Laboratory Analysis of a Carbonate-Bicarbonate Mixture 

  Na₂CO₃             + 2 HCl                  ---->             2 NaCl              + H₂O            + CO₂  

FW = 106 grams

FW = 58.5 grams  2 X FW = 117 grams

  
You can see from the reaction that 106 grams of Na₂CO₃ will produce 117 grams of NaCl.  That is a 10% increase in weight.   If the sample was 1 gram of pure Na₂CO₃ that weight gain is what  you would observe and the final weight would be 1.10 grams.   A graph will illustrate the weight change, note that the product weight is always a little greater than the sample weight: 
 

 

Bicarbonate reaction: 

  NaHCO₃                + HCl                    ---->              NaCl                + H₂O            + CO₂  

FW = 84 grams

FW = 58.5 grams

  
Now you can see from the reaction that 84 grams of NaHCO₃  will produce 58.5 grams of NaCl.  That is a 30% decrease in weight.   If the sample was 1 gram of pure NaHCO₃ that weight loss is what you would observe and the final weight would be 0.70 grams.  A graph will illustrate the weight change, note that now the product weight is always less than the sample weight: 
 

A sample containing a so called "mixture" could be pure carbonate so the product would weigh more than the sample or it could be pure bicarbonate so the product would weigh less than the sample.  A sample contain both Carbonate and Bicarbonate could gain or lose weight depending on the ratio of Carbonate and Bicarbonate in the sample.  But no sample can gain more weight than the pure Carbonate and no sample can lose more weight than the pure Bicarbonate. 

A graph can show the range of results which can make sense for a real mixture: 
 

Another graph can show the same data as weight gain or loss.  Only the region in blue is possible for a real mixture of Carbonate and Bicarbonate: 
 

There are two ways that your experimental data can be analyzed to determine the actual weight percentage (wt %) of Carbonate.  One is a graphical and the other an algebraic method.