Percentage Composition

Lesson 28

Warm up

Submit Mole Conversion packages in basket, and copy the following upon arrival

Write-Pair-Share• What is the percent of the total composition of

each ingredient in the following recipe (masses are fictitious):– 2 bananas (100 g)– 1 cup of flour (100 g)– 1 cup of sugar (150 g)

Results?

Reflection Question:

Why may results have varied during your warm up?

Learning Objectives

1. I know the meaning of “Percent Composition”

2. I can calculate the percent composition of a particular chemical

3. I can determine empirical formulas using percent composition

Percent Composition

Let’s look at some examples and create our own definition

Percent Composition

• This class is: _____% Male and ______% Female

Write-Pair-Share

Definition and calculation for percent composition…

Determining the Percent Composition by Mass of Chemicals

Example: Determine the percent composition by mass of each atom in the chemical, CO2

How could we determine the percent composition if we do not know the mass of an individual Carbon and Oxygen atom?

Determining the Percent Composition by Mass of Chemicals

ASSUME THAT WE ARE DEALING WITH 1 MOLE OF THE SUBSTANCE!

Step-by-Step

Step 1: Find the molar mass of the individual atoms.

Step-by-Step

Step 2: Find the number of grams of each component, multiplying the molar mass by number of moles of each atom

Step-by-Step

Step 3: Find the total mass of the compound

Step-by-Step

Step 4: Divide the mass of each component (step 2) by the total mass of the compound (step 3), and multiply by 100

You Practice!

Complete page 9 of Mole Conversion workbook

Determining Formula using Percent Composition

Refresher…

Empirical Formula: the simplest whole number ratio of one element to another in a compound

Example: Octane is C8H18. But that is not the simplest ratio. The simplest ratio is C4H9, so this is its empirical formula

% Composition Formula

Example problem: What is the empirical formula of a compound that is 79.8% carbon and 20.2% H?

If we assume that we have 100.g of compound, the percentages convert to grams.

Step 1: Convert % to grams

79.8% C becomes 79.8g C and 20.2% H becomes 20.2g H.

Step 2: Convert grams to moles

Step 3: Convert Ratio to Whole Number

Since all compounds have simple whole number ratios of one element to another, we must convert the ratio 6.64:20 to a whole number one. The easiest way is to divide through by the smallest number, so… [show calc]

Step 4: Write the Empirical Formula

The ratio is 1:3, resulting in an empirical formula of CH3. You may encounter examples where the ratio works out to something like 1:2.5 or 1:3.5. If that is the case, the ratio actually becomes 2:5 or 2:7.

You Practice!

Complete Page 12 to practice

Learning Objectives

1. I know the meaning of “Percent Composition”

2. I can calculate the percent composition of a particular chemical

3. I can determine empirical formulas using percent composition

Post-Assessment

1. Determine the percent composition of each atom in water

2. Determine the empirical formula of a substance with 70.0% iron and 30.0% oxygen.