What is percent
composition?

 Proportion or ratio of each element in a chemical
compound
 Expressed as grams of element per 100 grams of
chemical compound
Ex. 1 % Composition of
MgCO3

 Find formula mass
 Find % of each element in the compound, must all
add up to 100%
 Part
Whole
Ex. 2 Find the amount
(in g) of N in 46.34g of

Ammonium nitrate
Percent Composition
Practice

1. Al2O3 Find the percentages of Al and oxygen in this
compound.
2. Ammonium phosphate. What is the percentage of
hydrogen in this compound? What is the amount (in
grams) of hydrogen?
2 types of chemical
formulas

1) Empirical




Simplest formula for a molecular compound
Lowest whole number ratio of atoms in a compound
Many compounds have the same formula
Ex. CH2O
2) Molecular
 Unique to a specific chemical compound
 Displays the actual # of atoms of each element in a
chemical compound
 “true formula”

If we are given just the %
composition of each element in a
compound—how do we get the
molecular formula ????
Pathway to the
Molecular Formula

1) Find the empirical formula
1) Use BOTH the empirical formula AND molecular
mass to find the molecular formula.
Finding the empirical
formula---
1) Assume a 100g sample of chemical compound.
Convert % of each element into grams.
2) Convert grams
moles for each element
3) Divide each mole amount by the smallest amount.
4) Multiply each number by smallest integer to get
whole numbers.

Ex. 1 Adipic acid has
49.32% C, 43.84% O,
and 6.85% H. Find the
empirical formula.
Ex. 1 The Sequel

 Find the molecular formula.
 Need the empirical formula AND molecular mass.
 Adipic molecular mass = 146 g/mol
Empirical Formula
Practice

1. A compound has an empirical formula of C2OH4 and
a molar mass of 88 g/mol. Find the molecular formula.
2. Rubbing alcohol contains 60.0%C, 13.4% H, and the
remaining mass is from oxygen. What is the empirical
formula for rubbing alcohol?
3. Caffeine is found to contain 49.48% C, 5.19% H,
28.85% N, and 16.48% O by mass. The molecular mass
of caffeine is 194.2 g/mol. Find the empirical and
molecular formula of this compound.
Homework

 For Friday
 Read pp. 83-89, p. 117 #39, 41, 43
 For Thursday
 Read over lab procedure