1. Percent Composition and Empirical Formula
Third week

2. Percent Composition % is based on the mass of an item
That is Grams to the rest of you!
The equation is:
Mass of item X 100
Total Mass
Molar mass may be used to calculate % composition.

4. Answer 2 Cl = 2 (35.5) = 71 amu 2 F = 2 (19) = 38 amu
1 C = 1 (12) = amu
Molar mass = g/mol
% comp or % mass of Cl in compound =
(71/121)x100=58.68

5. What is the percent mass of oxygen in Ascorbic Acid?

6. Answer: 6 Carbons = 72 amu 8 Hydrogens = 8 amu 6 Oxygens = 96 amu
Molar mass = 176 g/mol
% composition =
(96/176) X100 = 54.55

7. Empirical Formula
Empirical Formula is the ratio of moles of the elements that are contained in a substance (ionic or molecular).
OR is the lowest whole number ratio of a chemical formula.
Remember:
Chemical Formula gives the actual ratio of elements in a chemical.

8. Using % Composition to calculate Empirical Formula
First- find the % composition of each element.
Second- assume a sample of 100 grams
Third- find the grams of each element
Fourth- convert grams to moles
Fifth- Calculate mole ratio. Divide all moles by the smallest number of mole to make the smallest equal to one. Round all to the closest whole number.

11. Determining the molecular formula from the Empirical Formula
You can determine the molecular formula if you know the empirical formula and the molar mass.
Find the empirical molar mass.
Divide it by the molar mass.
Multiply the subscripts of the empirical formula by the quotient.
Worksheet