Section 2:
Mass and the Mole
Which one weighs more? Why?
Same number different masses
Same concept in chemistry…
1 mole = 6.022 x 1023 atoms (single element)
But… atoms are not all the same size. That is why…
1 mol of carbon atoms = 12 g C (Periodic Table)
1 mol of iron atoms = 55.85 g Fe (Periodic Table)
Same number…different mass
1
Calculating Moles and Mass
1 mol of element = atomic mass of element
1 mol of compound = formula or molar mass of compound
Example
1 mol of Fe = 55.847 g
1 mol of Na = 22.990 g
1 mol of carbonic acid (H2CO3) = 62.024 g (Add Them Together!)
1 mol of sodium chloride (NaCl) = 58.443 g (Add Them Together!)
2
Example
Step 1
Step 2
Step 3
Calculate the formula mass for ammonia (NH3)
Write down the different elements and their
atomic mass
N
14.007g
H
1.008g
Count how many of each element is present
and multiply the quantity by the atomic mass
N
14.007g x 1 = 14.007 g
H
1.008g x 3 = 3.024 g
Add all the totals together
Formula mass = 14.007 + 3.024 = 17.031 g
3
Solving the problems Using the Tools
1st…Follow your rules.
1. What do I know?
2. What do I want?
3. How do I get there?(Tools)
• You have 2 tools in your tool box!
Avogadro’s #
1 mol Al
6.022 x 1023 particles Al
Molar Mass
6.022 x 1023 particles Al
1 mol Al
1 mol Al
107.87g of Ag
107.87g of Al
1 mol Al
4
PRACTICE USING THE TOOLS
Example: Determine the mass in grams of 3.57 mol Al
Follow your rules…
1. What do I know?
2. What do I want?
3. How do I get there?(Tools)
3.57 mols of Al
“Given”
1 mol Al
6.022 x 1023 particles Al
=
6.022 x 1023 particles Al
1 mol Al
1 mol Al
26.98 g of Al
9.63
g of Al
26.98 g of Al
1 mol Al
5
PRACTICE USING THE TOOLS
Example: Determine the number of moles 25.5 grams Ag.
Follow your rules…
1. What do I know?
2. What do I want?
3. How do I get there?(Tools)
25.5 g of Ag
“Given”
1 mol Al
6.022 x 1023 particles Al
=
6.022 x 1023 particles Al
1 mol Al
1 mol Al
107.87g of Ag
0.236
mols of Ag
107.87g of Al
1 mol Al
6
Example How many mols of oxygen are in 52 g of
oxygen
52 g- -O-2
x
1 mol O2
31.998 g- -O-2
= 1.625
1.6 mol
mol
O2O2
Example How many mols of calcium are in 37 g of
calcium
-37 -g-Ca
x
1 mol Ca
== 0.92
0.9232
molmol
CaCa
40.08 -g -Ca
-Example How many mols of sodium chloride are in
100 g NaCl
--100 g- -NaCl
x
1 mol NaCl
58.443 g- -NaCl
---
= 1.711
2 mol mol
NaClNaCl
7
Example How many grams of hydrogen are in 1.67
mol of hydrogen
1.67 mol
---H
- -2
x
2.016 g H2
= 3.367
3.37 ggHH2 2
---H
-2
1 mol
Example How many grams of carbon are in 0.85
mol of carbon
0.85 mol
- - - -C
x 12.011 g C
---C
1 mol
= 10.
10.209
g Cg C
Example How many grams of ammonia are in
2.3 x 105 mol ammonia
- - - NH
- - -3
2.3 x 105 mol
x
17.031 g NH3
6 g6 NH
= 3.917
x 10
g NH
3.9 x 10
3 3
1 mol
- - - NH
-- 3
8
Converting Mass and atoms
Golden Rule…GET TO MOLES!!!!
• Then you can do what ever you want!
Mass (g)
Moles (mol)
Particles
Example: How many atoms are in 55.2 g Li
55.2 g of Li
“Given”
=
=
1 mol Li
6.022 x 1023 particles Li
6.022 x 1023 particles Li
1 mol Li
4.79 x 1024
1 mol Li
6.94 g of Li
7.95
mols of Li
atoms of Li
6.94 g of Li
1 mol Li
9
Example An extra strength aspirin tablet contains
0.500g of aspirin (C9H8O4). How many
molecules of aspirin are in one tablet
0.500 -gC
- 9-H- 8-O-4-
1 mol
- - -C-9H- 8-O-4
x
x
6.022 x 1023 molecules C9H8O4
- - -C-9H- 8-O-4
1 mol
- - g- -C-9H
- -8O
-4
180.16171
= 1.67
1.671xx10
102121molecules
moleculesCC9H
9H
8O
8O
44
Example In a small firework, 3.467 x 1022 atoms Sr
are used to make the firework burn red. How
many grams of strontium are in the firework.
3.467 x 1022
-atoms
- - - - Sr
--
x
1-mol
- - -Sr- - - -Sr
-6.022 x 1023 -atoms
x
87.620 g Sr
- - -Sr1 mol
= 5.044 g Sr
10