Chemistry · Rung 8 of 24

The mole and the sums

A mole is 6.022 × 10²³ particles, Avogadro's number. It is not a magic figure: it is chosen so that the mass of one mole in grams matches the atomic mass on the table. A mole of carbon is twelve grams, a mole of water eighteen. That turns the balance into a way of counting atoms, which is all a chemist ever wants to do. Three sums follow and never stop coming back: moles = grams ÷ molar mass; the ratio between two substances is the ratio of their coefficients in the balanced equation; and the limiting reagent is the one that runs out first, found by dividing each one's moles by its coefficient and taking the smallest. The actual yield almost never reaches the theoretical one, and that ratio is the percentage yield.

Where this sits

Comes after
Reactions and balancing
Opens up
Solutions and gases, Limiting reactant and yield

Exercises from this rung

Generated by the app, from easiest to hardest

  1. How much does one mole of water weigh?

    • 18 grams
    • 180 grams
    • 10 grams
    • 1 gram

    Why Molar mass is summed off the table: two hydrogens at 1.008 and one oxygen at 16.00 give 18.02 g/mol. That is, eighteen millilitres of water — three spoonfuls — is one mole, six hundred thousand trillion molecules. It is the figure worth memorising, because water turns up in half of all problems.

    easier

  2. Why does one mole of carbon weigh exactly 12 grams?

    • Because the mole was defined so the mass in grams matches the atomic mass
    • Because carbon atoms weigh one gram per twelve
    • By coincidence
    • Because carbon has 12 protons

    Why Avogadro's number was not chosen to be round, it was chosen to be useful: it is calibrated so that a mole of any substance weighs in grams what its atomic mass says. That way the bench balance counts atoms with no further sums. Carbon, incidentally, has six protons, not twelve; twelve is its mass number, protons plus neutrons.

    in between

  3. The sums say you should get 10 grams and you get 8. What is the yield?

    • 80 %
    • 8 %
    • 20 %
    • 125 %

    Why Percentage yield is what you got divided by the theoretical amount, times a hundred: 8 ÷ 10 = 0.8, or 80 %. It is almost never 100: product is lost in filtering, some never reacts, and side reactions sometimes eat the reagent. A yield above 100 is not a triumph, it is a sign the product is wet or dirty.

    harder