n(Al3+) = a/4 g ÷ 27g/mol = a/108 mol
根據電荷守恒可得:2*n(SO42-) = 3*n(Al3+) ;
n(SO42-) = a/72 mol
c(SO42-) = a/72 mol ÷ 4V/1000 L = 125a/36V mol/L
稀釋後溶液中硫酸根離子的物質的量濃度是125a/36V mol/L