Let x g be the weight of of Na2CO3. Then, weight of NaHCO3=(15−x) g.
Moles of NaCl produced =11.058.5=0.188 mol
NaCl is produced by the reaction of (x106) mol of Na2CO3 and (15−x)84 mol of NaHCO3.
Each mol of Na2CO3 produces 2 mol of NaCl.
∴2x106+15−x84=0.188
On solving, we get x=13.5 g Na2CO3
NaHCO3=(15−1.35)=13.6 g
% Na2CO3=1.3515×100=9.0 % Na2CO3
Hence, the percentage of sodium carbonate in the sample is 9.0%.