Chapter 10 - The Mole & Avogadro’s Number

Measuring Matter

Sections 10.1 10.3

Measuring Matter/Counting Units

•One pair = 2

(boots, shoes)

•One dozen = 12 (eggs, donuts, roses)

•One gross = 144 (pencils)

•One ream = 500 (sheets of paper)

Measuring a Mole – units used to count particles

• Atoms are so tiny chemists count them using a unit of measurement called a “mole”

• 1 mole = 6.02 x 1023 (Avogadro’s Number) atoms/particles/ions/formula units/molecules

• This is a very large number because it measures extremely tiny items

• 602,000,000,000,000,000,000,000

How Big is a Mole?• One mole of pennies would allow every person on

Earth to spend a million dollars an hour, day and night, and still have half of it unspent when they died

• One mole of popcorn would cover every dry surface of the Earth to a depth of six milesOne mole of stacked pennies would reach to the moon

and back 16 times• One mole of marshmallows would cover the United States to a depth of 650 miles

History of Avogadro’s NumberAvogadro: 1811 – equal volumes of gases at the same temperature and pressure contain equal numbers of molecules

Cannizzaro: 1861 – developed atomic weights for known elements

Loschmidt: 1865 – determined the number of atoms present in a given volume of air

Ostwald: 1902 – developed the concept of the mole

1971 – the true value, 6.02x1023, was actually determined and is called Avogadro’s number since the entire string of events started with his findings in 1811

The Law of Conservation of Mass

• According to the Law of Conservation of Mass,

Mass of reactants = mass of productsMass of reactants = mass of products

Mass of starting substances = mass of ending substancesMass of starting substances = mass of ending substances• So the production of one water molecule requires 2

hydrogen atoms and 1 oxygen atom.Therefore:

two atoms of hydrogen + one atom of oxygen 1 water molecule

And

12.04x1023 hydrogen atoms + 6.02x1023 oxygen atoms 6.02x1023 water molecules

The Law of Definite Proportions and Ratios of Atoms

Law of Definite Proportions – atoms combine in small whole number ratios to form compounds

Example: Water- two atoms of Hydrogen will always combine with one atom of Oxygen to form one molecule of water

The atomic mass ratio of hydrogen to oxygen is 1 amu : 16 amu

The gram mole ratio between hydrogen and oxygen is 1 g : 16 g

So, chemists can determine how many atoms are present in a sample.

One gram of hydrogen would contain exactly 6.02x1023 atoms, or Avogadro’s Number of atoms. Two grams of hydrogen atoms would contain 12.04x1023 hydrogen atoms.

Also, 16 gram moles of oxygen would contain 6.02x1023 atoms.

Therefore: 1 gram of hydrogen = 6.02x1023 atoms = 1 mole

2 grams of hydrogen = 12.04x1023 atoms = 2 moles

16 grams of oxygen = 6.02x1023 atoms = 1 mole

32 grams of oxygen = 12.04x1023 atoms = 2 moles

Relationships between moles, Relationships between moles, atoms and the gram moleatoms and the gram mole

•2 atoms of hydrogen + 1 atom of oxygen = 1 water molecule

•2 amu of hydrogen + 16 amu’s of oxygen = 18 amu’s of water molecules

•2 g of hydrogen + 16 grams of oxygen = 18 grams of water molecules

•12.04x1023 atoms H + 6.02x1023 atoms O = 6.02x1023 atoms H2O molecules

•2 moles of H atoms + 1 mole of O atoms = 1 mole of water molecules

In Conclusion:In Conclusion:

Moles Particles Moles Particles ConversionConversion

ParticlesAtoms, molecules, ions, formula units

x 6.02 x 1023

÷ 6.02 x 1023

Moles

EXAMPLE PROBLEMS:EXAMPLE PROBLEMS:

1) Convert 3.5 moles to atoms

2) Convert 18.06 x 1023 atoms to moles

Convert from moles to particlesConvert from moles to particles3) 2.7 moles of lithium

4) 1.8 moles of sodium chloride

5) 5.3 moles of bromine

6) 4.7 moles of potassium oxide

Convert from particles to molesConvert from particles to moles7) 4.32 x 1024 atoms of calcium

8) 2.7 x 1020 atoms of copper

9) 2.5 x 1024 molecules of sodium chloride

10) 5.32 x 1023 molecules of calcium chloride

You try these…You try these…

11) 25 moles to particles

12) 4.08 x 104 particles to moles

13) 0.002 moles to particles

14) 2.3 x 1016 particles

HomeworkHomeworkComplete on another sheet of Complete on another sheet of

paper showing your work!paper showing your work!Particles to moles

5) 4.32 X 1024 6) 2.70 X 1020 7) 2.50 X 1024 8) 5.32 X 1023

Moles to particles

1) 2.7 moles

2) 1.8 moles

3) 5.3 moles

4) 4.7 moles