# USING MOLES AND MOLAR MASS TO FIND PERCENT COMPOSITION

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```					USING MOLES AND MOLAR MASS
TO FIND
PERCENT COMPOSITION,
EMPIRICAL FORMULAS
AND
MOLECULAR FORMULAS
% COMPOSITION
% Composition =

Mass of element
Mass of Compound   X 100
If given % composition, you can find the empirical formula by:

Step 1: Assume you have 100g.

This way, your percentages will be the number of
grams. Example: 40% of a compound is carbon. 40% of 100 g is
40 grams.

Step 2: Change grams to moles.

Since empirical formulas are mole comparisons, we need to put our numbers in
moles first.

Step 3: Compare the number of moles of each to the smallest number of moles by
dividing by the smallest number.
Step 4: Round your ratio to the nearest
whole number as long as it is “close.”

For example, 1.99987 can be rounded to
2.

Step 5: Write the formula using your ratio
numbers as subscripts.
To find the molecular formula,

you must be given the actual molar mass
of the compound.

Find the ratio of the actual mass to the
empirical mass.
For example, if it is one, the empirical formula is
equal to the molecular formula. If it is 2, the
subscripts must be multiplied by two.
DEFINITIONS:
The empirical formula is the simplest formula for
a compound.
A molecular formula is the same as or a multiple
of the empirical formula, and is based on the
actual number of atoms of each type in the
compound.
For example, if the empirical formula of a
compound is C3H8 , its molecular formula may be
C3H8 , C6H16
AN EXAMPLE:

Let's determine the empirical formula for a
compound with the following elemental
composition:

40.00% C, 6.72% H, 53.29% O.
STEP 1:

The first step will be to assume exactly
100 g of this substance.

This means in 100 g of this compound,
40.00 g will be due to carbon, 6.72 g will
be due to hydrogen, and 53.29 g will be
due to oxygen.
STEP 2: CHANGE GRAMS TO MOLES
STEP 3: COMPARE MOLAR RATIOS AND
DIVIDE BY THE SMALLEST NUMBER
The ratio of C:H:O has been found
to be 1:2:1, thus the empirical
formula is: CH2O
Suppose we know that the
molecular weight of this
compound is 180 g/mol. With this
information, the molecular formula
may be determined
The formula weight of the empirical
formula is 30 g/mol. Divide the
molecular weight by the empirical
formula weight to find a multiple:
The molecular formula is a multiple
of 6 times the empirical formula:
C(1 x 6) H(2 x 6) O(1 x 6)

which becomes

C6H12O6
Example Problem #1

A compound was analyzed and found
to contain 13.5 g Ca, 10.8 g O, and
0.675 g H. What is the empirical
formula of the compound?
each element, given in the
problem.
Convert the mass of each element to
moles using the molar mass from the
periodic table
Divide each mole value by the smallest
number of moles calculated. Round to
the nearest whole number.
This is the mole ratio of the
elements and is represented by
subscripts in the empirical formula
Example Problem #2
NutraSweet is 57.14% C, 6.16% H,
9.52% N, and 27.18% O.

Calculate the empirical formula of
NutraSweet and find the molecular
formula. (The molar mass of
NutraSweet is 294.30 g/mol)
each element, given in the problem.
Convert the mass of each element to
moles using the molar mass from the
periodic table.
Divide each mole value by the smallest
number of moles calculated. Round to
the nearest whole number.
This is the mole ratio of the elements and is
represented by subscripts in the empirical formula.
If the number is too far to round (x.1 ~ x.9), then
multiply each solution by the same
factor to get the lowest whole number multiple
Now, we can find the molecular
formula by finding the mass of the
empirical formula and setting up a
ratio:

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