COUNTING ATOMSChapter 3.4

OBJECTIVES:

1. Compare the quantities and units for atomic mass with those for molar mass.

2. Define mole, and explain why this unit is used to count atoms

3. Calculate either mass with molar mass or number with Avogadro’s number given an amount in moles

ATOMIC MASS

Mass of an atom expressed in atomic mass units ( amu )

Originally used b/c the mass of proton and neutron were approximately 1.0 amu each. Example: Copper – 63 62.940 amu Copper – 64 64.928 amu

From periodic table: Copper is given atomic mass of 63.546 amu

This is an average of naturally occurring isotopes

INTRODUCTION TO THE MOLE

Mole : Defined as the number of atoms in exactly

12 grams of carbon – 12. SI base unit for amount of substance Used as Counting unit

Instead of 12 eggs, we say 1 dozen eggs We use 1 mol of carbon or 2 mol of iron

MOLAR MASS

Mass in grams of ONE mole. Units – g/mol Find from periodic table

Oxygen – 15.9994 amu Round to 16.00 amu - USE two decimal places

So the molar mass of oxygen is 16.00 g/mol

Find the molar masses for Aluminum Uranium Chlorine

26.98 g/mol

238.03 g/mol

35.45 g/mol

AVOGADRO’S NUMBER

Number of particles in 1 MOLE of a substance 6.022 x 1023

60 220 000 000 000 000 000 000 How big is this number?

Entire population of Earth – 6 billion 6.00 x 109 or 6 000 000 000

DETERMINE MASS FROM MOLES

1. Determine mass in grams of 3.50 mol of copper

3.50 mol

Step 1:

Write what is given

Step 2:Cancel out units

x g

mol

=63.55 1

222 g

Step 3:Calculate

From periodic table* Multiply everything on top* Divide by everything on bottom

PRACTICE PROBLEMS

1. What is the mass in grams of 1.00 mol of uranium?

2. What is the mass in grams of 00.50 mol of uranium?

3. Calculate the number of moles of 0.850 g of hydrogen atoms. What is the mass in grams of 0.850 mol of hydrogen atoms?

4. Calculate the mass in grams of 2.3456 mol of lead. Calculate the number of moles of 2.3456 g of lead.

238 g

1.2 g

0.84 moland 0.86 g

486.0 g

and 0.01132 mol

DETERMINE NUMBER OF ATOMS FROM MOLES

1. Determine the number of atoms in 0.30 mol of fluorine atoms

0.30 mol

Step 1:

Write what is given

Step 2:Cancel out units

xmol

=6.02 x 1023

Step 3:Calculate

Avogadro’s Number* Multiply everything on top* Divide by everything on bottom

1.8 x 1023 F atoms

atoms

1

PRACTICE PROBLEMS

1. How many atoms are in 0.70 mol of iron?

2. How many moles of silver are represented by 2.888 X 1023 atoms?

3. How many moles of osmium are represented by 3.5 x 1023 atoms?

4.2 x 1023 atoms

0.4796 mol

0.58 mol