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A chemist has a problem no other scientist has quite so sharply: the thing being counted is invisible and the thing being measured is a weight. You can put 11 grams of dry ice on a balance. You cannot put 1.5 × 10²³ molecules on anything.
The mole is the bridge. One mole is simply a fixed count of particles — 6.022 × 10²³ of them, called Avogadro's number — chosen so that the count weighs a convenient number of grams. One mole of carbon-12 atoms weighs exactly 12 grams. One mole of water molecules weighs 18 grams. That correspondence is the whole trick.
Once you accept it, every numerical question in the topic is a conversion between three currencies: grams, particles, and — for gases only — litres. Three formulas move between them, and the next card is all three. Learn which currency the question gives you and which it wants back; the arithmetic is trivial after that.
1. How many moles are present in 11 g of carbon dioxide? (C = 12, O = 16)
2. What volume does 0.5 mole of any ideal gas occupy at STP?
3. The molar mass of calcium carbonate, CaCO₃, is: (Ca = 40, C = 12, O = 16)
4. How many molecules are present in 9 g of water? (H = 1, O = 16)
5. A compound has the empirical formula CH₂O and a molar mass of 180 g/mol. Its molecular formula is:
6. Which statement about the figure 22.4 litres is correct?
7. Which of the following contains the greatest number of ATOMS?
8. The mass in grams of one mole of a substance is numerically equal to: