1. Do now
Take out lab from last week

2. Review The Mole!
Empirical formula

3. Calculating Empirical Formulas
Empirical Formula: consists of the symbols for the elements combined in a compound, with subscripts showing the smallest whole-number mole ratio of the different atoms in the compound.
The empirical formula does not always equal the molecular formula

4. Worked Example Which of the following are empirical formulas?
N2O4
Pb(NO3)2
The elements of these formulas are shown in the simplest whole number ratio.

5. What is the empirical formula for a compound containing 32
What is the empirical formula for a compound containing 32. g of sulfur with 32 g of oxygen?
32 g S 1 mol S = 1 mol S
32 g S
32 g O mol O = 2 mol O
16 g O
1:2 mole ratio means the formula is SO2

6. 12. 3 g of hydrated magnesium sulfate MgSO4·xH2O gives 6
12.3 g of hydrated magnesium sulfate MgSO4·xH2O gives 6.0 g of dehydrated salt MgSO4 on heating. Deduce the value of x
6.0 g MgSO4 1 mol MgSO4 = mol MgSO4
120. g MgSO4
6.3 g H2O mol H2O = mol H2O
18 g H2O

7. 12. 3 g of hydrated magnesium sulfate MgSO4·xH2O gives 6
12.3 g of hydrated magnesium sulfate MgSO4·xH2O gives 6.0 g of dehydrated salt MgSO4 on heating. Deduce the value of x
= mol MgSO4 = 1
0.05 mol
= mol H2O = 7
0.05 mol H2O
1:7 mole ratio means the formula is MgSO4·7H2O

8. Calculating Empirical Formulas
If you are given the % of each element in the compound you can determine the empirical formula.

9. Example (Pg 229 textbook)
The percentage composition of diborane is 78.1% B and 21.9% H. What is the emperical formula?
BxHy Basically we are asking what is x and y?
1. Assume you have 100 g of this material.
This means you have 78.1 g B and 21.9 g H

10. Convert from grams to moles
Use molar mass the other way
78.1 g B x 1 mol B = mol B
10.81g B
21.9 g H x 1 mol H = 21.7 mol H
1.01 g H

11. Find the formula Divide by the smallest # of moles to get the ratio
78.1 g B x 1 mol B = mol B/7.22 mol = 1
10.81g B
21.9 g H x 1 mol H = 21.7 mol H/7.22 mol = 3.01
1.01 g H
The final result then is B1H3 or BH3

12. Molecular formula
Formulas which show all the atoms present in a molecule.
Can deduce molecular formula if you know empirical formula and the molar mass.

13. Calomel is a compound once used in the treatment of syphilis
Calomel is a compound once used in the treatment of syphilis. It has the empirical formula HgCl and a molar mass of gmol-1. What is its molecular formula?
Calculate mass of empirical formula:
= gmol-1
Molar mass of compound = factor
molar mass of empirical formula
472.08/ = 2 so multiply formula by 2
Hg2Cl2