Guided Notes Section 7.2: Relative Atomic Mass and Chemical Formulas
1. Most elements are a mixture of _________________.
a. Isotopes are atoms of the ___________ element that have _____________
numbers of _______________.
b. Because they have different numbers of _____________, isotopes have different
_______________ _________________.
c. The period table reports _____________ atomic mass, which is a
_______________ average of the atomic mass of an element’s _____________.
d. If you know the abundance (how much there is) of each isotope, you can calculate
the average ______________ mass of an element.
2. Calculating Average Atomic Mass: Sample Problem E
The mass of a Cu-63 atoms is 62.94 amu, and that of a Cu-65 atoms is 64.93 amu.
Using the data below, find the average atomic mass of copper. (amu = atomic mass unit)
abundance of Cu-63 = 69.17%
abundance of Cu-65 = 30.83 %

Change each percentage to a ______________ by dividing it by _________.
Cu-63 = 69.17% / 100% = .6917
Cu-65 = 30.83% / 100% = .3083

Multiply each decimal by it’s _____________ ____________.
Cu-63 = (.6917) (62.94 amu) = 43.54 amu
Cu-65 = (.3083) (64.93 amu) = 20.01 amu

Average atomic ___________ is found by _________ the two together:
43.54 amu + 20.01 amu = 63.55 amu (which is the atomic mass of Cu shown on the PT)
3. Additional Example: Calculate the average atomic mass for gallium if 60.00% of its
atoms have a mass of 68.926 amu and 40.00% have a mass of 70.925 amu.
4. A compound’s chemical formula tells you which ____________, as well as __________
much of each, are present in a compound.
a. Formulas for ______________ compounds show the elements and the number of
___________ of each element in a molecule.
b. Formulas for _____________ compounds show the ______________ ratio of
______________ and ____________ in any pure sample.
5. Formulas for _______________ ions show the _______________ ratio of cations and
anions.
a. They also show the ______________ and the number of ___________ of each
element in each ion.
b. For example, the formula KNO3 indicates a ratio of one ________ cation to one
__________ anion.
6. Calculating Molar Mass of Compounds: Sample Problem F
Find the molar mass of barium nitrate.

Barium is a ________ cation and nitrate is a _________ anion.

You need ________ nitrate ions to balance with the _________ barium cation.

The simplest formula is: ___________________

To find the molar mass of Ba(NO3)2, ________ the atomic masses of each
__________ together:

Barium: 137.33 g/mol = 137.33 g/mol

Nitrogen: 2 (14.01 g/mol) = 28.02 g/mol

Oxygen: 6 (16.00 g/mol) = 96.00 g/mol
Molar mass of Ba(NO3)2 = 261.35 g/mol
7. Additional Practice: Calculate the molar mass of (NH4)2SO3