1. Atoms Combine in a Constant Ratio to make Compounds
NaCl always is 1 Na and 1 Cl
1 mole of NaCl contains 1 mole of Na and 1 mole of Cl
1 mole of CaCl2 contains 1 mole Ca and 2 moles Cl
How many moles of Br are in 2 moles of AlBr3?
(3 Br per molecule X 2 moles = 6 moles Br)

2. Molar Mass This is the mass of one mole of something
Calculate it by adding up the molar masses of all the parts
Find the molar mass of Mg(NO3)2
What elements are present?
How many atoms of each element are there?
What is the molar mass of each element?

3. Molar Mass of Mg(NO3)2 1 Mg atom per molecule 2 N atoms per molecule
6 O atoms per molecule
1 Mg * 24.31g/mol = g/mol
2 N * g/mol = g/mol
6 O * g/mol = g/mol
g/mol

4. You Know Mass? You Know Moles!
Mass and moles are interconvertable using the molar mass.
170.0 g NaNO3 is ? moles
Find the molar mass:
X = g/mole
Calculate the moles:
170.0 g / g/mole = moles

5. You Know Moles? You Know Mass!
Moles and mass are interconvertable using the molar mass.
1.83 moles of KMnO4 = ? grams
Find the molar mass:
X = g/mole
Calculate the mass:
1.83 moles X g/mole = 289 g

6. Percent Composition
A certain percent of the molar mass comes from each atom in the molecule.
You calculate this by comparing the mass of just the atom (or group of atoms) to the total mass of the compound.
% = mass of atoms X 100 %
mass of compound

7. % Oxygen in KNO2 Remember: % = mass of atoms X 100 % mass of compound
mass of oxygen = 2 X = g/mol
molar mass = X = g/mol
% O = X 100 % = %
85.11

8. Each Element Adds Mass
Just like the straw that breaks the camel’s back, every tiny piece of a molecule has mass.
You already know this from calculating molar masses, but can you apply it?
How much of the mass of 0.38 g of CaCO3 comes from the oxygen atoms?
We’ll try two methods here.

9. Method 1 - Percent The molar mass of CaCO3 is: The percent oxygen is:
X = g/mol
The percent oxygen is:
(48.00 / ) X 100 % = %
And then we can find the mass:
X 0.38 g = 0.18 g

10. Method 2 – Molar Mass The molar mass of CaCO3 is:
X = g/mol
Find the moles of oxygen:
0.38 g / g/mol = mol CaCO3
mol X 3 oxygen = moles O
Convert to mass:
0.011 moles X g/mole = 0.18 g

11. Compare: Both methods give the same result (0.18 g)
Which method seems easier?
In a similar problem, method 2 has a distinct advantage.
How many moles of oxygen are there in 0.38 g of CaCO3?
Look back – we already have this in Method 2
Method 1 would require an additional step

12. Practice
Find the mass of nitrogen in moles of magnesium nitrate.
Hint – Mg(NO3)2
4.000 moles N
14.01 g/mol
14.01 g/mol X moles =
56.04 g N

13. More Practice
Find the mass of oxygen in 24.1 g of copper (II) hydroxide.
7.90g
Find the moles of chlorine in 87.6 g of vanadium (V) chloride.
1.92 moles
Find the mass of cyanide in moles of lithium cyanide.
2.12 g

14. Empirical Formula Determine the simplest formula:
Begin with the relative mass of the elements
Divide by the molar mass of each
Find the simplest whole number ratio
Write the formula
Example: % Ni, 45.04% S

15. 54.96% Ni, 45.04% S
54.96g X 1mol/58.69g = mol
45.04g X 1mol/32.07g = mol
/ = X 2 = 2
1.404 / =  X 2 = 3
Ni2S3
To get a whole number ratio, we had to multiply by 2