piatra vanata : CuSO4·5H2O
250 g p.v.................160 g CuSO4.................90 g H2O
20g.............................x.....................................y
x = 12,8g CuSO4 y = 7,2g H2O
in solutia initiala : md= c·ms/100 = 15·350/100 = 52,5g
in solutia finala : md = 52,5 + 12,8 = 65,3g