Unit 5Percent Composition, Empirical Formulas & Reactions

CHAPTERS 10 & 11

Percent Composition of a Compound

SECTION 10.3

Math Warm Up

What is 73% of 150?

109.5

What percentage of 6.5 is 3.1?

48%

Percent Composition1. Calculating % from experimental data

Based on mass (not moles or # of particles)

For a compound it depends on the formula:

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

%100% xMass

MassMass

Compound

Element

2. Calculating % from formulas and molar mass

% mass = molar mass of element X 100%

molar mass of compound

What is the percent composition of the elements in Propane (C3H8)?

3. Percent composition as a conversion factor

How many grams of Carbon and Hydrogen are in 82.0 g of C3H8?

Hydrated Crystals

CoCl2●6H2O Hydrated Crystal

CoCl2 Anhydrous Crystal

Example Problem

What is the percent of water in Cobalt (II) chloride hexahydrate?

Warm-up 1/19

Hydrogen peroxide has the formula H2O2. What percent is it H, and what percent is it O?

Empirical Formulas

Formula Units ARE Empirical Formulas

Examples

NaCl

(NH4)2SO4

Molecular Formulas are not necessarily Empirical Formulas

Examples

C2H6

H2O2

H2O

• Lowest whole number ratio of elements in a compound

Identify each compound as Molecular or Ionic. Which ones are also “Empirical Formulas”?

Mg3(PO4)2

C6H12O6

CO

CaCl2

C6H6

H2O

(NH4)2CO3

Calculating Empirical Formulas

1. Find mass of each element in a compound

2. Convert masses to moles

3. Using the element with the smallest number of moles – Calculate the whole number “mole ratio” of each element (Divide each mole value by the smallest)

Example: 52.9% Aluminum 47.1% Oxygen

52.9 g Aluminum 47.1 g Oxygen52.9 g Al (1mol/26.98 g) = 1.96 mol Al 47.1 g O (1 mol/16.0g) = 2.94 mol O

52.9 g Aluminum 47.1 g Oxygen52.9 g Al (1mol/26.98 g) = 1.96 mol Al 47.1 g O (1 mol/16.0g) = 2.94 mol O

1.96 mol Al / 1.96 = 1 mol Al 2.94 mol O/ 1.96 mol = 1.5 mol O

Empirical Formula: Need to multiply each by 2 to turn into whole numbers Al2O3

Example

What is the empirical formula for a compound which contains 41.1% iron, 23.6% sulfur and 35.3% oxygen?

First assume each % is a mass in g’s (corresponding to a total substance mass of 100.00 g). Find the atomic mass of each element from the periodic table and use it to convert the mass in grams to moles.

41.1 g Fe Fe: 55.85 g/mol (41.1g)/(55.85g/mol) = 0.736 mol Fe

23.6 g S S: 32.07 g/mol (23.6g)/(32.07g/mol) = 0.736 mol S

35.3 g O O: 16.00 g/mol (35.3g)/(16.00g/mol) = 2.21 mol O

Next, convert the mole values to lowest whole number mole ratios. Do this by dividing each mole value by the smallest mole value of the group. These whole numbers tell the empirical formula ratios.

0.736 mol Fe 0.736mol/0.736mol = 1

0.736 mol S 0.736mol/0.736mol = 1

2.21 mol O 2.21 mol/0.736 mol = 3

Warm-up 9/20

What is the empirical formula of a compound that is 40.7% Carbon, 5.1% Hydrogen, and 54.2% Oxygen?

What is the molar mass of this formula?

Calculating Molecular formula

Divide the molar mass of the molecular formula by the molar mass of the empirical formula. The answer represents the factor the empirical formula needs to multiplied by.

Molecular Formulas

Look at Problem # 15a on Worksheet 1

15. Determine the molecular formula for a compound: (Hint: you will need to calculate the empirical formula first)

a. 94.1% O and 5.9% H: Molar mass = 34 g/mol

Ch 11Describing Chemical Reactions

Introduction to Chemical Equations

ALL chemical reactions follow 2 laws!

Law of Conservation of mass:

In a chemical reaction, the total mass of the reactants and the total mass of the products must be the same.

Law of Conservation of Atoms:

In a chemical reaction, atoms can not be created or destroyed.

Chemical Reactions are about the arrangement of Atoms, Not the changing of Atoms

The only thing about atoms that changes to drive chemistry is the location of electrons

Reaction Vocabulary:

Reactants: Starting Atoms, Molecules, or Compounds

Products: Ending Atoms, Molecules, or Compounds

Yields: Progress of reaction – Gives “→”

Catalyst: Substance that speeds up chemical reactions

Additional Information

States of Matter

Shown as (s), (l), (g), (aq) after formulas

Catalyst

Shown above Yield arrow

If Heat is added show with or H above Yield arrow

CatalystReactants Products

Example:

Aqueous hydrogen peroxide will decompose into gaseous forms of oxygen and water in the presence of manganese dioxide as a catalyst

)(2)(2)(222

gg

MnO

aqOHOOH

Balancing Chemical EquationsFollowing the Law of conservation of ATOMS You must NEVER change the # of Atoms in a formula by changing the

subscript numbers

To change # of atoms (or molecules or compounds), add a coefficient

Coefficients are numbers placed in front of a formula.

Coefficients change the # of atoms by a multiplying effect

Example:

Hydrogen gas combusts with Oxygen gas to form water.

1. Get Formulas correct

2. Organize REACTANTS vs. PRODUCTS

3. BALANCE

Balancing Reaction HINTS

Look for Polyatomic Ions!

Save elements O2 and H2 for the end.

If you assume the coefficient of a large particle is 1 and come to a dead end, change it to a 2.

Mg3(PO4)2 + NaCl MgCl2 + Na3PO4

C2H6 + O2 CO2 + H2O

Reaction Types

1. Combination Reaction:

2 or more substances as Reactants combine to form 1 Product

Also Called “Synthesis”

2. Decomposition Reaction:

1 Reactant breaks down into 2 or more Reactants

Most require energy to take place (take in Energy)

3. Single-Replacement Reaction:

1 element replaces another element in a compound

Reactivity of Metals and Non-Metals is important:

Use activity series (Page 361).

4. Double-Replacement Reaction:

An Exchange of positive ions between 2 Ionic Compounds

Both Reactants are Ionic Compounds

Often takes place in solution with water.

Sometimes a Product is a solid that will not dissolve in water and forms a PRECIPITATE

5. Combustion Reactions

A Reactant combines with Oxygen (other Reactant)

Usually produce energy (give off energy)

Usually involves a Molecular Compounds with (C,H, & or O) and Oxygen

Often have Water and Carbon dioxide as Products

“Burning Hydrocarbons”

Wood, Gas, Fuel, Sugars (Remember Cellular Respiration?)

May also be a Combination (synthesis) Reaction if only one Product forms

“Burning Elements”

Combination:

A + B AB

Decomposition:

AB A + B

Single Replacement:

AB + C CB + A

Double Replacement:

AB + CD AD + CB

Combustion:

? + O2 H2O + CO2

Predicting Reaction Types Examples

1. A single Reactant: H2O2

2. A metal (Na) and an Ionic Compound (MgCl2) React:

3. C2H6 reacts with O2:

4. Two Ionic compounds react [KCl & Mg(NO3)2]

5. A single product: H2O