Notes: Chemical Composition & Formulas
Percent Composition
Mass percent = weight percent
 The proportion of a given atom in a compound, expressed as a %
 Based on atomic masses
Mass fraction
for a given
=
element
Mass of the element present in 1 mole of compound
Mass of 1 mol of compound
 The mass fraction is converted to a mass percent by multiplying
by 100%
Example:
The formula for ethanol is C2H5OH. Calculate the mass percent of each element
in this compound.
 Step 1: Determine the molar mass
 Step 2: Determine the mass fraction & mass percent for each
 Step 3: Check your work by making sure the % of each atom add up to
approximately 100% (It may not be exact because of rounding)
Formulas of Compounds: Empirical Formulas
Definition = the formula of a compound that expresses the smallest
whole-number ratio of atoms present
If two solutions are mixed and a solid product is formed, how can you
figure out what the solid is?
 Write the chemical reaction equation
 Compare the properties of the solid to known substances
 To obtain the formula of an unknown compound, we use the
measured masses of the elements present to determine the
formula.
* Remember that a formula tells us the relative numbers of atoms
Example:
A mystery compound contains only the elements carbon, hydrogen, and oxygen and its
mass is 0.2015 g. The masses of each element are found to be 0.0806-g of carbon,
0.01353-g of hydrogen, and 0.1074 g of oxygen.
Step 1: Convert to moles
Step 2: Divide by the smallest # of moles of atoms to determine the formula ratio.
What do these numbers tell us?
 The compound contains the same number of C and O atoms
 There are twice as many H atoms as C or O atoms.
The substance can be represented by the empirical formula CH2O
Steps for determining Empirical Formulas:
Step 1: Obtain the mass of each element in grams.
Step 2: Determine the number of moles of each atom.
Step 3: Divide by the smallest number of moles to determine the formula ratio.
If any number in the ratio is not a whole number, complete step 4.
Step 4: If any numbers are not whole numbers, multiply by the smallest integer
that will convert all of them to whole numbers.
Sample Problems:
1. Determine the empirical formula for a compound composed of 71.65% Cl,
24.27% C, and 4.07% H.
2. A sample of Nickel with a mass of 0.2636 g is heated and reacts with oxygen
in the air to produce 0.3354 g of Nickel oxide.
 To find the mass of oxygen that reacted with Nickel, subtract the
original mass of Nickel from the total product mass of nickel oxide.
Molecular Formulas
Definition: The formula of a compound that indicates the actual number of
atoms of each element present.
To determine a compound’s molecular formula, we must know the empirical
formula and its molar mass.
Example:
A white powder is analyzed and found to have an empirical formula of P2O5. The
compound has a molar mass of 283.88 g. What is the compound’s molecular
formula?
1) Determine the empirical formula mass
2 mol of P = 2(30.97) = 61.94 g
5 mol of O = 5(16.00) = 80.00 g
Mass of 1 mol P2O5 = 141.94 g
2) To determine the molecular formula, divide the molar mass by the empirical
formula mass.
The molecular formula is: (empirical formula)n or (n x empirical formula), SO…
Sample Problems:
1. A compound with the empirical formula of C2H5O was found to have a molar
mass of approximately 90 g. What is the molecular formula for the
compound?
2. Determine the molecular formula of a compound that contains 26.7% P,
12.1% N, and 61.2% Cl and has a molar mass of 580 g/mol.
Molecular Formulas #2
You CAN find the molecular formula using the percent composition and molar
mass of a compound.
Steps:
1) Using the mass % and molar mass, determine the mass of each element in
1 mole of the compound
2) Determine the number of moles of each element present in 1 mole of the
compound
3) Round numbers to whole integers. These numbers represent the
subscripts of the molecular formula.
Example:
Caffeine contains 49.48% Carbon, 5.15% Hydrogen, 28.87% Nitrogen, and
16.49% Oxygen by mass and has a molar mass of 194.2 g/mol. Determine the
molecular formula for caffeine.