M,Na2SO4= (2X23gNa+32gS+4X16gO)/mol=142g/mol
-calculez masa celor 2,5mol
m= nxM
m= 2,5molx142g/mol= 355g sulfat de sodiu; aceasta este masa care se afla dizolvata in solutie, notata md
c= mdx100/ms, unde ms=md+m,apa
, ms= 355+555=> 910g
c= 355x100/ 910=........................calculeaza !!!!