1. Calculating
Empirical Formula

2. Empirical Formula
Tells the simplest whole number molar ratio of elements in a compound

3. To Calculate Empirical Formula
Assume 100 g of compound
Convert percent composition of each element to grams
Find the number of moles of each element
Use moles as subscripts for formula
Find the simplest whole number ratio by dividing all subscripts by smallest subscript

4. Sample Problem 7-12
A compound contains 32.38% sodium, 22.65% sulfur, and 44.99% oxygen. What is the empirical formula?
Assume 100 g of the compound and convert each percent to grams:
32.38g Na
22.65 g S
44.99 g O

5. Sample Problem 7-12 Continued
Calculate moles:
32.38 g Na x 1 mole/22.99 g = 1.408
22.65 g S x 1 mole/32.07 g =
44.99 g O x 1 mole/16.00 g = 2.812
Use moles as subscripts
Na1.408S0.7063O2.812
Divide by smallest subscript
Na1.408/0.7063S0.7063/0.7063O2.812/0.7063
Na1.993S1O3.981
Round to get final answer: Na2SO4

6. Sample Problem 7-13
Analysis of a g sample of a compound that contains only phosphorous and oxygen indicates the phosphorous content is g. What is the empirical formula?
Find the mass of oxygen:
g – g = g O
Find the moles:
P: g x 1 mole/30.97 g = mole P
O: g x 1 mole/16.00 g = mole O

7. Sample Problem 7-13 Continued
Use moles as subscripts:
P O
Divide by smallest mole
P0.1431/ O /0.1431
P1O2.497
Multiply to get whole number ratio
P2O5