Chapter 7

Chemical Quantities

The Mole (Friend or foe)

What is a mole?

1. SI base unit to measure the amount of a substance

2. The amount of a substance that contains the same number of

particles as the number of atoms in exactly 12 g of carbon-12

What is Avogadro’s number?

1. the number of particles in a mole

2. 1 mole = 6.02 x 1023 particles

3. particles are: atoms, ions, molecules, formula units

• How do you use these numbers?

1. 1 mole of C = 6.02 x 1023 atoms C

2. 1 mole of NaCl = 6.02 x 1023 formula

units NaCl

3. 1 mole of H2O = 6.02 x 1023 molecules

H2O

4. 1 mole of Ca+2 = 6.02 x 1023 ions Ca+2

• What is molar mass?

1. the mass in grams of one mole of a pure substance (element or compound)

2. use the periodic table to find the mass

3. What is the molar mass of helium?

4.0 g He

mole

4. will be used as a conversion factor

5. Examples:

a. What is the molar mass of 1 mole of Al?

27.0 g Al

mole

b. What is the molar mass of 1 mole of CO2? C = (1)(12.0) = 12.0

O = (2)(16.0) = 32.0

44.0 g/mole

Converting from moles to grams

1. What is the grams of 3.50 mole of Cu?

2. There are 11.9 g of Al, how many moles of Al are there?

3. How many moles are in 25.0 grams of H3PO4?

• Converting moles to molecules

1.How many molecules are in 5.2 moles of NaCl?

2.How many moles are in 3.12 x 1015 Mg+2 ions?

• Convert grams to molecules.

1.How many molecules are in 3.14 grams of NaOH?

2.How many grams are in 4.12 x 1012 molecules of Rh?

3. How many molecules are in 523 g of KI?

Percent Composition

1. Definition – the percent by mass of each element in the

compound

2. Formula: % composition = mass of element 100 mass of compound

3. Ex.

Find the percent composition of copper (I) sulfide.

Cu2S

Cu = 2(63.5) = 127.0

S = 1(32.1) = 32.1

159.1g/mole

%Cu = 127.0 100 = 79.8%

159.1

%S =

Empirical Formula

1.Shows the simplest whole-number ratio of the atoms in a compound

2. Examples:

a.) C2H8 =

b.) Al2O3 =

3. Examples:

Analysis shows a compound to contain 26.56% K, 35.41% Cr, 38.03% O. Find the empirical formula.

Molecular Formula

1. Indicates the ratio of the atoms

2. Indicates the actual number of atoms

3. n = Molecular mass

empirical mass

4. (empirical formula)n = molecular formula

5. Examples:

An organic compound is found to contains

92.25% C and 7.75% H. If the molecular mass

is 78, what is the molecular formula?