Why is a balanced chemical equation needed for stoichiometric calculations?
This is a good question. Balanced chemical equations are needed to determine the coefficients (numbers) that are placed before each chemical species in a chemical equation (see the general example below):
2A + B --> C
When we have the coefficients in place we can then see the relative number of each chemical species needed to perform the chemical reaction. Without balancing the equation we would just be guessing at how the different chemical species are related to each other. And stoichiometry depends on knowing the exact relationship between species. For example, we would not be able to calculate how many grams of C would be produced from 5 grams of A if we did not know that 2 moles of A are required for every mole of C. When balancing equations always remember to make sure that the total atom count for each element is the same on both sides of the equation.