1. Unit 3 Notes Part 2

2. Weighted Average…
In a class, tests are 70% of your grade, labs are 15%, and daily grades are 15%. If you have the following averages, what is your overall grade in Chemistry?
Test:50
Lab: 60
Daily: 95

3. Atomic Mass – a weighted average mass of the atoms in a naturally occurring sample of the element.
Atomic Mass Unit (amu) – is defined as one-twelfth the mass of a carbon-12 atom.

4. Example: A sample of cesium is 75% 133Cs, 20. % 132Cs, and 5. 0% 134Cs
Example: A sample of cesium is 75% 133Cs, 20.% 132Cs, and 5.0% 134Cs. Calculate its average atomic mass.
0.75 x
133 =
99.75 or 1.0 x 102 x
0.20
132 =
26. x
0.050
134 =
6.7
or 133 amu

5. Compare this value to that of the periodic table. Is it accurate?
Calculated Value
Periodic Table
amu
amu
I would say…YES!

6. Questions: Draw a chemical symbol and circle the atomic mass.
How do you calculate atomic mass?