Percent Composition: The percent by mass on an element in a compoundIs the number of grams of the element in a formuladivided by the total mass of the formula.

Steps to determine Percent Composition1. Find the molar mass of the formula

- Ex. H2O H = 2g O = 16g total = 18g2. Divide each element by the total and multiply by 100% - Ex. 2g = .11 x 100 = 11% 18g

Empirical Formula: The Empirical Formula of a compound shows the smallest whole number ratio of the atoms in the compound

Which of the following are empirical formulas?

Sodium peroxide, NaOTerephthalic acid, C8H6O4

Phenobarbital, C12H12N2O3

Now that you can identify an empirical formula, let uslearn to calculate one.

There are three steps in calculating an empirical formula

Let us try an exampleEx. Calculate the empirical formula of a compound

that contains 67.6% Hg, 10.8% S, 21.6%O

1. Change the percentage to grams and convert the mass to moles for each element in the formula

67.6g Hg x 1 mole Hg = .34 mol Hg 201g Hg

10.8g S x 1 mole S = .34 mol S 32g S

21.6g O x 1 mole O = 1.4 mol O 16g O

2. Find the lowest molar value and divide it into itself and all the other molar values

67.6g Hg x 1 mole Hg = .34/.34 = 1 201g Hg

10.8g S x 1 mole S = .34/.34 = 1 32g S

21.6g O x 1 mole O = 1.4/.34 = 4 16g O

3. The number just calculated becomes the subscript of that atom in the formula.

67.6g Hg x 1 mole Hg = .34/.34 = 1 = Hg1

201g Hg

10.8g S x 1 mole S = .34/.34 = 1 = S1

32g S

21.6g O x 1 mole O = 1.4/.34 = 4 = O4

16g O

Therefore, the empirical formula is HgSO4

Look familiar? Mecuric Sulfate!

So lets review

List the steps for percent composition. Try the sampleproblems on pages 306,307

List the steps for empirical formula calculations. Try thesample problems on page 310

Work on the handouts – if you lost them they are on theweb-page

Now let us learn how to calculate molecular formulas:

-The molecular formula of a compound is either the same as its empirical formula or a aimple whole number multiple of the empirical formula.

Ex. CH empirical, C2H4 molecular

Remember an empirical formula can not be reduced but amolecular formula can be reduced.

Determining Molecular Formulas:

1. Calculate the empirical formula

2. Calculate the molar mass of the empirical formula

3. Divide the mass of the molecular formula by the molar mass of the empirical formula

4. Use that value to multiply with each subscript of the empirical formula.

Remember our example of an empirical formula for HgSO4?

Well the sum of this formula is 297g what would the Molecular formula be if the molecular mass was 594g?

594g/297g = 2 therefore

Hg2(SO4)2 becomes Hg2(SO4)2