Fosfat

Ja mislim da se zadatak resava na sledeci nacin

NaH2PO4--------->Na+ +H2PO4-

n(NaH2PO4)=m/Mr=12/120=0.1 mol

n(H2PO4-)=n(NaH2PO4)=0.1 mol

c(H2PO4-)=n/V=0.1/0.25=0.4 mol/l

Na2HPO4---------->2Na+ + HPO42-

n(Na2HPO4)=m/Mr=1.42/142=0.01 mol

n(HPO42-)=n(Na2HPO4)=0.01 mol

c(HPO42-)=n/V=0.01/0.25=0.04 mol

H2PO4- +H2O--------->HPO42- +H3O+

K={H+}*{HPO42-}/{H2PO4-}

2*10(-7)={H+}*0.04/0.4

{H+}=2*10(-7)*0.4/0.04=2*10(-6) mol/l