称取碳酸钠试样0.2600g.溶于水后用T(HCl)=0.007640g/mL的盐酸标准溶液滴定.用去22.50mL
求Na2CO3的质量分数
[M(HCl)=36.46g/mol,M(Na2CO3)=106.00g/mol]