Thursday, April 4th: “A” Day
Friday, April 5th: “B” Day (ACT seminar)
Agenda
Collect Homework: pg. 21/22 practice worksheet
Continue Section 7.1: “Avogadro’s Number &
Molar Conversions”
Homework :
“Grams/Moles Calculations” worksheet
Concept Review: “Avogadro’s Number &
Molar Conversions”
Continue 7.1: “Avogadro’s Number
and Molar Conversions”
6.022 X 1023 atoms
1 mole
Molar Mass Links Moles to Grams
Molar mass: the mass in grams of one mole of an
element or compound.
Molar mass is numerically equal to the atomic
mass of elements (Fe, Al, etc.) and the formula
mass of compounds (H2O, CH4, etc. ) and diatomic
elements. (O2, F2, etc.)
** The units for molar mass are **
grams/mole
Molar Mass Example #1
What is the molar mass of water, H2O?
1. Use the period table to find the atomic mass of
each element in the compound.
2. Add them up to get the molar mass of the
compound:
H = 2(1.01 g/mol)
O = 16.00 g/mol
Molar mass of water = 18.02 g/mol
Molar Mass Example #2
What is the molar mass of sulfuric acid, H2SO4?
H = 2(1.01 g/mol)
S = 32.07 g/mol
O = 4(16.00 g/mol)
Molar mass of sulfuric acid =
98.09 g/mol
Molar Mass Example #3
 What is the molar mass of sodium carbonate,
Na2CO3?
Na = 2(22.99 g/mol)
C = 12.01 g/mol
O = 3(16.00 g/mol)
Molar mass of sodium carbonate =
105.99 g/mol
So, why do we need to know the molar
mass…
Molar mass can be used as a conversion
factor to change from mass (in grams)
to amount (in moles) !!
Changing Moles
Mass (in grams)
Determine the mass in grams of 3.50 mol of
copper, Cu.
1. Start with what you know: 3.50 mol Cu
2. Use the molar mass of Cu as the conversion
factor to change mol Cu
grams Cu.
3.50 mol Cu X 63.55 g Cu =
1 mol Cu
222 g Cu
(3 sig figs)
Changing Moles
Mass (in grams)
Example #1
What is the mass in grams of 10.5 mol ammonia,
NH3?
1. Start with what you know: 10.5 mol NH3
2. Use the periodic table to calculate the molar
mass of NH3 to use as a conversion factor:
N: 14.01 g/mol
H: 3(1.01) g/mol
17.04 g/mol
Changing Moles
Mass (in grams)
Example #1, cont.
3. Multiply by the conversion factor to change mol
NH3
grams NH3.
10.5 mol NH3 X 17.03 g NH3 =
1 mol NH3
179 g NH3
(3 sig figs)
Changing Moles
Mass (in grams)
Example #2
How many grams are in 1.26 moles of aluminum
hydroxide, Al(OH)3?
1. Start with what you know: 1.26 moles Al(OH)3
2. Use the periodic table to calculate the molar
mass of Al(OH)3 to use as a conversion factor:
Al: 26.98 g/mol
O: 3(16.00 g/mol)
H: 3(1.01 g/mol)
78.01 g/mol
Changing Moles
Mass (in grams)
Example #2, cont.
3. Multiply by the conversion factor to change
mol Al(OH)3
grams Al(OH)3.
1.26 mol Al(OH)3 X 78.01 g Al(OH)3 =
1 mole Al(OH)3
98.3 g Al(OH)3
(3 sig figs)
Do you think we can go the other
way and change mass in grams to
amount in moles..
Of course we can, and that’s what
we’re going to do…
Changing Mass (in grams)
Moles
How many moles of helium are contained in 0.255
g He?
1. Start with what you know: 0.255 g He
2. Use the molar mass of He as the conversion
factor to change mass He
mol He
0.255 g He X 1 mole He
4.00 g He
=
.0638 mol He
(3 sig figs)
Changing Mass (in grams)
Example #1
Moles
How many moles are there in 68 g of
copper (II) hydroxide, Cu(OH)2?
1. Start with what you know: 68 g Cu(OH)2
2. Use the periodic table to calculate the molar
mass of Cu(OH)2 to use as a conversion factor:
Cu: 63.55 g/mol
O: 2(16.00 g/mol)
H: 2(1.01 g/mol)
97.57 g/mol
Changing Mass (in grams)
Example #1, cont.
Moles
3. Multiply by the conversion factor to change
mol Cu(OH)2
grams Cu(OH)2.
68 g Cu(OH)2 X 1 mol Cu(OH)2 =
97.57 g Cu(OH)2
.70 mol Cu(OH)2
(2 sig figs)
Changing Mass (in grams)
Example #2
Moles
How many moles are there in 1,200 g of
ammonia, NH3?
1. Start with what you know: 1,200 g NH3
2. Use the periodic table to calculate the molar
mass of NH3 to use as a conversion factor:
N: 14.01 g/mol
H: 3(1.01 g/mol)
17.04 g/mol
Changing Mass (in grams)
Example #2, cont.
Moles
3. Multiply by the conversion factor to change
mol NH3
grams NH3.
1,200 g NH3 X 1 mol NH3 =
17.04 g NH3
70. mol NH3
(2 sig figs)
Homework
You may work with a partner to complete the
“Grams/Moles Calculations” worksheet.
You must show all work to get credit!