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Counting Atoms

Learn how to count atoms using mass, understand the concept of moles, and discover the significance of Avogadro's number in chemistry.

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Counting Atoms

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  1. Counting Atoms

  2. Using Mass to Count Things • Atoms are so tiny that it is impossible to count the number of atoms in even 1 gram of matter. • We use mass to “count” atoms.

  3. Counting by mass • You have a pile of nails that weighs 1227 g. • One nail has a mass of 0.450 grams. • How many nails are in the pile? • (1227g) / 0.450 g per nail) = 2726.6 nails. Let’s call it 2730 nails.

  4. What is the mole? Not this kind of mole!

  5. The MOLE • Unit used to “count” numbers of atoms. • Count-by-mass technique. • Defined as the number of atoms in 12.0 grams of C-12. This is the STANDARD! • 12.0 g of C-12 has 6.0221415 X 1023 atoms.

  6. The MOLE • So 1 mole of any element has 6.02 X 1023 particles. • This is a really big number – It’s so big because atoms are really small!

  7. 6.02 X 1023 = Avogadro’s Number • One mole of water = 6.02 X 1023 water molecules. • One mole of gorillas is 6.02 X 1023 gorillas. • One mole of He atoms is 6.02 X 1023 He atoms. • One mole of anything is 6.02 X 1023 of that thing.

  8. Similar Words • Pair: 1 pair of shoelaces = 2 shoelaces. • Dozen: 1 dozen oranges = 12 oranges. • Gross: 1 gross of spider rings = 144 rings. • Ream: 1 ream of paper = 500 sheets of paper. • Mole: 1 mole of Na atoms = 6.02 X 1023 Na atoms.

  9. How did they find out that 12.0 g of C-12 has 6.02 X 1023 C atoms? • Experimentally by X-Ray diffraction studies

  10. How Big is a Mole?

  11. 1 mole of C-12 = 12.0 grams of C-12 = 6.02 X 1023 atoms of C-12 0.5 mole of C-12 = 6.0 grams of C-12 = 3.01 X 1023 atoms of C-12 Equalities

  12. 1 mole of C-12 = 12.0 grams of C-12 = 6.02 X 1023 atoms of C-12 0.25 mole of C-12 = 3.0 grams of C-12 = 1.50 X 1023 atoms of C-12 Equalities

  13. 1 mole of C-12 = 12.0 grams of C-12 = 6.02 X 1023 atoms of C-12 2 moles of C-12 = 24.0 grams of C-12 = 12.04 X 1023 atoms of C-12 = 1.204 X 1024 atoms of C-12 Equalities How does this help with some other element?

  14. Atomic Weights are Relative

  15. Observation • As long as the number of C and H is the same, the original 12-to-1 mass ratio is preserved. • What is the mass ratio of 425 C atoms compared to 425 H atoms? 12 to 1

  16. Conversely • If the mass ratio is the same, then the number of atoms in two samples is the same. • Given 72 g of C and 6 g of H: • The mass ratio is 12 to 1. So … • These 2 samples have the same number of atoms.

  17. What is the mass of 1 mole of H? • Has to have 6.02 X 1023 H atoms • It’s the same as the number of C atoms in 1 mole of C so the mass ratio is preserved. • Mass ratio for C to H is 12 to 1. • 1 mole of C has a mass of 12 grams. • So 1 mole of H is 1 gram.

  18. Try that again! • Comparing two things: If the number of each is the same, the mass ratio is preserved. • What is the mass of 1 mole of magnesium?

  19. 1 mole of magnesium • 1 atom of C = 12.0 u. 1 atom of Mg = 24.3 u. • 2 atom of C = 24.0 u. 2 atom of Mg = 48.6 u. • 3 atom of C = 36.0 u. 3 atom of Mg = 72.9 u. • 1 mole of C = 12.0 grams. • 1 mole of Mg = 24.3 grams. Mg atoms are a bit more than 2X as heavy as C atoms. Same # of atoms, so mass ratio is preserved.

  20. Molar mass for any element: • To find the molar mass of any element, take the atomic mass and replace a.m.u. with grams. • 1 mole of Ne = 20.2 g = 6.02 X 1023 atoms • 1 mole of Ar = 39.9 g = 6.02 X 1023 atoms • 1 mole of Kr = 83.80 g = 6.02 X 1023 atoms

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