Ok we have a mix of Sodium bicarbonate (NaHCO3) and Sodium carbonate (Na2CO3), we do not know the relation. Then we take a sample of 5 gram. Then we burn that at 300 Celsius for like ten minutes. Then the following reaction will happen: 2NaHCO3 -> Na2CO3 + CO2 + H2O Then we measure the weight of the sample after we have burned it and we get 3.71 gram. Then we need to calculate many gram Sodium Carbonate we started out with, the number of gram in the samble. And that I do not know how to. I do hope you can follow if not then please tell me and I will try to make it more understandable.