Unit 3: Bonding and Nomenclature
Content Outline: Calculating with Empirical Formulas (3.10)
I.
Empirical Formula
A. This is a type of formula that shows the element symbols combined in a compound, with subscripts
showing the smallest whole-number mole ratio(not the actual number present) of the different
elements.
1. For ionic compounds they are usually the same.
For example, C6H12O6 has an empirical formula of CH2O… hence the term Carbohydrate.
NaCl (ionic) has an empirical formula of NaCl.
II. Calculating Empirical Formula:
A. Step 1: Assume you have a 100.0 gram sample of your compound.
Step 2: Convert your percentages to gram measurements.
For example, 32.38 % Na  32.38 grams of Na
22.65% S  22.65 grams of S
44.99% O  44.99 grams of O
Step 3: Take the gram amount and convert to moles by using the conversion moles over grams
32.38 g of Na x 1mole of Na = 1.408 mol Na
22.99g Na
22.65 g of S x 1 mole of S = 0.7063 mol of S
32.07 g of S
44.99 g of O x 1 mole of O = 2.812 mol of O
16.0 g of O
Step 4: Divide the mole value for each element by the smallest mole value to convert to a ratio.
Na = 1.408/ 0.7063 = 1.993
S = 0.7063/0.7063 = 1.0
O = 2.812/0.7063 = 3.981
Step 5: Round each ratio value up or down to the nearest whole number.
Na = 1.993 rounds to 2; S = 1; O = 3.981 rounds to 4
Step 6: Combine the ratio values into an empirical number (values are subscripts).
Na2SO4
III. Calculating Molecular Formulas from Empirical formulas
A. Remember, the empirical formula is the lowest possible ratio of elements. The molecular formula is
the actual formula for a molecular compound.
1. An empirical formula may or may not be a correct molecular formula.
B. The relationship between an empirical formula and a molecular formula can be expressed as:
X(empirical formula) = molecular formula
X represents a whole number multiplier. (Number need to convert empirical to actual).
For example C6H12O6 is expressed as 6(CH2O)
IV. Calculating Molecular Formula Mass from Empirical Formula Mass
A. The relationship can be expressed as:
X(Empirical Formula Mass) = Molecular Formula Mass
X represents a whole number multiplier
B. Step 1: You must be given a known formula mass of a compound.
Step 2: Divide the given formula mass by the Empirical Formula Mass.
 This gives you the multiplier value.
Step 3: Multiply the empirical formula using the multiplier value to get the molecular Formula
Step 4: Use the Molecular Formula to calculate the Molecular Formula Mass