POH=PKb2-lg[HPO42-]/[H2PO4-]
POH=6.6=PKb2 那么c(Na2HPO4)=cNaH2PO4)
m(Na2HPO4)=142×0.01/2=0.72g
m(NaH2PO4)=120×0.01/2=0.6g