Empirical Formulas
Formulas
•
___________________________________________ - a compound contains elements in a
certain fixed proportions (ratios), regardless of how the compound is prepared or
where it is found in nature.
•
If you have one molecule of methane gas, you will always have 1 carbon atoms and
4 hydrogen atoms.
3 Steps to Determining Chemical Formulas
1. Determine the percent composition of all elements.
2. _____________________________________________________________________________
3. Find the true number of atoms/ elements in the compound (Molecular Formula)
Step 2. Empirical Formula
is the formula that gives the _____________________________________. It does not necessarily tell
you the exact number of each type of atom.
Steps to Determine Empirical formula:
1.
2.
3.
4.
5.
List given values.
Write the mass percent as mass (mass percent is just mass per 100g sample).
Convert each mass to moles.
State the mole ratios.
Calculate lowest whole number ratio.
Problems
1. The percent composition of a compound is 69.9 % iron and 30.1% oxygen. What is the empirical formula of a
compound?
CHEMICAL QUANTITIES
CHEMISTRY 20
Empirical Formulas
2. The percentage composition of a compound is 21.6% sodium, 33.3% chlorine, and 45.1% oxygen. What is the
empirical formula of the compound?
Empirical Formula Assignment
1. Determine the empirical formula for each compound below.
(a)
64.5% silver; 8.2% nitrogen; 28.2% oxygen [AgNO3]
(b)
14.3% nitrogen; 4.1% hydrogen; 81.6% bromine [NH3Br]
(c)
24.7% potassium; 34.7% manganese; 40.5% oxygen [KMnO4]
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CHEMISTRY 20
Empirical Formulas
(d)
56.6% potassium; 8.68% carbon; 34.7% oxygen [K2CO3]
(e)
9.92% carbon; 58.7% chlorine; 31.4% fluorine [CCl2F2]
(f)
32.4% sodium, 22.5% sulfur and 45.1% oxygen [Na2SO4]
(g)
25.3% copper, 12.9% sulfur, 25.7% oxygen and 36.1% water [CuSO4  5H20]
CHEMICAL QUANTITIES
CHEMISTRY 20
Empirical Formulas
2.Analysis of a sample of sulfuric acid shows it to contain 0.17g of hydrogen, 2.82g of sulfur,
and 5.67g of oxygen. What is the empirical formula for this compound? [H2SO4]
3.Analysis of a salt results in the following composition: 3.47g of sodium; 2.12g of nitrogen; and
7.27g of oxygen. What is the empirical formula for this salt? [NaNO3]
4.An ore containing zinc, carbon, and oxygen and weighing 485.25g is analyzed and found
to contain 46.59g of carbon and 186.37g of oxygen. What is the empirical formula for this
compound? [ZnCO3]
CHEMICAL QUANTITIES
CHEMISTRY 20