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How to Count Atoms (when they are really really small)

How to Count Atoms (when they are really really small). Review: 1 12 C atom = 12.0000 amu = 1.9926x 10 -23 g (from mass spectroscopy experiments) So. Goal: a number that is capable of expressing numbers of atoms in convenient terms.

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How to Count Atoms (when they are really really small)

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  1. How to Count Atoms(when they are really really small) Review: 1 12C atom = 12.0000 amu = 1.9926x 10-23 g (from mass spectroscopy experiments) So

  2. Goal: a number that is capable of expressing numbers of atoms in convenient terms. H and O react in simple numbers of atoms to form water (H2O): 2 H : 1 O There is not a simple relation between the masses that react. Yet – masses are what we can readily measure in a laboratory. To get a quantity capable of expressing numbers of atoms, Define: 1 mole = number of atoms in 12.0000 g of 12C.

  3. We now have enough information to calculate the number of atoms in one mole of carbon: But this is also the number of atoms in one atomic mass, expressed in grams, of any element. This important number is known as Avogadro’s Number, NA.

  4. Both a mole and a dozen express quantities of things by a collective number. A dozen eggs weighs more than a dozen ping-pong balls because each individual unit is heavier.

  5. 16.0 amu 12.0 amu O C 192.0 amu in carton 144.0 amu in carton (A dozen oxygen atoms weigh more than a dozen carbon atoms) Take the same number of atoms as at the left. The total mass will be 16.0 grams Take enough cartons to have 6.02 x 1023 atoms The total mass will be 12.0 gram Moral: a mole of oxygen weighs more than a mole of carbon because each individual oxygen atom weighs more.

  6. So now, atomic masses have two interpretations: 1 atom of C weighs 12.0000 amu 1 mole of C weighs 12.0000 g Some conversion factors: 1 mole of C = 12.00 g 1 mole of C = 6.02 x 1023 atoms C

  7. Now we can calculate the mass of any atom: 1 atom of U = 238 amu

  8. Now we can calculate the mass of any atom: 1 atom of U = 238 amu Find the number of carbon atoms in 3.0 g C.

  9. A mole road-map Atomic mass NA atoms Moles grams

  10. This can be applied to compounds, which have a molecular mass (in amu) or a molar mass (in grams). For instance: In one molecule of H2O, there are 2 H atoms = 2 x 1 amu = 2 amu 1 O atom = 1 x 16 amu = 16 amu 18 amu 1 mole of H2O has 1 mole of O atoms and 2 moles of H atoms. 1 mole of water weighs 18 g and has 6.02 x 1023molecules

  11. For Ca(OH)2: 1 Ca = 40.1 amu 2 O = 32.0 amu 2 H = 2.0 amu 74.1 amu 1 mole of Ca(OH)2 has a mass of 74.1 g Ca(OH)2 is an ionic compound so we can’t talk about molecules. Sometimes the simplest formula – which indicates only the ratio of the ions – is referred to as a formula unit.

  12. How many atoms of O are there in 5.0 g of CO2?

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