设NaHCO₃有xmol，
2NaHCO₃

△   .

Na₂CO₃+CO₂↑+H₂O△m
2mol                           62g
xmol                           0.31g
x=

2mol×0.31g

62g

=0.01
m（NaHCO₃）=0.01mol×84g/mol=0.84g，
w（NaHCO₃）=

0.84g

1.9g

×100%=44.2%．
答：原混合物中NaHCO₃质量分数为44.2%．