NAME:_____________________________________ DATE:_______________ PERIOD:_____ #:_____
Empirical and Molecular Formulas Notes
Empirical Formula
An empirical formula consists of the symbols for the elements combined in a compound, with subscripts showing the smallest whole-number mole ratio of the different atoms in the compound.
For ionic compounds the formula unit is ________________ as the empirical formula.
For covalent compounds the molecular formula can be ________________ as the empirical formula or ___________________ of the empirical formula.
Calculating Empirical Formulas
Step 1: Assume you have 100g of the compound and calculate grams of each element.
Step 2: Convert mass of each compound to moles by dividing by the molar mass.
Step 3: Divide each calculated mole value by the smallest calculated mole value.
Step 4: Round calculated numbers to nearest whole number to generate formula.
NOTE: **If final answers are not whole numbers, you must multiply each element by the same
number until you have all whole numbers (or very close).**
Sample Problems 1 + 2
1.
Find the empirical formula for a compound that contains 78.1% boron and 21.9% hydrogen.
Step 1:
Step 2:
Step 3:
Step 4:

Empirical and Molecular Formulas Notes
2.
A sample of a compound is found to contain 4.433g of phosphorus and 5.717g of oxygen. Determine the empirical formula.
Step 1:
Step 2:
Step 3:
Step 4:
Calculating Molecular Formulas
Step 1: Calculate the mass of the compound using the empirical formula.
Step 2: Divide the molar mass by the empirical formula mass.
Step 3: Multiply each subscript in your empirical formula by your answer from Step 2.
Sample Problem 3:
3.
Experimentation shows that the molar mass of the compound in Sample Problem 2 is
283.89 g/mol. What is the compound’s molecular formula?
Step 1:
Step 2:
Step 3:

Empirical and Molecular Formulas Notes
Mixed Practice Problems
Odd numbers have the correct solution for you. SHOW ALL WORK to receive full credit.
1.
Find the empirical formula for a compound that contains 63.52% iron and 36.48% sulfur. FeS
2.
Find the empirical formula for a compound that contains 85.64% C and 14.36% H.
3.
Find the empirical formula of a compound found to contain 26.56% potassium, 35.41% chromium, and the remainder oxygen. K
2
Cr
2
O
7
4.
Find the empirical formula for a compound that is 32.37% Na, 22.65%S, and 44.99% O.
5.
Analysis of a 20.0g sample of a compound that contains only calcium and bromine is found to have 4.00g of calcium. What is the empirical formula of this compound? CaBr
2

Empirical and Molecular Formulas Notes
6.
Find the empirical formula for a sample of a compound that contains 5.211g of carbon, 1.314g of hydrogen and 3.475g of oxygen.
7.
Determine the molecular formula of the compound with an empirical formula of CH and a molar mass of 78.110 g/mol. C
6
H
6
8.
A sample of a compound with a molar mass of 34.00 g/mol is found to consist of 0.44 gH and
6.92 g O. Find it’s molecular formula.
9.
The molar mass of a compound is 92 g/mol. Analysis of a sample of the compound indicates that it contains 0.606 g N and 1.390 g O. Find it’s molecular formula. N
2
O
4
10.
What is the molecular formula of the molecule that has an empirical formula of CH2O and a molar mass of 120.12 g/mol?