Unit 5 Section 2: Percent Composition

Unit 5 Section 2: Percent Composition

Multiple Choice

How many atoms of iodine are in a mole of iodine?

• 53

• 63.55g

• 126.9

• 6.02 x 10²³

Multiple Choice

The number 6.02 x 10²³ is called...

• The Magic number

• Bohr's number

• The Molar number

• Avogadro's number

Lesson Objectives

• Use molar mass to determine the percent composition of a compound

• Use a given mass of a compound to determine the percent composition of the whole compound

• Use Percent Composition to predict the amounts of individual elements in a compound

Chemical Ratios

• The Law of Definite Proportions: Every Compound has a fixed ratio of atoms that does not change

  • Water is always H₂O, 2 hydrogens, and 1 Oxygen

    • 2:1 Ratio

  • Carbon Dioxide always has 1 Carbon and 2 Oxygen

    • 1:2 Ratio

Dropdown

Sodium Chloride has the formula NaCl. It has a ratio of ​

Select answer

Na : ​

Select answer

Cl

Dropdown

Butane has the chemical C₄H₁₀. It has a chemical ratio of ​

Select answer

Carbon : ​

Select answer

Hydrogen

Percentage Composition

• Percentage Composition: the percentage by mass of each element in a compound​

  • It is sometimes useful to know how much of a particular element is in a compound

Percent Composition Sample Problem 1

• ​Find the percentage composition of copper(I) sulfide Cu₂S

• First step: find the total molar mass of the compound Cu₂S

  • ​2 Cu= 2 x 63.55

  • ​1 S= 1 x 32.07

Percent Composition Sample Problem 1

Find the percentage composition of Tin (IV) Oxide (SnO₄)

• ​​Identify the individual masses

  • 1*Tin: 118.710

  • 4*Oxygen:​ 4(16)

• Calculate the Total atomic Mass

Find the percentage composition of Tin (IV) Oxide (SnO₄)

Why Use Percentage Composition?

• Percent Composition can also be used to determine the formulation of ionic compounds based solely on the mass of a substance tested

  • By comparing the mass of one sample to the mass of the whole, you can determine the amount of other components in the compound

Percent Composition from mass sample 1

• When a 13.60 g sample of a compound containing only magnesium and oxygen decomposes, 5.40 g of oxygen is obtained. What is the percent composition of this compound?

  • Step 1: Identify what you know

    • Total Mass: 13.60 g

    • Mass of Oxygen: 5.40

    • Mass of Magnesium: ?

  • Step 2: Find the mass of your unknown

    • Magnesium=Total-Oxygen= 13.60-5.40

Percent Composition from mass sample 1

• When a 13.60 g sample of a compound containing only magnesium and oxygen decomposes, 5.40 g of oxygen is obtained. What is the percent composition of this compound?

  • Known:

    • Total Mass: 13.60 g

    • Mass of Oxygen: 5.40

    • Mass of Magnesium: 8.20

  • Step 3: Plug your values into your percent composition equation

Using Percent Composition as a Conversion Factor

• You can use percent composition to calculate the number of grams of any element in a specific mass of a compound

•  If you have a 100 gram sample, the percent composition can tell you how much of each element is in that sample.

Using Percent Composition as a Conversion Factor #1

• Example: 11.11 % of water is hydrogen. How much hydrogen is in 20 grams of water?

Percent Composition as a Conversion Factor #2

• Calculate the mass of carbon and the mass of hydrogen in 82.0 grams of propane (C₃H₈).

  • Step 1: Calculate your mass of propane

Percent Composition as a Conversion Factor #2

• Calculate the mass of carbon and the mass of hydrogen in 82.0 grams of propane (C₃H₈).

  • Step 1: Calculate your mass of propane

    • C₃H₈ =44 g/mol

  • Step 2: Identify the percent composition of both Carbon and Hydrogen in Propane

Percent Composition as a Conversion Factor #2

• Calculate the mass of carbon and the mass of hydrogen in 82.0 grams of propane (C₃H₈).

  • Step 1: Calculate your mass of propane

    • C₃H₈ =44 g/mol

  • Step 2: Identify the percent composition of both Carbon and Hydrogen in Propane

    • C= 81.82 %, H= 18.18%

  • Step 3: Use the percent Composition to convert 82 grams of propane into grams of carbon and grams of hydrogen