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

That is an extremely common question when first using stoichiometry. The answer is that you are converting based on number, not on mass.

Think of a sample problem like this - you have 1.0 kg of hamburger patties and 0.50 kg of hamburger buns (sorry, I'm Canadian - hence the metric values.) How many complete hamburgers could you make? Hopefully you recognize that you would first need to know the mass of an individual burger patty and bun in order to answer that question.

In chemistry, we base our conversions on the balanced chemical equation, but it is solely based on number. 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, for example. However, the mole is just a number, albeit a really big one. Big, because atoms and molecules are way too small to count, so we mass large numbers of them instead, 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. We then use the molar mass to convert out final answer into grams.