In this experiment, back titration is being used as calcium carbonate is an insoluble salt.
To determine the amount of calcium carbonate in the toothpaste used, a known mass of toothpaste is reacted with HCl in excess. When HCl reacts with calcium carbonate, the product is calcium chloride, carbon dioxide and water.. Since hydrochloric acid is in excess, only some of the acid reacts with calcium carbonate to produce calcium chloride, water and carbon dioxide. Hence, the excess HCl is back titrated with NaOH. The reacted amount of HCl is determined by subtracting the original amount of HCl by the amount of HCl that is in excess.
From the stoichiometric equations above, the mole ratio between HCl and CaC is 2:1. Hence, the number of mole of CaC can be found by taking the number of moles of HCl divided by two. The mass of calcium carbonate can be determined by multiplying its mole by its molar mass. The percentage by mass of calcium carbonate in toothpaste is then calculated by dividing the mass of CaC by the mass of the toothpaste, multiplied by 100.
The experiment was repeated two to obtain an accurate result. The percentage obtained should be around the literature value of 20%