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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