Empirical and Molecular Formulas Notesheet
1) Empirical formulas are like reduced fractions.
2) 10/12 can be reduced to ______. Mg2O2 can be reduced to _______. C6H12O6 can be reduced to ________.
3) Write your own one-sentence definition for each of the following:
A.
Empirical formula
B.
Molecular formula
4) Learning Check: A. What is the empirical formula for C4H8?
B. What is the empirical formula for C8H14?
C. Which are possible molecular formulas for CH2O?
1) C2H4
1) C4H7
1) CH2O
2) CH2
2) C6H12
2) C2H4O2
3) CH
3) C8H14
3) C3H6O3
5) We can use the percent composition to find the ______________________ ______________________.
6) The empirical formula may or may not be the same as the actual ___________________________ formula.
7) Example: glucose
Molecular formula = _______________ Empirical formula = _________________
What is their common element ratio? _________________________________________
What is the scaling factor? ______
8) Finding Empirical Formula from Percent Composition
a. You need to assume a few things:
 The total mass of the compound is 100.0g.
 The percent composition of the element is equal to the mass in grams of the element.
9) Example 1: Determine the empirical formula of an oxide of sulfur that has a percent composition of 40.05% S and 59.95% O.
a. In 100 g of the compound, ________ g are S and ________ g are O
b. Next, find the amount of mol for each element.
1 𝑚𝑜𝑙 𝑆
1 𝑚𝑜𝑙 𝑂
40.05 𝑔 𝑆
c.
d.
e.
f.
__________𝑔 𝑆
=
59.95 𝑔 𝑂
________𝑔 𝑂
=
Divide all numbers by the smallest number.
So, S has a subscript of one:
 1.249 mol S / 1.249 = _____ mol S
Then, divide the mol O by the same number to find its subscript in the empirical formula.
 3.747 mol O/ 1.249 = _____ mol O
Then write your empirical formula using your subscripts: ________
10) Example 2: A blue solid has 36.84% nitrogen and 63.16% O. What is its empirical formula?
11) Molecular formula = __________________ formula x _______________ factor
12) To find the scaling factor:
1) Determine the _______________ mass (total mass of all elements in empirical formula)
2) Divide ________________ mass by empirical mass
13) Example 3: Chemical analysis of succinic acid indicates it is composed of 40.68% C, 5.08% H, and 54.24 % O, and has a molar mass of
118.1 g/mol. Determine the empirical and molecular formulas for succinic acid.
1) Convert the percent for each element into moles
2) Next, divide each mol amount by the smallest mol amount.
3) Write Empirical Formula: _________________
4) We need to find the empirical mass using the masses of each element.
Empirical Mass = _________________.
5) Now, divide the molar mass by the empirical mass to determine the scaling factor.
6) Multiply subscripts by scaling factor and write the Molecular Formula: ___________________
Empirical and Molecular Formulas Practice Problems
STEPS TO FOLLOW:
 Change the percent grams to moles
 Find the mole rations by dividing by the smallest number
 If whole number ratios are not obtained, try multiplying by 2 or 3
 Put the whole number, associated with the element, as a subscript in the chemical formula
Or try the Rhyme!!
% to mass; Mass to mol; Divide by small; Multiply ‘till whole
Find the empirical formulas based on the given percent compositions. Use separate paper!
1.
75% carbon; 25 % Hydrogen
2.
52.7% Potassium;
47.3% Chlorine
3.
22.1% Aluminum;
25.4% Phosphorous;
52.5% Oxygen
Determine the Empirical Formula for the following: You need separate paper!
4.
Propane is a hydrocarbon, a compound composed only of carbon and hydrogen. It is 81.82% carbon
and 18.18% hydrogen. What is the empirical formula?
5.
What is the empirical formula for a compound that contains 10.89% magnesium, 31.77% chlorine, and
57.34% oxygen?
6.
Fat makes up a major portion of all soaps. A fat used in many soaps is 76.5% carbon, 12.2% hydrogen,
and 11.3% oxygen. What is its empirical formula?
7.
Strychnine, a deadly poison, has a molecular mass of 334 amu and a percentage composition of 75.42%
carbon, 6.63% hydrogen, 8.38% nitrogen, and 9.57% oxygen. What is the emirical formula of
strychnine?
Determine the molecular formula for the following:
8.
A compound was found to be composed of 49.98 g carbon, and 10.47 g hydrogen. The molar mass of
the compound is 58.12 g/mol. Determine the molecular formula.
9.
A compound is 27.4% S, 12.0% N and 60.6 % Cl. If the compound has a molar mass of 351 g/mol,
what is the molecular formula?
10.
Determine the molecular formula for ibuprofen, a common headache remedy. Analysis of ibuprofen
yields a molar mass of 206 g/mol and a percent composition of 75.7% C, 8.80% H, and 15.5% O.
11.
Aspirin is 60.0% Carbon, 4.5 % Hydrogen, and 35.5% Oxygen. Calculate its empirical and molecular
formulas. The molar mass of aspirin is 180.15742 g/mol.
Write the molecular formulas of the following compounds:
12.
A compound with an empirical formula of C2OH4 and a molar mass of 88 grams per mole.
13.
A compound with an empirical formula of C4H4O and a molar mass of 136 grams per mole.
14.
A compound with an empirical formula of CFBrO and a molar mass of 254.7 grams per mole.
15.
A compound with an empirical formula of C2H8N and a molar mass of 46 grams per mole.