H3PO4 ----> H2PO4- + H+
设H3PO4]的初始浓度为X,那么,
[H2PO4-]=[H+]= X-10-3 M,[H2PO4-]=[H+]=10-3 M
Ka=7.5E-3=[H2PO4-][H+]/[H3PO4]= 10-6/(X-10-3)
求出：X= 1.133x10-3 M