Stoichiometry

A balanced equation tells you how particles trade places, not how masses compare. The coefficients count molecules or formula units, so they also count moles, since a mole is just a fixed number of particles. That means the equation gives you an exact mole-to-mole exchange rate between any two substances in the reaction. Mass isn't fixed by the coefficients because different substances have different molar masses. One mole of C3H8 and three moles of CO2 are not the same mass, even though they're in a 1:3 ratio of particles.

The path is always grams to moles to moles to grams. Start with the mass you know and divide by its molar mass to get moles. Use the coefficients from the balanced equation to switch to moles of the substance you want, multiplying by the ratio of its coefficient to the known substance's coefficient. Then multiply by that substance's molar mass to land back in grams. Skipping the trip through moles and dividing masses directly, or using mass ratios instead of mole ratios, gives a wrong answer because the equation never promises those masses are proportional.

This grams-moles-grams path is the engine behind everything else in the unit: limiting reactant problems just run this path twice, once for each reactant, and percent yield compares this calculated theoretical mass to what was actually collected. Once this conversion chain is automatic, those extensions are just extra bookkeeping steps layered on top.