Calculations Involving Chemical Quantities
The Percent Composition of Unknown Compounds:
We can calculate the composition of unknown compounds either by laboratory data or
by the chemical formula
Recall percentage composition:
% element = melement/ msample X 100% (note: mass units should be identical, and will
cancel out)
Example 1: a 500 mg tablet of Aspirin C9H8O4 contains 300.00 mg carbon and 8.08 mg
hydrogen. The remaining mass is oxygen. Determine the percentage composition of
Aspirin.
Known:
Unknown:
Solution:
Example 2: researchers have isolated a compound that contains only nitrogen and
oxygen; they found that a 4.60 g sample of this compound contains 1.40 g of nitrogen.
Find the percentage composition of this compound. Find the empirical formula of this
compound ******
Known:
Unknown:
Solution:
Finding Empirical Formula Using Percentage Composition:
Use this chart to help you with decimals.
Decimal/
fraction
Whole number
0.25= 1/4
0.33= 1/3
0.50 = 1/2
0.67= 2/3
0.75= 3/4
4(0.25)=1
2 (0.33)= 1
2 (0.50)= 1
3 (0.67)= 2
4 (0.74) = 3
Multiply
subscripts
by:
4
3
2
3
4
Example 3: Determine the empirical formula of a compound that contains 52.2% carbon,
6.15% hydrogen and 41.7 % oxygen.
Known:
Unknown:
Solution:
Example 4: the percentage composition of ascorbic acid (vitamin C) is 40.9% carbon,
4.55% hydrogen. The remainder is oxygen. Determine the empirical formula of ascorbic
acid.
Known:
Unknown
Solution:
Example 5: In a percentage composition investigation a compound was decomposed
into its elements: 20.0 g of calcium, 6.0 g of carbon and 24.0 g of oxygen. Determine
the empirical formula of this compound.
Known:
Unknown
Solution
Finding composition from chemical formula:
Example 6: Determine the percentage composition of calcium hydroxide Ca(OH)2
Known:
Unknown:
Solution:
Finding Molecular formula from Empirical Formula:
x= Molar mass of compound/ molar mass of empirical formula
Example 7: determine the molecular formula of a compound with empirical formula CH2
and molar mass 84.18 g/mol
Known:
Unknown:
Solution:
Using Percentage Composition to find Molecular Formula of an Unknown Compound:
Example 8: Determine the molecular formula of vitamin C (ascorbic acid). This
compound contains 40.5% carbon, 4.6% H, 54.5% O. Its molar mass is 176.14 g/mol
Known:
Unknown:
Solution: (5 steps)
Example 9. A hydrocarbon containing 85.6% carbon and the remainder hydrogen has a
molar mass of 42.09 g/mol.