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

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 (H 2 O): 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 g of 12 C.

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, N A.

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.

1.0 amu H 12.0 amu in carton 16.0 amu O amu in carton Take enough cartons to have 6.02 x atoms The total mass will be 1.0 gram Take the same number of atoms as at the left. The total mass will be 16.0 grams (A dozen carbon atoms weigh more than a dozen hydrogen atoms) Moral: a mole of oxygen weighs more than a mole of hydrogen because each individual oxygen atom weighs more.

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

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.

A mole road-map Moles gramsatoms NANA Atomic mass

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 H 2 O, 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 H 2 O has 1 mole of O atoms and 2 moles of H atoms. 1 mole of water weighs 18 g and has 6.02 x molecules

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.

How many atoms of O are there in 5.0 g of CO 2 ?

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