1. Write the formulas that go with the following names:
Warm-Up
Write the formulas that go with the following names:
Tricarbon Pentaoxide
Osmium (III) Sulfide
Magnesium Nitride
Dinitrogen Tetraoxide
Lead (IV) Fluoride
Calcium Bromide
Pentacarbon Hexaphosphide

2. Chemistry: Empirical Formula
Moles
Whitaker
21 October 2013

3. Empirical formula is the simplest chemical formula
Ex. CH4

4. Calculating the empirical formula from the percent composition
Steps
Assume the percent = grams.
Convert each amount of grams to moles.
Divide each number of moles by the smallest number of moles.
If all the numbers are not whole numbers, multiple everything by a number that will get you a whole number. (This may take a few tries!)
The whole numbers become subscripts in the formula.

5. Percent to mass
Mass to mole
Divide by small
Multiply ‘til whole

6. Example
A compound is 32.38% sodium, 22.65% sulfur, and 44.99% oxygen. What is the empirical formula?

7. Example 2
A compound is 40.0% Carbon; 6.71% Hydrogen; % Oxygen. What is its empirical formula?

8. Practice 89.94% C; the rest is H 56.34% P; 43.66% O 43.64% P; 56.36% O
56.34% P; 43.66% O  
43.64% P; 56.36% O  
40.9% C; 4.58% H; 54.5% O  

9. Molecular Formula
Molecular formula is the chemical formula that can be reduced.
Molecular Formula- the formula for a compound in which the subscripts give the actual number of each element in the formulas it truly exists.
Ex. C2H8

10. Molecular Formula Empirical Formula H2O CH3COOH CH2O C6H12O6
Notice two things: 1. The molecular formula and the empirical formula can be identical. 2. You scale up from the empirical formula to the molecular formula by a whole number factor.
Molecular Formula
Empirical Formula
H2O
CH3COOH
CH2O
C6H12O6

11. Now, finding the molecular formula
Find the empirical formula
Calculate the molar mass (formula mass)
Divide the given mass of the substance by the molar mass
Multiply each subscript of the empirical formula by the answer to #3

12. Example
You have grams of a substance with an empirical formula of CH2. What is the molecular formula?