculez masa de oxid pur
p= m,px1oo/m,i---> m,p=400x80/100= 320g CuO PUR
M,CuO=81g/mol
A,Cu= 65g/mol
-deduc masa de cupru, din ecuatia chimica
1mol.....................1mol
CuO + H2------> Cu+ H2O
81g.....................65g
320g......................x=m,Cu= 256,8g