Percent Composition

Percent Composition by Mass in a Compound

Aim: How can we determine the percentage composition by

mass of elements in a compound?

Essential Questions

1. What does “percent composition” mean?

2. How do we calculate the percent due to each element from the formula mass?

3. How do we calculate the percentage of water in a hydrate?

Key Words: percentage, formula mass, component, element, compound, hydrate, anhydrous.

Do NowFind the formula mass of mercueric (II)

chlorine

How Can We Calculate the Percent Composition of Each

Element Compared to the Formula Mass?

Define Percent Composition:

Calculate Percent Composition:

Percent CompositionDefinition: The percent composition of a substance represents the composition as a percentage of each element compared with the total mass of the compound.

Formula:

What are Hydrates?Ionic compounds often contain a fixed amount of water in their crystal structure. These molecules are indicated as part of the formula. Ex: CuSO4 5H2O.

Crystals which contain attached water molecules are called hydrates.

Substances without water are known as anahydrous.

What is the Percent by Mass of Water in Na2CO3

10H2O?

Calculating the Percentage of Water in a Hydrate

When calculating the percentage of water in a hydrate, treat the entire water molecule as a single unit/component.

Step 1: Calculate the formula mass of the crystal.

Step 2: Calculate the percent of water in the compound.

Learning Check1. What is the percent by mass of nitrogen in NH4NO3?

Learning Check 2. What is the percent by mass of water in CaSO4

2H2O?

Homework1. Redo HW #3, exercises 1-5 using concepts

learned in classroom today.

2. Calculate the percent by mass of water in

Na2B4O 10H2O

(Notes)II. Percent Composition

A. Anhydrous Substances - Substances without water trapped inside are known as anahydrous.

1. Definition: The percent composition of a substance represents the composition as a percentage of each element compared with the total mass of the compound.

2. Formula:

Example

NotesB. Hydrous Substances – Crystals which contain

attached water molecules are called hydrates.

1. Ionic compounds often contain a fixed amount of water in their crystal structure. These molecules are indicated as part of the formula. Ex: CuSO4 5H2O.

2. When calculating the percentage of water in a hydrate, treat the entire water molecule as a single unit/component.

a. Step 1: Calculate the formula mass of the crystal.

b. Step 2: Calculate the percent of water in the compound.

Example