氢氧化镁Ksp = 1.9×10^-13，氨水Kb=1.8x10^-5混合成20mol溶液，cNH3.H2O=0.005molcOH-=（0.005x1.8x10^-5）^0.5=0.0003molL-cMg2+=0.05molL-Q=cMg2+*cOH-^2=4.5x10^-9>Ksp沉淀
