S. 5.2 – The Mole
The mole:
Ex.
Molar mass (g/mol):
Ex.
Element
N
F
P
V
Ni
Atomic mass shown
on Periodic table
14.0067
18.998403
30.97376
50.9415
58.69
Molar mass of
element
14.0067 g
18.998403 g
30.97376 g
50.9415 g
58.69 g
Finding the molar mass of a compound.
- Look up the molar masses of the elements within the compound using the periodic table.
- Add up the masses and express in grams.
- Round the molar masses to two decimal places. The more decimal the places the more precise your
answer is.
Steps:
1) Determine how many atoms of each element are within the compound.
2) Determine the molar mass of each element.
3) Multiply the molar mass of each element by how many atoms of that element are within the
compound.
4) Add up the numbers produced in step three.
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Ex. 1: Calculate the molar mass of C6H4Cl2.
1) 6 C atoms, 4 H atoms, 2 Cl atoms
2) C = 12.01 g, H = 1.01 g, Cl = 35.45 g.
3) C = 6 × 12.01 g = 72.06 g,
H = 4 × 1.01 g = 4.04 g,
Cl = 2×35.45 g = 70.9 g.
4) 72.06 g + 4.04 g + 70.9 g = 147 g = molar mass = 1 mol C6H4Cl2
It is easier to set up the equation as:
6 C = 6 × 12.01 g = 72.06 g
4 H = 4 × 1.01 g = 4.04 g
2 Cl = 2 × 35.45 g = 70.9 g
molar mass = 147 g C6H4Cl2
Ex. 2: Calculate the molar mass of Mg3(PO4)2
We have 2 PO4
Ex. 3: Calculate the molar mass for Cu(NO3)2·6H2O
Questions:
- Pg. 80, # 6 – 7.
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Calculations Relating the Number of Moles and The Mass of Substance.
Why we do calculations.
- THE TEACHER MAKES US.
- Can determine the mass produced in a reaction.
- Can determine the amounts of reactants required to react so neither is in excess. Chemicals are
expensive and a waste is no good.
Two Possible Conversion Factors.
1 mol A
molar mass of A
or
molar mass of A
1 mol A
Steps:
1) Find the molar mass of the compound.
2) Determine the initial, conversion statement equation.
3) Determine the unknown (grams or moles)
Ex. 1: What is the mass of 5.21 mol NH3
1) Molar mass of NH3 =
2) Mass of NH3:
Ex. 2: What is the mass of 1.46·102 NaBr?
1) Molar mass of NaBr =
2) Mass of NaBr:
Ex. 3: Determine the number of moles of H2O there are in 1000 g of H2O.
1) Molar mass of H2O is
2) Moles of H2O is:
Ex. 4: Determine the number of moles of C2H5OH there are in 355 g C2H5OH.
1) Molar mass of C2H5OH is
2) Moles of C2H5OH is:
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Finding molar mass from amount of substance and mole of substance.
- Calculated by dividing the mass of a substance by the number of moles contained in the
substance.
Ex. If 0.140 mol of acetylene gas has a mass of 3.64 g, what is the molar mass of acetylene?
molar mass =
Questions:
- Pg. 82, # 8 – 10.
Calculations relating the number of moles and the volume of gas.
- Due to Avogadro’s hypothesis the molar volume of a gas at standard temperature and pressure
(STP) is 22.4 L.
STP = 0 °C and 101.3 kPa.
Molar Volume: The volume occupied by one mole of gas.
1 mol of any gas at STP = 22.4 L
Two Possible Conversion Factors.
1 mol A
22.4 L A
or
22.4 L A
1 mol A
Steps:
1) Write out the initial, conversion and unknown equation.
2) Do the math.
Ex. 1: What is the volume occupied by 2.30 mol of O2(g) at STP?
Ex. 2: How many moles of any gas is contained in a 25 L sealed container at STP?
Questions:
- Pg. 83, # 11 – 12.
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Converting moles to particles.
- Due to Avagadro’s hypothesis it is possible to convert the number of moles of a substance into the
number of particles present.
Avagadro’s number: The number of particles contained in one mole of substance.
Avagadro’s number = 6.02  1023
Two Possible Conversion Factors.
6.02 1023 particles A
1 mol A
or
6.02 1023 particles A
1 mol A
Ex. 1: How many particles are there in 0.252 mol CO.
Ex. 2: How many moles of NO3 are there in 9.88·1020
Remember that the we are dealing with particles which equals molecules, atoms.
Questions:
- Pg. 84, # 15 18.
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