需要知道Ka值。Na3PO4+HCl--->Na2HPO4+NaCl；Na2HPO4+HCl--->NaH2PO4+NaCl；NaH2PO4+HCl--->H3PO4+NaCl。[H+][H2PO4-]/[H3PO4]=Ka1[H+][HPO42-]/[H2PO4-]=Ka2[H+][PO43-]/[HPO42-]=Ka3
