4.2 g of baking soda on strong ignition in an open container leaves a residue of mass

2NaHCO₃ $\stackrel{∆}{\to }$Na₂CO₃ + H₂O + CO₂ 

Molar mass of Na₂CO₃ is 106 gm and molar mass of NaHCO₃.

According to the given reaction,

168 gm on heating leaves residue = 106 gm

4.2 gm on heating leaves residue = ?

= $\frac{4.2\times 106}{168}=$ 2.65 gm