Percent Composition
__________________________ is the percentage by mass of each element in a
compound.
How to find percent composition:
1.
2.
3.
4.
5.
Write a correct ____________ for the compound.
Find the _________________ of the compound. (calculated from the periodic table)
Divide the total atomic mass of the ________________ by the molar mass of the compound.
Multiply by __________ to convert your results to a percent.
Express your answer to __________decimal places and don’t forget the % sign.
Formula:
Molar mass of element in sample of compound x 100 =percent composition of element in compound
Molar mass of sample of compound
Example 1: Find the percent composition of oxygen in potassium oxide.
Example 2: Find the percent composition of copper (I) sulfate.
Better source of copper or sulfer?
Empirical Formula (simplest formula)- The empirical formula of a compound is a
formula that shows the ratio of elements present in the compound. The ratios are
denoted by subscripts next to the element symbols.
Example:
Glucose has a molecular formula of C6H12O6.
It contains 2 moles of hydrogen for every mole of carbon and oxygen.
The empirical formula for glucose is CH2O.
Steps for Determining an Empirical Formula
1. Start with the number of grams of each element, given in the problem.
o If percentages are given, assume that the total mass is 100 grams so that
the mass of each element = the percent given.
2. Convert the mass of each element to moles using the molar mass from the
periodic table.
3. Divide each mole value by the smallest number of moles calculated.
4. 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 #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?
Start with the number of grams of 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.
Caffeine has the following percent composition: carbon 49.48%, hydrogen 5.19%,
oxygen 16.48% and nitrogen 28.85%. Its molecular weight is 194.19 g/mol. What is its
molecular formula? What is the empirical formula? (By the way, this is repeat of
example #3, with the addition of the last question.)
Now for the solution using the new technique:
Step One - multiply the molecular weight by the percent composition:
carbon:
194.19
hydrogen: 194.19
oxygen: 194.19
nitrogen: 194.19
x
x
x
x
0.4948
0.0519
0.1648
0.2885
=
=
=
=
96.0852
10.07846
32.0025
56.0238
Step Two - divide each answer by the atomic weight:
carbon:
96.0852 ÷ 12.011 = 7.9997
hydrogen: 10.07846 ÷ 1.008 = 9.998
oxygen: 32.0025 ÷ 15.9994 = 2.000
nitrogen: 56.0238 ÷ 14.0067 = 3.9997
Step Three - round off to closest whole number ratio
carbon:
8
hydrogen: 10
oxygen: 2
nitrogen: 4
The molecular formula is C8H10N4O2.
Step Four - remove common factor to get the empirical formula.
The common factor between 8, 10, 4 and 2 is 2. The empirical formula is C4H5N2O.
Try it yourself:
What are the empirical and molecular formulas for a compound with 86.88% carbon and
13.12% hydrogen and a molecular weight of about 345?