1. Calculations w/ Atomic Mass and Molar Mass

2. Chemistry Question
Previously in this chapter, we have discussed the concept of atomic mass.
Chemistry Question
Based on what you know about atomic mass, what do you think is meant by the term “molar mass?” How does atomic mass differ from molar mass?

3. Atomic Mass vs. Molar Mass
The mass of one single atom of an element
Molar Mass
The mass of one mole of a pure substance (either an element or a compound)
Remember Avogadro’s number
1 mole = 6.02 x 1023

4. Units of Mass Atomic mass unit (amu)
Chemists use the amu to describe atomic mass, as the mass of one atom is so small
An amu is defined as exactly 1/12 the mass of an atom of carbon-12
Therefore, the mass of one atom of C-12 is 12 amu exactly
Why this seemingly random determination?

5. Molar Mass The mass of one atom of carbon-12 is exactly 12 amu
In addition, the mass of one mole of carbon-12 atoms is exactly 12 g
This means that the mass listed on the periodic table is BOTH:
The average atomic mass of the element (in amu)
The molar mass of the element (in grams)
Unless otherwise stated, always round masses from your periodic table to two decimal places (i.e., use 1.01 g/mol of H or g/mol of O)

6. Finding the Molar Mass of a Compound
How can we determine the molar mass of H2O?
Think about having one mole of H2O molecules.
How many moles of H atoms do you have?
Two
How many moles of O atoms do you have?
One
Find the molar masses of each element and add them together.
1.01 (2) = g/mol

7. Types of Particles Atoms Molecules Formula Units C, Cu, Fe
All non-metals in the chemical formula
O2, CO2, C6H12O6
Formula Units
Have a metal in the chemical formula
NaNO3, LiCl, Mg3(SO4)2

8. Check Your Understanding
You have g of Na3PO4
How many moles of Na3PO4 do you have?
moles of Na3PO4
How many formula units of Na3PO4 do you have?
9.174 x 1022 F.U. of Na3PO4
How many moles of oxygen atoms do you have?
moles of O
How many grams of oxygen do you have?
9.754 g O
How many oxygen atoms do you have?
3.670 x 1023 atoms of O