Limiting Reactant and Percent Yield

A reaction stops when one reactant is gone, even if the flask is still full of the other. That first reactant to disappear is the limiting reactant, and it sets a hard ceiling on how much product can form. The other reactant, the excess reactant, just sits there unreacted once the limiting one runs out. You cannot tell which is limiting by comparing masses or even volumes directly, because the balanced equation almost never calls for a 1:1 ratio of moles.

To find the limiting reactant, convert each reactant's mass to moles, then divide each by its coefficient in the balanced equation. Whichever gives the smaller result is limiting, because it cannot supply enough moles to match the ratio the equation demands. Once you know the limiting reactant, all product calculations run off its moles, using the mole ratio from the balanced equation to get moles of product, then molar mass to get grams. This gram amount is the theoretical yield, the maximum possible under perfect conditions.

Real reactions almost never hit that maximum. Side reactions, incomplete reactions, and product lost during purification all eat into the actual yield, the mass you actually collect and measure. Percent yield reports how close you came: actual yield divided by theoretical yield, times 100. This topic sits right after balancing equations and mole-to-mole stoichiometry, and it is really just that same math with an extra decision step tacked on at the front, then a reality check tacked on at the end.