1. Ch. 7: Chemical Formulas and Compounds
7.4 Determining Chemical Formulas

2. Empirical Formula
formula containing the simplest whole-number ratio of atoms
not necessarily the CORRECT molecular formula
Why?
Ex. BH3 and B2H6
have same empirical formula
have different molecular formulas

3. Calculating Empirical Formulas from Percent Composition
convert percentage to grams by assuming there is 100 g total of sample
convert grams to moles for each element using molar mass
identify the smallest mole value
divide each mole value by that smallest value

4. Example 1
Quantitative analysis shows that a compound contains 32.38% Na, 22.65% S, and 44.99% O.
Find the empirical formula.
Assuming you have 100 g of sample total
32.38 g Na
22.65 g S
44.99 g O

5. Example 1 Na2SO4 sodium sulfate Convert each of those to moles
Divide by the smallest
/ ≈ 2
/ ≈ 1
/ ≈ 4
Na2SO4
sodium sulfate

6. Calculating Empirical Formula from mass composition
don’t have to assume you have 100 g since you have an actual amount
follow all other steps the same way

7. Example 2
Analysis of a g sample of a compound containing only P and O, is known to contain g of P. Find the empirical formula.
Find the mass of all components
mtotal = mP + mO so mO = mtotal – mP
mass of O : = 5.717g

8. Example 2 P2O5 diphosphorus pentoxide
Convert all mass values to moles using molar mass
Divide by the smallest mole value
To get rid of the decimal, multiply by integer
/ ≈ 1
/ ≈ 2.5
P2O5
diphosphorus pentoxide

9. Molecular Formulas
the molecular formula is the actual formula for a compound
could be same as the empirical formula but doesn’t have to be same
to find molecular formula:
need empirical formula
need actual molar mass (or formula mass)
compare the molar mass of empirical formula to actual molar mass

10. Example 3
In the last example the empirical formula was found to be P2O5. Experimentation shows that the molar mass is actually g/mol. Find the molecular formula.

11. Example 3 Find molar mass of empirical formula
2( ) + 5( ) =
Divide the actual molar mass by the empirical formula’s molar mass
/ ≈ 2
Multiply this number by each subscript in empirical formula
P4O10