1. Percent Composition, Empirical Formulas, Molecular Formulas

2. Percent Composition

3. So… Percent Composition Part _______ Percent = x 100% Whole
Percent Composition – the percentage by mass of each element in a compound
Part
_______
Percent =
x 100%
Whole
So…
Percent composition
of a compound or =
molecule
Mass of element in 1 mol
____________________
x 100%
Mass of 1 mol

4. Percent Composition Molar Mass of KMnO4
Example: What is the percent composition of Potassium Permanganate (KMnO4)?
Molar Mass of KMnO4

5. Percent Composition Molar Mass of KMnO4 = % K % Mn % O
Example: What is the percent composition of Potassium Permanganate (KMnO4)?
Molar Mass of KMnO4 =
% K
% Mn
% O

6. Percent Composition
Determine the percentage composition of sodium carbonate (Na2CO3)?
Molar Mass
Percent Composition
% Na =
% C =
% O =

7. Using Percent as a converting factor

8. Finding mass of an element:
Directions:
Step #1: Find Molar mass of Compound
Step #2: Find % of Specific Element.
Step #3: Use % to find mass of element in sample.

9. Percent Composition
Calculate the mass of nitrogen in 85.0 g of the amino acid lysine, C6H14N2O2.

10. Review - Percent Composition
Determine the percent composition of ethanol (C2H5OH)?
_______________________________________________
Determine the mass of carbon in a 198-g sample of sodium oxalate (Na2C2O4)?

11. Empirical and Molecular Formulas
Determining Formulas

12. Formulas
Percent composition allows you to calculate the simplest ratio among the atoms found in compound.
Empirical Formula – formula of a compound that expresses lowest whole number ratio of atoms.
Molecular Formula – actual formula of a compound showing the number of atoms present
Examples:
C4H10
- molecular
C6H12O6
- molecular
C2H5
- empirical
- empirical
CH2O

13. Formulas Is H2O2 an empirical or molecular formula?
Molecular, it can be reduced to HO
HO = empirical formula

14. Calculating Empirical Formula
An oxide of aluminum is formed by the reaction of g of aluminum with g of oxygen. Calculate the empirical formula.
1. Determine the number of grams of each element in the compound.
2. Convert masses to moles.

15. Calculating Empirical Formula
An oxide of aluminum is formed by the reaction of g of aluminum with g of oxygen. Calculate the empirical formula.
3. Find ratio by dividing each element by smallest amount of moles.
4. Multiply by common factor to get whole number. (cannot have fractions of atoms in compounds)

16. Calculating Empirical Formula
A g sample of cobalt reacts with g chlorine to form a binary compound. Determine the empirical formula for this compound.

17. Calculating Empirical Formula
When a g sample of iron metal is heated in air, it reacts with oxygen to achieve a final mass of g. Determine the empirical formula.
Fe = g
O = g – g = g
1 mol Fe
2.000 g Fe
= mol Fe
55.85 g Fe
1 mol O
0.573 g O
= mol Fe
16.00 g
1 : 1
FeO 17

18. Calculating Empirical Formula
A sample of lead arsenate, an insecticide used against the potato beetle, contains g lead, g of hydrogen, g of arsenic, and g of oxygen. Calculate the empirical formula for lead arsenate.
1 mol Pb
g Pb
= mol Pb
207.2 g Pb gH
1 mol H
= mol H
1.008 g H
1 mol As
g As
= mol As
74.92 g As
1 mol O
0.4267g Fe
= mol O
16.00 g O

19. Calculating Empirical Formula
A sample of lead arsenate, an insecticide used against the potato beetle, contains g lead, g of hydrogen, g of arsenic, and g of oxygen. Calculate the empirical formula for lead arsenate.
mol Pb
= mol Pb
mol H
= 1.00 mol H
PbHAsO4
mol As
= mol As
mol O
= mol O

20. Calculating Empirical Formula from % Composition
The most common form of nylon (Nylon-6) is 63.38% carbon, 12.38% nitrogen, 9.80% hydrogen and 14.14% oxygen. Calculate the empirical formula for Nylon-6.

21. Calculating Molecular Formula
A white powder is analyzed and found to have an empirical formula of P2O5. The compound has a molar mass of g. What is the compound’s molecular formula?
Step 3: Multiply
Step 1: Molar Mass
P = 2 x g = 61.94g
O = 5 x 16.00g = g g
(P2O5)2 =
P4O10
Step 2: Divide MM by
Empirical Formula Mass g
= 2
141.94g

22. Calculating Molecular Formula
A compound has an experimental molar mass of 78 g/mol. Its empirical formula is CH. What is its molecular formula?
(CH)6 =
C = g
H = g
13.01 g
C6H6
78 g/mol
= 6
13.01 g/mol