1. Unit 8: The Mole
Molar Mass Notes

2. What is molar mass? It is the TOTAL mass of the entire compound.
It is calculated by adding up the AMU of every single atom in the compound.
What is AMU, again? Where do we find that?
It is the mass of the substance in 1 mole of that particular substance.
We use the unit grams/mol

3. How is molar mass calculated?
Take the AMU of each element, in the substance, from the periodic table and add them all together.
If there are multiple atoms of the same element multiply how many atoms of that element there are by the elements AMU. CaCl2 (40 + (35 x 2)) = 110g/mol
You have your molar mass.
FOR MOLAR MASS WE ONLY USE SUBSCRIPTS
NOT COEFFECIENTS

4. Determining the molar mass of compounds:
Add the molar masses of EVERY atom in the compound together.
Find the molar mass of H2O
H= = (1) x 2 hydrogen atoms g
O=15.994= (16) x 1 oxygen atom g
Molar mass of H2O g/mol
*You can use whole numbers instead of decimals but you will need to round!

5. Find the molar mass: 1. Copper: 6. Sulfur: 2. Aluminum: 7. Oxygen:
3. Hydrogen: Sodium chloride:
4. Nitrogen: Potassium iodide:
5. Lead: Beryllium chloride:

6. What is Percent Composition?
The percent by mass
of each element in a compound
*Percent – a part over the whole x 100

7. How to determine the % comp of each element:
1. Calculate the molar mass of the entire substance.
2. Calculate the individual mass of each element.
(remember to account for every atom of that element)
3. Divide the part by the whole
Part: the mass of the element, that you are calculating the % comp for.
Whole: the mass of the entire substance
4. Multiply by 100
5. This is the percent of the individual element
(do this for each element in the compound)

8. How to calculate % comp:
If we have Mg3(PO4)2 and we want to know the mass % of Magnesium:
What is the molar mass of just Mg? 3 x 24g = ___________________
What is the molar mass of the entire substance of Mg3(PO4)2? = _______
Part: __________________
Whole: _____________________________
Part/whole = 72/262 x 100 = 27%

9. How to check your work: To check your work add up all the percentages:
Mg – 27%
P – 24%
O – 49%
=________________
It should be right around 100…within a number or 2 (this is because we are rounding to the nearest whole number)

10. Practice
Lithium Sulfate:
Iron (III) oxide: