Guided Notes: Math in Chemistry
Percent Composition, Empirical and Molecular Formulas
Percent Composition

Used to _____________________ out chemical formulas

The __________________________ by mass of each element in a compound
Two types of Problems
1. Masses are Given
2. No masses are Given
Masses are Given
Steps to Solve Problem:
1. Add given ___________________ to get total mass for the compound
2. ______________ individual mass by the total mass
3. Multiply by 100 to get the ___________________
Example: Masses Given
Find the percent composition of a sample that is 30g Mg and 8.0g O.
No Masses Given
Steps to Solve Problem:
1. Assume you have ____ ______________ of the compound
2. Calculate the __________________ _____________ of each element in the compound by
multiplying the subscript by the molar mass of the element
3. Divide the molar mass for the element by the ________________ molar mass of the compound
4. Multiply by 100 to get the percent
Example: No Masses Given
Calculate the percent composition of oxygen in water
Propane (C3H8), the fuel commonly used in gas grills, is one of the compounds obtained from
petroleum. Calculate the percent composition of propane.
Empirical Formula
Smallest whole number _____________ of the atoms of the elements in a compound
Steps to solve problem
1. Find ______________ (or %) of each element.
2. Find ___________________ of each element (divide given mass by molar mass)
3. ___________________ answers by the smallest # to find subscripts
4. When necessary multiply _________________________ by 2,3, or 4 to get whole #’s.
Example:
A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the
empirical formula of the compound?
Step 1: percent to mass (assume 100.0g)
Step 2: mass to mole
Step 3: divide by smallest
Step 4: multiply until whole
1,6-diaminohexane is used to make nylon. What is the empirical formula of this compound if it is 62%
C, 13.8% H, and 24.1% N?
Molecular Formula
The “true formula” or actual number of atoms in a compound
Can be either the same as its experimentally determined empirical formula, or it is a simple whole
number multiple of its empirical formula
Steps to solving problems
1. Find the empirical formula
2. Find the empirical formula mass
3. Divide the molecular mass by the empirical mass
4. Multiply each subscript by the answer from step 3
Example:
Calculate the molecular formula of a compound whose molar mass is 60.0g/mol and empirical formula
is CH4N.
Step 1: Find empirical formula
Step 2: find empirical formula mass
Step 3: divide molecular mass by empirical mass
Step 4: multiply each subscript by the answer from #3
The compound methyl butanoate smells like apples. Its percent composition is 58.8% C, 9.8% H, and
31.4% O and its molar mass is 102 g/mol. What is its empirical formula? What is its molecular
formula?
You find that 7.36g of a compound has decomposed to give 6.93g of oxygen. The only other element
in the compound is hydrogen. If the molar mass of the compound is 34.0 g/mol, what is its molecular
formula?