1. Ch. 7 Determining the Formula of a Compound
Adv Chemistry
11/20/12

2. Empirical Formula vs. Molecular Formula
The lowest whole-number ratio of the atoms in a compound
The true number of atoms of each element in the formula of a compound
C3H15N3
CH5N
Note:
If the compound is an ionic compound, its empirical formula is usually the same as its normal molecular formula. (Ex: Na2O)
Formulas for molecular compounds might already be in their lowest ratio. Ex: NaCl, MgCl2

3. First! We need to know Percent Composition
The relative amounts of each element in a compound are expressed as the percent composition, which is the percent by mass of each element in the compound.
General Formula:
Mass of element in 1 mole of compound x 100%
Molar Mass of Compound

4. How to Find the Percent Composition
Step 1: Find the molar mass of the compound
Step 2: Divide the mass of each element by the molar mass of the compound and multiply by 100 to get the percentage

5. Example 1 What is the percent composition of each element in Ca(NO3)2?
Step 1: Find molar mass
Step 2: Plug and chug into general formula
Check: The percentages should add up to %

6. Example 2
Calculate the percent composition of a compound that is made of 29.0 grams of Ag with 4.30 grams of S.
Reminder: SIG FIGS 

7. Example 3
How many grams of Silver can be recovered from a 500 grams sample of AgNO3?

8. Determining Empirical Formulas
Step 1: Assume you have a 100 g sample, so now the percentages become gram amounts.
Step 2: Convert grams to moles by dividing by the molar mass
Step 3: Divide each mole by the lowest number of moles to get the mole ratios
Step 4: Write the formula according to step 3. The ratios represent subscripts for each element in the compound.

9. Example 4
Calculate the empirical formula of a compound composed of 38.67% C, 16.22% H, and 45.11% N.

10. Example 5
Find empirical formula for a compound composed of 69.9% Fe and 30.1% O?

11. Empirical to Molecular Formula
Since the empirical formula is the lowest ratio, the actual molecule would weigh more by a whole number multiple.
Divide the actual molar mass by the empirical formula mass – this gives you the whole number to increase each coefficient in the empirical formula.
True formula molar mass = Whole # ratio
Empirical Formula molar mass

12. Example 6
Caffeine has a molar mass of 194 g and its empirical formula is C4H5N2O. What is its molecular formula?

13. Add these problems to the back of your handout and complete
6) If the empirical formula of a compound is CH2 and its true molecular mass is 70 g/mol, what is its molecular formula? 7) A compound is 54.5% carbon, 9.1% hydrogen, and 36.4% oxygen. Its molecular mass is 88 g/mol. What is its true molecular formula? 8) What is the empirical formula for a compound that contains 25.9% Nitrogen and 74.1% Oxygen?