7.1
LAST NAME________________________ FIRST NAME________________ DATE ________ PERIOD
7.1 Molar Mass of an Element and Compound
MOLAR MASS OF AN ELEMENT
What is the Mass of the following? (Use your Periodic Table)
1 mole of C
12 grams
2 moles of C
2×12 g/mole =
1 mole of Silver
2 moles of Silver
1 mole of Gold
1 mole of Iron
2 moles of Iron
3 moles of Iron
24 grams
1 mole of Sodium
2 moles of Sodium
CALCULATING MOLAR MASS FOR A MOLECULE/COMPOUND
The molar mass of a molecule is the molar mass of all of the parts of the molecule added together. For
example, the molar mass for a water molecule H2O is the molar mass of 2 Hydrogen and 1 Oxygen.
Mathematically this would be done in the following method:
1 molecule of water is H2O. It has 2 hydrogen atoms and a single oxygen.
H 2O
Element
# in one
Molecule
Molar Mass of
Element
Subtotal
H
O
2
1
1.00 gram/mole
16.00 gram/mole
2.00 gram/mole
16.00 gram/mole
18.00 gram/mole
Calculate the molar mass for the following compounds
H2SO4
Element
# in one
Molecule
Molar Mass of
Element
Subtotal
H
S
2
1
gram/mole
gram/mole
gram/mole
gram/mole
O
4
gram/mole
gram/mole
gram/mole
NaNO2
Element
# in one
Molecule
Molar Mass of
Element
Subtotal
gram/mole
gram/mole
gram/mole
gram/mole
gram/mole
gram/mole
gram/mole
Pt(O2)2
Element
# in one
Molecule
Molar Mass of
Element
Subtotal
gram/mole
gram/mole
gram/mole
gram/mole
gram/mole
gram/mole
gram/mole
CaCO3
Mg(CN)2
AlPO4
WORK CHECK LIST:
Did you set up the tables properly?_____
Do ALL your values have Units? ___________
HAVE YOUR WORK TO THIS POINT CHECKED___________
Percent Composition – Percent Composition tells a scientist the percentage that a molecule
is made of each element. Notice that most of the mass of water is from the elment Oxygen so the
Oxygen’s percentage is the higher of the two. Remember that a percent is a fraction of the part
divided by the whole.
% 𝐶𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛 =
𝑝𝑎𝑟𝑡
× 100%
𝑤ℎ𝑜𝑙𝑒
Calculate the Molar Mass and Percent Composition of the following Compounds.
EXAMPLE: Calculating the % Composition of a Water Molecule:
You try to Calculate the % Composition of Oxygen ( about 89%)
1. KC2H3O2
2. Na2CO3
3. AlPO4
WORK CHECK LIST:
Did you set up the tables properly?_____
Do ALL your values have Units? ___________
HAVE YOUR WORK TO THIS POINT CHECKED___________
SOME ANSWERS TO CHECK YOUR WORK.
Molar Mass Questions:
H2SO4 =98 g/mole, NaNO2=69 g/mole , Pt(O2)2 = 258 g/mole , CaCO3 =100 g/mole , Mg(CN)2 =76 g/mole,
AlPO4 =122 g/mole
% Composition Questions:
KC2H3O2
Na2CO3
AlPO4
- MM = 98 g/mole
- MM = 106 g/mole
-MM = 122 g/mole
39.8 %K, 24.5 %C, 3.1 %H, 32.6%O
43.4 %Na, 11.3%C, 45.3 %O
22.1 %Al, 25.4 %P, 52.5%O