1. Ch 6 Chemical Composition
In Chemistry, one of the most vital skills is to be able to calculate mass relationships from a balanced chemical equation.
In This chapter we will be learning the skills to calculate the amount of reactant or product in a unit called the mole. And how to convert that quantity to a more familiar measured quantity mass.
Intense focused study and practice are essential for your success this chapter. This Chapter is foundational for the remainder of the year!

2. Atomic Masses
Individual atoms are fare too small to be accurately weighed
The mass of atoms is based on a relative scale
The standard used for atomic mass is the carbon-12 atom. The mass of one C-12 atom is defined as 1 amu (atomic mass unit)
All of the mass values on the periodic table are based upon this comparison

3. Isotopic Abundance
Masses listed on the periodic table are values representing the weighted average of all of the stable isotopes of that element.
The reason for this is that if you have a sample of material, all of the isotopes are likely present in the abundance given.
For example, chlorine has two stable isotopes, Cl-35 and Cl-37, present in 75.53% abundance and 24.47% respectively.
The listed value for the atomic mass of chlorine is amu, which is the weighted average of the atomic mass of those two isotopes. Notice the value is closer to the Cl-35 isotope because that species is more abundant.

4. Counting by Weighing
The number of atoms in a measured sample can be determined if the mass of any individual atom in known
Although individual atoms may vary for an atom (isotopes) we can use the average mass which is listed on the periodic table in amu.
Dividing the measured mass by the mass of any individual atom will give the number of those atoms in the sample.