In class, we did a decomposition reaction of sodium hydrogen carbonate, which makes water, carbon dioxide, and sodium carbonate. How do you know when sodium hydrogen carbonate is completely reacted?

Answer 1

You weigh the NaHCO3 to start, let it heat at the specified T, place in a desiccator to cool, weigh. Repeat heat and cooling. Weigh again. If the two weights after heating are the same, all of the NaHCO3 has been converted to Na2CO3.
Note: One must be careful with this because Na2CO3 can be decomposed to Na2O and CO2 and the loss of CO2 will show up as a loss in weight; the optimum T for NaHCO3 to Na2CO3 is about 275 C to 300 C. The decomposition of Na2CO3 is higher than that.