Chemistry · Rung 21 of 24

Limiting reactant and yield

A balanced equation is a recipe, and with a recipe the same thing happens as in a kitchen: what is left over is left over and what runs out first is in charge. The limiting reactant is not the one there is least of but the one that runs out first according to the coefficients, and confusing the two is the classic mistake. In A + 2B → C, each A needs two B: with four A and six B it is B that limits, even though there are more moles of B than of A. You find it by dividing each one's moles by its coefficient and taking the smallest. What is left over of the other does not disappear: it is still there at the end, unreacted, and separating the product from the leftovers is half the work in a lab. From the limiting reactant you get the theoretical yield, which is a ceiling: in practice product is lost filtering, transferring and drying, some never reacts, and sometimes another reaction happens alongside. The ratio of what comes out to that ceiling is the percentage yield, and ninety per cent in a long synthesis is already excellent.

Where this sits

Comes after
The mole and the sums

Exercises from this rung

Generated by the app, from easiest to hardest

  1. You should get 40 g and you get 32 g. What is the yield?

    Answer 80

    Why 32 out of 40 is 80%. It is never a hundred: product is lost filtering, transferring and drying it, some never reacts, and sometimes another reaction happens alongside. A ninety per cent yield in a long synthesis is excellent, and a drug with ten steps can end up under twenty.

    easier

  2. You should get 20 g and you get 12 g. What is the yield?

    Answer 60

    Why 12 out of 20 is 60%. It is never a hundred: product is lost filtering, transferring and drying it, some never reacts, and sometimes another reaction happens alongside. A ninety per cent yield in a long synthesis is excellent, and a drug with ten steps can end up under twenty.

    in between

  3. In X + 2Y → 2Z, you start from 5 mol of X. How many moles of Z come out at most?

    Answer 10

    Why The equation's coefficients are a recipe: each X gives 2 Z, so 5 × 2 = 10 mol. That is the theoretical yield, the ceiling: in the lab you always get less, never more.

    harder