To obey the Law of Conservation of Mass, the sum of all individual elements of a compound is equal to the mass of the compound. So, if HCN has a mass of 7.83 grams, then
7.83 g = mass of H + mass of C + mass of N
We know the masses of H and N to be 0.290 g and 4.06 g, respectively. Hence, we can find for the mass of C:
7.83 = 0.29 + mass of C + 4.06 mass of C = 3.48 g
As an extension to the Law of Conservation of Mass, there is also a Law of Definite Proportions. According to Dalton's atomic theory, a compound is formed from a fixed ratio of its individual elements. From our previous calculations, we know that the mass ratio of H to C to N is 0.29 g: 3.48 g:4.06 grams. The ratio could also be expressed in percentages. Let's find the mass percentage of Carbon in HCN to be used later:
mass % of Carbon = (3.48 g/7.83 g)*100 mass % of Carbon = 44.44%
So, if you collect a different mass of HCN, say 3.37 g, the corresponding mass of Carbon is equal to:
Mass of Carbon = (3.37)(44.44%) Mass of Carbon = 1.498 g
