1. Mass Relationships in Chemical Reactions: STOICHIOMETRY
Chapter 3
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2. Atomic mass is the mass of an atom in atomic mass units (amu)
Micro World
atoms & molecules
Macro World
grams
Atomic mass is the mass of an atom in atomic mass units (amu)
By definition:
1 atom 12C “weighs” 12 amu
On this scale
1H = amu
16O = amu

3. The average atomic mass is the weighted
average of all of the naturally occurring
isotopes of the element.

4. Average atomic mass of lithium:
Naturally occurring lithium is:
7.42% 6Li (6.015 amu)
92.58% 7Li (7.016 amu)
Average atomic mass of lithium:
7.42 x x 7.016
100
= amu

5. Problem 3.5, Page 110
The atomic masses of 35Cl (75.53 amu) and 37Cl (24.47) are amu and amu, respectively. Calculate the average atoamic mass of chlorine.

6. Average atomic mass (6.941)

7. Avogadro’s number (NA)
The Mole (mol): A unit to count numbers of particles
Dozen = 12
Pair = 2
The mole (mol) is the amount of a substance that
contains as many elementary entities as there
are atoms in exactly grams of 12C
1 mol = NA = x 1023
Avogadro’s number (NA)

8. Molar mass is the mass of 1 mole of in grams marbles atoms
eggs
shoes
Molar mass is the mass of 1 mole of in grams
marbles
atoms
1 mole 12C atoms = x 1023 atoms = g
1 12C atom = amu
1 mole 12C atoms = g 12C
1 mole lithium atoms = g of Li
For any element
atomic mass (amu) = molar mass (grams)

9. One Mole of: S C Hg Cu Fe

10. molecular mass (amu) = molar mass (grams)
Molecular mass (or molecular weight) is the sum of
the atomic masses (in amu) in a molecule.
SO2 1S
32.07 amu 2O
+ 2 x amu
SO2
64.07 amu
For any molecule
molecular mass (amu) = molar mass (grams)
1 molecule SO2 = amu
1 mole SO2 = g SO2

11. formula mass (amu) = molar mass (grams)
Formula mass is the sum of the atomic masses
(in amu) in a formula unit of an ionic compound.
NaCl
1Na
22.99 amu
1Cl
amu
NaCl
58.44 amu
For any ionic compound
formula mass (amu) = molar mass (grams)
1 formula unit NaCl = amu
1 mole NaCl = g NaCl

12. What is the molar mass of H2O, of CH4

13. RELATIONSHIPS 1 mole = 6.022 x 1023 atoms or molecules
Formula identifies number of atoms in a molecule
Weight of one mole = molecular formula weight

14. 1 mol C3H8O molecules = 8 mol H atoms
How many H atoms are in 72.5 g of C3H8O ?
1 mol C3H8O = (3 x 12) + (8 x 1) + 16 = 60 g C3H8O
1 mol C3H8O molecules = 8 mol H atoms
1 mol H = x 1023 atoms H
1 mol C3H8O
60 g C3H8O x
8 mol H atoms
1 mol C3H8O x
6.022 x 1023 H atoms
1 mol H atoms x =
72.5 g C3H8O
5.82 x 1024 atoms H

15. What is the formula mass of Ca3(PO4)2 ?
1 formula unit of Ca3(PO4)2
3 Ca
3 x 40.08
2 P
2 x 30.97
8 O
+ 8 x 16.00
amu

16. Problem 3.26
How many molecules of ethane (C2H6) are present in grams of C2H6?

17. Problem 3.27
Calculate the number of C, H, and O atoms in 1.50 g of glucose (C6H12O6) a sugar.