Lesson Objectives• Calculate percent composition from a

chemical formula• Determine the empirical and molecular

formulas for a substance

Percent Composition, Empirical, and Molecular Formulas

• Percent composition describes the contribution of each element to the total mass of a compound– Calculations done for one mole of a substance– Ratio of one part of the compound to the whole

compound

Percent Composition

• To determine the percent composition, you need to know– Formula of the

compound– Molar mass of each

element in the compound

– Molar mass of the compound

Ex) Find the percent

composition of CaCl2

Calculating Percent Composition

• Empirical formula – formula that shows the simplest whole number ratio of elements in a compound

• Molecular formula – formula of one molecule or formula unit of a compound as it actually exists– Can be a multiple of the empirical formula

Ex) N2O4 is a molecular formula with an empirical formula of NO2

– Sometimes the empirical and molecular formulas are the sameEx) NH3 is the molecular formula and the empirical formula of ammonia

Empirical and Molecular Formulas

N

H HH

• Reduce a molecular formula to an empirical formula by dividing the subscripts by a factor that produces the lowest whole number ratio of subscripts

Ex) C6H12

Empirical formula is CH2

Ex) H2SO4

Empirical formula is the same as the

molecular, H2SO4

Empirical Formula from Molecular Formula

• Molecular formula can be a multiple of the empirical formula1. Determine the

molar mass of the empirical formula

2. Divide the molar mass of the molecular formula by the molar mass of the empirical formula to find the factor

3. Multiply the empirical formula by the factor

Ex) What is the molecular formula of a

compound with an empirical formula of

CH2O and molar mass 180.18 g/mol?

Molecular Formula from Empirical Formula and Molar Mass

• Percent composition gives a ratio of the masses of the elements in a compound1. Convert masses into

moles• Assume a 100 g sample

2. Divide each mole value by the smallest mole value• Whole number products

are the subscripts for the empirical formula (see rounding rules)

Ex) A compound is 82.8% carbon and 17.2%

hydrogen. The molar mass is 58.14 g/mol. Find

the empirical and molecular formulas.

Empirical and Molecular Formulas from Percent Composition and Molar Mass

• If a number ends with a decimal of .01 or .99 then round to the nearest whole number

• Sometimes the decimals will not be close enough to a whole number to round– Create whole

numbers by multiplying all the mole values by the smallest possible factor

• Look for these decimals during calculations– Ends in .5

• Equivalent of ½ – Multiply by 2

– Ends in .33 or .66• Equivalents of or

– Multiply by 3

– Ends in .25 or .75• Equivalents of ¼ or

¾ – Multiply by 4

Rounding in Empirical Formula Calculations

Back to Problem

• Percent composition gives a ratio of the masses of the elements in a compound1. Convert masses into

moles• Assume a 100 g sample

2. Divide each mole value by smallest mole value• Whole number products

are the subscripts for the empirical formula

3. Divide the molar mass of the molecular formula by the molar mass of the empirical formula to find the factor

4. Multiply the empirical formula by the factor

Ex) A compound is 82.8% carbon and 17.2%

hydrogen. The molar mass is 58.14 g/mol. Find

the empirical and molecular formulas.

Empirical and Molecular Formulas from Percent Composition and Molar Mass

Analysis of a chemical used in photographic developing fluid indicates a chemical composition of 65.45% C, 5.45% H, and 29.09% O. The molar mass is found to be 110.0 g/mol. Determine the molecular formula.

Practice Problem