1L 0.2mol/L的NaCl溶液含有0.2molNaCl,含有0.2molNa+
1L 0.2mol/L的Na2SO4含有0.2molNa2SO4,含有0.4molNa+
n(Na+)=0.2+0.4=0.6mol
c(Na+)=n/V=0.6/2=0.3mol/L