题目

 Tom was about to close the windowhis attention was caught by a bird. A．when B．If  C．and D．till 答案：A有Na2SO4和Fe2(SO4)3的混合溶液，已知其中Fe3＋的物质的量浓度为0.4 mol/L，SO的物质的量浓度为0.7 mol/L，则此溶液中Na＋的物质的量浓度为          (　　) A.0.1 mol/L              B.0.15 mol/L C.0.2 mol/L              D.0.25 mol/L