Date: ______________
Notes: Quantitative Relationships in Compounds
Text Reference: Chapter 10
Mathematical relationships in chemistry are based upon molar relationships.
Percent Composition
Percent composition is an expression of the percentage by mass of each element present in a
compound.
% composition = mass of element x 100%
molar mass
Steps for Determining Percent Composition from a Given Formula:
Example: Find the percent composition of Cu2S
(1) Determine the molar mass of the compound.
(2) Determine the molar mass of each component of the compound.
(3) Divide the mass of the component by the molar mass of the compound. Multiply by
100% to obtain the percent composition for that component.
Practice 2) Find the percentage composition of a sample that is 28 g Fe and 8.0 g O.
Practice 3) How many grams of copper are in a 38.0-gram sample of Cu2S?
Empirical and Molecular Formulas
- An ____________ ____________ contains the atoms in the compounds in the simplest
whole number ratio. A _______________ ______________ contains the atoms in multiples
of the simplest ratio.
Practice 3) Determine the empirical formula for glucose, C6H12O6 : ______________
Determine the empirical formula for octane, C8H18 : _____________
Steps for Determining Empirical Formulas Using Percent Composition
(1) Assume you have 100 grams of sample. Convert all percentages directly to grams.
(2) Determine the number of moles of each component using molar mass of the
element.
(3) Determine the simplest ratio of moles in the compound by dividing all values by the
smallest mole value.
(4) Write the formula using the calculated whole number molar ratio.
Practice 4) Chemical analysis of 2-propanol, also known as isopropanol or rubbing alcohol,
indicates that it is 60.0% C, 13.4% H, and 26.6% O. Determine its empirical
formula.
- The molecular formula is simply a whole number multiple of the empirical formula.
Steps for Determining Molecular Formulas Given Empirical Formula
(1) Determine the molar mass of the empirical formula.
(2) Determine the ratio of the two molar masses by dividing the molecular mass by the
molar mass of the empirical formula.
(3) Multiply the subscripts of the empirical formula by the calculated ratio.
Practice 5) A compound has the following composition: 76.54% C, 12.13% H, 11.33% O.
If the molar mass of the compound is 282.45 g/mole, what is its molecular
formula?