Chapter 11 The Mole

• A chemists best friend

Counting units in groups is common

Dozen

Case

Gross

Mole - a particular number of atoms, ions, molecules or formula units

It is a large number

6.02 X 10 23

• Just like dozen = 12

• Mole = 6.02 x 10 23 particles

Use atomic masses to 3 significant digits or to the hundredths' place, which ever gives you more digits

Avogadro constant - 6.02 10 23

It is an SI standard (symbol = N ) A

The Mole-

6.02 10 “things”

In chemistry “things” are atoms, ions, molecules, or formula units.

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One mole of particles has a mass in grams equivalent to that of one particle in atomic mass units

1 mole of particles contains 6.02 10 particles

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Molar mass- the mass of one mole of molecules, atoms, ions, or formula units

Examples:

grams to moles

molecules to grams

grams to atoms

Percent Composition- used to identify unknowns

Example: Find the percentage composition of aluminum oxide

2 Al atoms = 53.96 u

3 O atoms = 48.00 u

101.96 u

% Al = 53.96 100 = 52.92%

101.96% O = 48.00 100 = 47.08%

101.96

Practice prob pg 331

Empirical Formulas

The decomposition of 11.47 g of a compound of copper and oxygen yields 9.16 g of copper. What is the empirical formula?

2.31 g oxygen

9.16 g Cu 1 mole Cu = .144 mole Cu

63.55 g Cu

2.31 g O 1 mole O = .144 mole O

16.00 g ORatio 1:1 hence CuO

A compound contains carbon 81.7% and Hydrogen 18.3%. Find the empirical formula

81.7 g C 1 mole C = 6.80 moles C

12.01 g C

18.3 g H 1 mole H = 18.1 moles H

1.01 g H

Ratio = 6.81 : 18.1

6.81 6.81 = 1:2.66 = 1:2

3 = 1: 8

3:8 C H

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3 33 8Exercises Pg 333

Molecular Formulas~ Must know molar massExample: analysis of a compound reveals this composition:

80.0% C 20.0% H

Molar mass = 30.0 g

What is the molecular formula?

80.0 g C 1 mole C = 6.66 mole C

12.01 g C

20.0 g H 1 mole H = 19.8 mole H

1.01 g H

C H1 3

C2H6

Ascorbic acid, vitamin C, is 40.9% carbon, 4.58% hydrogen and 54.5% oxygen. Its molar mass is 176.1 g/mole. What is the molecular formula?

Hydrates ~ water molecules are part of the crystal structure

Example: 5.20 g of hydrated BaI2 is heated. The dry sample has a mass of 4.76 g. What is the formula of the hydrate?

.44 g H O 2

5.20 g – 4.76 g = .44 g H O 2

4.76 g BaI 1 mole BaI = .0122 mole BaI

391.13 g BaI2 2

2

2

.44 g H O 1 mole H O= .024 mole H O

18.02 g H O

2 2

2

.0122 mole = 1.00 .024 mole = 2.0

.0122 .0122

2

1:2 Ratio BaI • 2H O22

Atomic mass unit ( u ) a.m.u.

H = 1.01 u

O = 16.00 u

C = 12.00 u

Molecular Mass- the sum of all the atomic masses of the atoms in a molecule

Formula Mass- used for ionic compounds