Download Unit 3: Bonding and Nomenclature Content Outline: Calculating

Unit 3: Bonding and Nomenclature
Content Outline: Calculating Molar Mass & Percent Composition (3.9)
I.
Molecular Mass
A. This is the sum mass (in AMUs or Daltons) for all atoms present within a molecule.
B. Step 1: Use the Periodic Table and the chemical formula to find the average atomic masses for each
element present.
Step 2: Calculate the total atomic mass for each element according to the elements subscripts.
Step 3: Sum (add) all the elements total atomic masses together to get the Formula mass.
Step 4: Express the quantity in AMUs or Daltons.
For example, H2O = 2 H at 1.01 AMU each and 1 O at 16.0 AMUs so 16.0 + 2.02 = 18.02 AMUs.
II. Formula Mass
A. This refers to the sum of the average atomic masses for all the atoms present within a chemical
formula for any given ionic compounds formula ( formula unit), or ion.
B. Step 1: Use the Periodic Table and the chemical formula to find the average atomic masses for each
element present.
Step 2: Calculate the total atomic mass for each element according to the elements subscripts.
Step 3: Sum (add) all the elements total atomic masses together to get the Formula mass.
Step 4: Express the quantity in AMUs or Daltons.
III. Molar Mass (MM)
1. Mole (mol)
a. This term is used to describe a “workable” amount of an element or molecule.
b. It is also references the number of atoms or molecules present in 1 mole.
i. Avogrado’s Number = 6.022 x 1023 atoms or molecules
c. Molar Mass(grams/mol) = Formula Mass or molecular mass in grams
2. This unit can be used as a conversion factor when working Chemistry problems.
a. number of moles x MM(g/mol) = Total Mass in grams
For example, 2 moles of water = 2 moles x 18.02 g/mol H2O(see above) = 36.02 grams/mol
IV. Quick Review of various Mole calculations:
A. Conversions from one unit to another unit involving the mole concept:
the basic concept is: Unit given x unit wanted = Unit wanted
unit given
The given unit cancels out and leaves you with the unit wanted.
1. Moles  Atoms/Molecules
a. You have 2.0 moles of Copper. How many atoms of Copper do you have?
1 mole = 6.022 x 1023 atoms so: 2.0 moles x 6.022 x 1023 atoms = 12.044 x 1023 atoms
1 mole
But using your rules for significant figures, it becomes 1.2 x 1024 atoms ( only laboratory
measurements determine significant figures)
2. Atoms/Molecules  moles
a. You have 1.806 x 1024 atoms of Zinc (Zn). How many moles of Zinc do you have?
Step 1: 1.8.06 x 1024 Atoms x
1 mole
= 3.000 moles
23
6.022 x 10 atoms
3. Grams  Moles
a.
You have 10.0 grams of Nickel (Ni). How many moles of Nickel do you have?
10.0 grams x
1 mole
= 0.170 moles
58.69 grams
4. Moles  Grams
a. You have 8.5 moles of Fluorine (F) gas. How grams of Fluorine do you have?
8.5 moles x 19.00 grams = 160.0 grams or 1.6 x102 grams
1 mole
V. Calculating Percent Composition from Molar Mass
A. This calculation allows for us to find the percentage (out of 100%) of one element from the total
molecule using Molar Mass.
For example, C6H12O6 What percentage of the total mass is Carbon?
B. Step 1: Use the Periodic Table and chemical formula to find the mass of the requested element.
Step 2: Use the chemical formula subscripts, to find the number of atoms present in one molecule or
formula unit. For example, C6H12O6 = 1 molecule, so for Carbon there are 6 atoms, for Hydrogen
there are 12 atoms, and for Oxygen there are 6 atoms present. In order to determine the mass in
grams view the formula as 1 mole. For Example, in 1 mole of C6H12O6, there will be 6 moles of
Carbon, 12 moles of Hydrogen and 6 moles of Oxygen.
Step 4: Calculate the Molar Mass for the molecule. (Mass of one mole of molecule)
For example, C moles of C = 6 x 12.0 g/mol = 72.0 g/mol
H moles of H = 12 x 1.01 g/mol = 12.1g/mol
O  moles of O = 6 x 16.0 g/mol= 96.0 g/mol
Mass of 1mole of C6H12O6 = 72.0 + 12.1 + 96.0 = 180.1 g/mol
Step 5: Divide the Elemental Mass of each element by the total Molar Mass.
For example, Carbon
72.0 g/mol of C
= 0.3998
180.1 g/mol C6H12O6
Step 6: Multiple the result of Step 5 by 100%
For example, 0.3998 x 100% = 39.98% (Just move the decimal two spots right.)
If need be you do this for H and O. To double check your math all 3 percentages should add up to
100%.