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 grams.
How many nails are in the pile?
(1227g) / g per nail) = 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 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. Equalities 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

12. Equalities 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

13. Equalities 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 = X 1024 atoms of C-12
How does this help with some other element?

14. Atomic Weights are Relative
C: # and mass
H: # and mass
Mass Ratio
1 C = 12.0 amu
1 H = 1.0 amu
12 to 1
2 C = 24.0 amu
2 H = 2.0 amu
12 to 1
3 C = 36.0 amu
3 H = 3.0 amu
12 to 1
4 C = 48.0 amu
4 H = 4.0 amu
12 to 1
5 C = 60.0 amu
5 H = 5.0 amu
12 to 1
10 C = amu
10 H = 10.0 amu
12 to 1

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. These 2 samples have the same number of atoms.
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.
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 = g = 6.02 X 1023 atoms