Percentage Composition

Proportion or ratio of each element in a

chemical compound

Percentage by mass of an element in a particular substance

Expressed as grams of element per 100 grams of chemical compound

Percent composition or Percent by Mass

Find formula mass Find % of each element in the compound,

must all add up to 100% Part Whole

Ex. 1 % Composition of MgCO3

1. Al2O3 Find the percentages of Al and oxygen in this compound.

2. Ammonium phosphate. What is the percentage of hydrogen in this compound? What is the amount (in grams) of hydrogen?

Percent Composition Practice

Empirical Formulas

1) Empirical

Simplest formula for a molecular compound Smallest whole number ratio of elements in a

compound Many compounds have the same formula Ex. CH2O

2) Molecular Unique to a specific chemical compound Displays the actual # of atoms of each element in

a chemical compound “true formula”

2 types of chemical formulas

If we are given just the % composition of each element in a compound—how do we get the molecular formula ????

1) Find the empirical formula

2) Use BOTH the empirical formula AND molecular mass to find the molecular formula.

Pathway to the Molecular Formula

1) Assume a 100g sample of chemical

compound. Convert % of each element into grams.

2) Convert grams moles for each element

3) Divide each mole amount by the smallest amount.

4) Multiply each number by smallest integer to get whole numbers.

Finding the empirical formula----

Ex. 1 Adipic acid has 49.32% C, 43.84% O,

and 6.85% H. Find the empirical

formula.

Find the molecular formula.

Need the empirical formula AND molecular mass.

Adipic molecular mass = 146 g/mol

Ex. 1 The Sequel

1. A compound has an empirical formula of C2OH4 and a molar mass of 88 g/mol. Find the molecular formula.

2. Rubbing alcohol contains 60.0%C, 13.4% H, and the remaining mass is from oxygen. What is the empirical formula for rubbing alcohol?

3. Caffeine is found to contain 49.48% C, 5.19% H, 28.85% N, and 16.48% O by mass. The molecular mass of caffeine is 194.2 g/mol. Find the empirical and molecular formula of this compound.

Empirical Formula Practice

Read pp. 83-89, p. 117 #21, 31, 32, 39, 40, 41

Homework