1. Final Review Day 2
Ch 11, 12 & 13

2. Percent Composition refers to the percentage by mass.
does not tell us the relative rate by which the atoms combine, or what portion of the volume is made up by an individual element.

3. Calculating Percent Composition
percent composition = part x 100
whole

4. What is the percentage composition of glucose (C6H12O6) ?
find the mass of the entire molecule
2. Use the formula to calculate the percent composition of each element

5. Formula Mass
C = 12.0 u x 6 atoms   = 72.0 u H = 1.01 u x 12 atoms = 12.1 u O = 16.0 u x 6 atoms   = 96.0 u                                                                          180 u

6. Answer 72.0 u % for Carbon = ---------------- x 100 = 40.0% 180 u
                               12.1 u % for Hydrogen = x 100 = 6.7 %                                180 u
                              96.0 u % for Oxygen =   x 100  = 53.3%                               180 u

7. Empirical Formulas
Empirical formulas: smallest whole number ratio of atoms present in a substance
Molecular formula: actual number of each type of atom present in a molecule
Many molecular compounds have different empirical and molecular formulas
N2O4 (molecular) NO2 (empirical)

8. Calculating Empirical Formulas
Percent composition data is commonly used to determine the empirical formula of a compound:
Four steps:
percent to mass
mass to mole
divide by smallest
multiple ‘til whole

9. Calculating Empirical Formulas
Example: An iron compound contains % Fe and % O. Calculate its empirical formula.
Step 1: % to mass
% Fe  g Fe
% O  g O

10. Calculating Empirical Formulas
Step 2: Mass to Moles
mol Fe = g Fe x mol Fe = mol Fe
g Fe
mol O = g O x mol O = mol O
g O

11. Calculating Empirical Formulas
Step 3: Divide by smallest
O: mol O = 1.5
mol Fe
Fe: mol Fe = 1.0

12. Calculating Empirical Formulas
Step 4: Multiply ‘til whole
FeO1.5 doesn’t make sense.
mol O
mol Fe
x 2 = mol O
Fe2O3
x 2 = mol Fe

13. Using Empirical Formulas to Find Molecular Formulas
Steps:
Find the empirical formula
Calculate the formula weight for the empirical formula.
MW = the whole number
FW (empirical formula) ratio between MW
and FW
Multiply the subscripts in the empirical formula by the whole number ratio.

14. Using Empirical Formulas to Find Molecular Formulas
Example: A certain substances has an empirical formula of CH2O. If its molecular weight is amu, what is its molecular formula?
Step 1:
Empirical Formula = given = CH2O

15. Using Empirical Formulas to Find Molecular Formulas
Step 2:
FW = 1 (12.0 amu) + 2 (1.0 amu) + 1 (16.0 amu)
= amu
Step 3:
Whole number ratio = amu = 6
30.0 amu
Step 3:
Molecular formula: C(1x6)H(2x6)O(1x6) = C6H12O6

16. The Mole–Volume Relationship
10.2
The Mole–Volume Relationship
At STP, 1 mol or, 6.02  1023 representative particles, of any gas occupies a volume of 22.4 L.
The quantity 22.4 L is called the molar volume of a gas.
This box, with a volume of 22.4 L, holds one mole of gas at STP.

17. 10.2
The Mole Road Map
The map shows the conversion factors needed to convert among volume, mass, and number of particles. Interpreting Diagrams How many conversion factors are needed to convert from the mass of a gas to the volume of a gas at STP?