1. Empirical Formula and Molecular Formula

2. For Ionic cmpds we can predict formulas based on common ion charges
Sodium + Oxygen together
Na+ O2-  Na2O
For molecular cmpds, charges are not involved, and this makes it harder to predict. If you know the % comp it can be done!!

3. Empirical Formula
Smallest whole number ratio of elements in a cmpd. (not necessarily the real formula!!)

4. Real Formula E.F. H2O H2O C6H12O6 CH2O
Can be same!

5. Steps Change % to grams Change grams to mole
Divide all by smallest number of moles
Use answers from step 3 as subscripts

6. What is the empirical formula for a compound that is 79. 9% C and 20
What is the empirical formula for a compound that is 79.9% C and 20.1% H
1) 79.9%  79.9g +20.1%  +20.1g 100% 100.0g 2) 3) 79.9g x 1 mol = 6.66mol C  6.66 = 1 12gC 20.1 g x 1 mol = 20.1 mol H 6.66= 3 1gH
4) CH3

7. Find the empirical formula for a cmpd that is 54.5gC, 13.6gH, 31.8gN

8. Find the empirical formula for a cmpd that is 54.5gC, 13.6gH, 31.8gN
54.5gC x 1molC = 4.54molC
12g
13.6gH x 1 mol H = 13.6molH 1g
31.8gN x 1mol N = 2.27mol N
14g
2.27 =2
2.27 =6
2.27 =1
C2H6N

9. Molecular formula
This formula has the actual number of atoms in the cmpd. (the EF could be the same or have less atoms)

10. Steps Find molar mass of the E. F. Divide M.F.’s molar mass
E. F. molar mass
3) If greater than 1, multiply all the subscripts in the E.F. by your answer in # 2
Given in problem

11. Ex.
What is the M.F. if: NO2 = E.F Molar mass is 92g/mol

12. M.F.’s molar mass E. F. molar mass
92 (MF) (14+2(16)= 46 (EF) =2 NO2 becomes N2O4