Why do you have to use moles when solving stoichiometry problems instead of just converting from grams to grams?

When someone uses stoichiometry for the first time, they frequently ask this question. The answer is that they are converting based on number rather than mass.

Let's take an example problem: you have 1.0 kg of hamburger patties and 0.50 kg of hamburger buns (sorry, I'm Canadian, that's why the values are in metric). How many complete hamburgers could you make? Hopefully you can see that the answer to that question would depend on knowing the mass of each burger patty and bun separately.

The balanced chemical equation serves as the foundation for all of our conversion calculations in chemistry. For example, one mole of substance A reacts with two moles of substance B to produce one mole of product C and one mole of product D. However, a mole is just a number—albeit a very large one—because atoms and molecules are far too small to count. Instead, we mass large numbers of them and use molar mass to convert to the NUMBER of moles of them. This number is then used in a ratio conversion based on the mole ratios in the balanced chemical equation. Finally, we use the molar mass to convert our final answer into grams.

Moles provide a direct link between the amount of a substance and the number of particles, facilitating balanced chemical equations and allowing consistent conversion between mass and moles.