The Math of ChemistryFactor-Label Method

the Molegram formula weightAvogadro NumberScientific Notation

Percent CompositionEmpirical Formulas

Hydrates

Begin working on HW 11 and 12

• To convert units, you must write down what you know, then use a conversion factor to convert it to what you want to know.

0.32 g = ____________mg

use 1000 mg = 1 g

0.32 g

1 g

1000 mg = 32 mg

What if you only know a similar conversion factor, not the exact one

you need?• 0.436 m3 = _____________cm3

– use 1m = 100 cm; then cube both sides.– 13 m3 = (100)3 cm3

– 1 m3 = 1,000,000 cm3

0.436 m3

1 m3

100000 cm3

= 436000 cm3

or4.36 x 105cm3

This procedure is called the factor label method

Use page 29 in your book to help you do problem 28 page 196

• On the blackboard, three students can earn a free homework pass by correctly solving and carefully explaining how to do #28 a and b OR #28 f or g

Quiz: Factor Label Method

convert the following:

1. 30 cm = ______m

2. 3600 sec =____hours

3. 10 cm3 = _____ m3

30cm (1 m)

(100 cm)= 0.30 m

3600sec (1 min)(1hour) (60 sec)(60 min)

1

10cm3 (13 m3)

(1003 cm3)= 0.01 m3

Molecular and Formula Mass

• Instead of using single atoms with masses on the order of 10-24g, we use moles– one mole = 1 Avogadro number of particles– 6.02 x 1023 particles = 1 Avogadro number.

• We can use the numbers on the periodic table as either atomic mass units or grams. When we use them as amu we are talking about quantities too small to measure.

• When we use moles, we are talking about 6.02 x 1023 atoms or ions or formula units or molecules.

Find the gfw of cyclopentane

C5H10

5 C

10 H

5 C = 5(12.0amu)

10 H = 10 (1.00amu)

5(12.0) + 10 (1.00)= 60.0 + 10.0 = 70.0 amu or

g/mole

Find the gfw of AlN

• First, identify the elements, and how many of each are in the compound.

• Then find the atomic masses on the periodic table

• Then, multiply how many of an atom by how much it weighs.

• Finally, find the sum of all the pieces of the compound.

27 amu

14 amu

1(27.0) and 1(14.0)

1(27.0) + 1(14.0)

41.0 g

1 Al and 1 N

In Groups

• You are responsible for finding the gram formula masses of each compound on the sheet of paper. Points will be awarded for the groups who have successfully calculated the greatest number of gfw.

• Students will come up to the board to explain how they found the gfw of a substance. Points will be awarded for the quality of the explanation.

Quiz: Find the gfw for the following compounds…

1. NaCl

2. Al2S3

3. Ca3(PO4)2

1. 23 + 35 = 58 g/mole

2. 2(27) + 3(32) =54 + 96 =

150

3. 3(40) + 2(31) + 8(16)

= 120 + 62 + 128

= 310 g/mole