向0.1mol\L的AL2(SO4)3溶液100mL中加入0.5mol\LNaOH溶液得到1.17沉淀

向0.1molL的AL2(SO4)3溶液100mL中加入0.5molLNaOH溶液得到1.17沉淀
解析：
NAOH溶液的体积可能:A.130ML或90ML (1)AI2(SO4)3 0.1mol(2)AI3+ + 3OH- ==AI(OH)3 n(AI3+)==1.17/78 ==0.015mol n(NaOH)==0.015*3 == 0.045mol V(NaOH)==0.045/0.5==0.09L(3)AI3+ + 3OH- ==AI(OH)3 n(NaOH)==1.17*3/78 == 0.045mol AI3+ + 4OH- ==AIO2- +2H2O n(NaOH)=(0.02-0.015)*4==0.02mol V(NaOH)=(0.045+0.02)/0.5 ==0.13L
其中,：n(NaOH)=(0.02-0.015)*4==0.02mol没懂,为什么要（0.02-0.015）*4?