Lecture 1

The Mole and Avogadro’s Constant

IB Chemistry Power Points

Topic 1

Quantitative Chemistrywww.pedagogics.ca

The Structure of Matter

Matter

Pure Substances

Mixtures

SolutionsElements Compound

s

Atoms

Covalent Ionic

Molecules Formula Units

Fundamental “particles” of pure substances

Elements - MetalsStructure of metallic elements

The metal solids “particles” consist of individual metal ions held together by mutual attraction for each others valence electrons.

Elements – Noble Gases

The fundamental particles of these elements are single atoms (monatomic).

The noble gases are the Group 8 (18) elements: He, Ne, Ar, Kr, Xe and Rn. They are extremely unreactive – considered to be inert.

Elements – Diatomic Molecules

The fundamental particles of these elements are molecules. Each molecule contains only one type of atom.

Seven elements (H, N, O, F, Cl, Br, and I) are called the diatomic elements because, as pure elements, they form molecules containing two atoms.

Different representations of Cl2

Elements – Other Structures

Other non-metallic elements are found in molecular form.

Carbon and silicon exist as giant covalent structures – networks of atoms held together by covalent bonds.

Compounds – Molecular Covalent

Covalent compounds consist of groups of two or more different types of atoms bonded together into particles called molecules.

Water is molecular covalent compound consisting of H2O molecules.

Do not confuse the forces BETWEEN molecules with the bonds WITHIN molecules

covalent bonds hold hydrogen and oxygen atoms together to make water molecules

intermolecular forces exist between adjacent H

2O

molecules

Compounds – Simple IonicIonic compounds consist of groups of ions held together by electrostatic attractions. Atoms lose or gain electrons to form cations and anions. The fundamental particle is called a formula unit.

Sodium chloride is an ionic compound. The chemical formula NaCl shows the 1:1 ratio of Na to Cl in compound

Compounds – Simple IonicTwo representations of NaCl

Now we can introduce

the Mole

Key Concept – “particles” is a general term referring to the fundamental components of a pure substance. This can be individual atoms, ions, small molecules, or formula units.

A mole is a unit of quantity.

A mole is 6.02 x 1023 things.

÷ 6.02 x 1023

6.02 x 1023 is known as Avogadro’s constant (NA)

Number of atoms,

molecules or fundamental

units

Number of moles (mol)

× 6.02 x 1023

Why the Mole?

Consider one molecule of water

How many molecules in 2000 mL of water?

6.7 x 1025 molecules

We count eggs by the dozen

We measure long periods of time in centuries.

6.7 x 1025 molecules is not a manageable number. Consider:

We measure long distances in our universe using light years.

The Mole

The mole is the SI unit for chemical quantity used to count the particles in a sample of pure substance.

There are many ways of measuring large quantities that utilize large units. The mole is one such unit.

One mole = 6.02x1023 particles.

“One mole of anything = 6.02x1023 units of that thing”

The Mole

How many molecules of water in 2000 mL?

6.7 x 1025 molecules

Or 111 mol meaning 111 moles of water molecules. This is a much more manageable number.

How many atoms are in 0.065 mol of copper?

3.9 x 1022

How many molecules are in 0.065 mol of CO2?

3.9 x 1022

How many formula units are in 0.065 mol of NaCl?

3.9 x 1022

How many moles is 9.03 x 1023 atoms of copper?

1.5

How many moles is 9.03 x 1023 molecules of CO2?

1.5

How many moles is 9.03 x 1023 formula units of NaCl?

1.5

How many oxygen atoms are in 1.4 x 10-7 mol of oxygen gas (O2)?(1.4 x 10-7) × NA = 8.428 x 1016 molecules of

O2

2 atoms of oxygen per molecule sofinal answer = 1.7 x 1017 atoms of oxygen (2 SF)

Given 1.76 x 1012 molecules of O2, how many moles of iron III oxide (Fe2O3) can be assembled?

122

23

122 3

1.76 10 molecules O 2 atoms/molecule6.02 10 3 atoms/molecule

1.95 10 moles Fe O

Fe2O3 is an example

of aChemical Formulae

What are chemical formulae?● short-hand notation

H2SO4 sulphuric acid

1 molecule of H2SO4

4 atoms of oxygen1 atom of sulfur2 atoms of

hydrogen

● tells us the types of elements in a compound

● gives the number of atoms of each type of element in one molecule of the compound

How many atoms in each?

C6H12O6 glucose

Mg(NO3)2 magnesium nitrateCu(SO4)2●5 H2O copper II sulfate hydrate

Relative Molecular (Formula) Mass (MR)RECALL: The average relative atomic mass (AR) for each element is noted on the Periodic Table. For example AR for copper is 63.55. Relative masses are based on 1/12th the mass of a 12C atom.

The relative molecular mass is the total mass of the atoms in one molecule or formula unit of a particular compound relative to carbon-12. For example H2SO4:

Hydrogen 2 atoms × 1.01 = 2.02Sulfur 1 atom × 32.07 = 32.07Oxygen 4 atoms × 16.00 = 64.00

MR 98.09

Molar Mass

Are you sitting comfortably?

Mass and the Mole: Why 6.02×1023?

Consider1 atom of sulfur AR = 32.07 = 5.326×10-23 grams

1 mole = 6.02×1023 was chosen because this was then number of carbon-12 atoms that has a mass of 12 grams.

Pure carbon (a mix of isotopes) has a mass of 12.01 g per mole.

We call this value MOLAR MASS

Consider1 mole of sulfur = 6.02×1023 atoms = 32.07 grams

6.02×1023 is a REALLY BIG numberConsider 6.02×1023 sheets of paper stacked.

How many round trips to the Moon would this stack of paper be equivalent to?

8×1010 (eighty billion ) roundtrips. Create, pick, find a mole analogy of your own. Show the math.

Aluminum AR = 26.98 gram atomic mass = 26.98 g

mol-1

Language issue

gram atomic massgram molecular massgram formula mass

Carbon dioxide (CO2) MR = 44.00 gram molecular mass = 44.00 g mol-1

Sodium chloride (NaCl) MR = 58.44 gram formula mass = 58.44 g

mol-1

MOLARMASS

We weigh chemical quantities in grams. The molar mass value for a substance allows us to determine the number of moles from a measured mass.

Consider 5.68 g of MgCl2

Molar mass of MgCl2 = 95.21 g mol-1

-1

massnumber of moles =

molar mass5.68 g

=0.0597 mol92.51 g mol

n

÷ 6.02 x 1023Number of atoms,

molecules or fundamental

units

MOLES× 6.02 x 1023

MASS

× molar mass

÷ molar mass

1. How many moles are in 45.0 g of water?2.50 mol

2. What is the mass of 1.75 mol of CO2?77.0 g

3. How many moles of oxygen atoms are in 300 g of CaCO3?

9 moles

4. What is the mass of 9.03 x 1023 atoms of copper?95.3 g

5. 106 g of glucose (C6H12O6) is how many moles?0.589 mol

6. What is the mass of the oxygen atoms in 106 g of glucose?

56.5 g