Download Unit 10 – The Mole

Unit 10 – The Mole
Calculating Formula, Atomic, and Molecular Masses (MOLAR MASS)
As you know, the mass of an element on the periodic table is a weighted average of all the naturallyoccurring isotopes of that element. Originally, we said that the unit for the mass of an element was
____________. From now on, we will be substituting ___________ as the unit for the mass of an
element or compound.
Steps for calculating MOLAR MASS:
1. Write the ___________________________.
2. Calculate the mass by adding up the masses of each element in the compound. If you have
more than one of an element, multiply how many you have by its mass before you add.
3. Make the unit ______________.
Substance
Name
Type of Particle
Go to Periodic Table
FM, AM, or MM?
1) K
2) KCl
3) SO2
4) Ca3(PO4)2
5) (NH4)2CO3
FM: _________________________________________
AM: _________________________________________
MM: _________________________________________
Different names for different types of particles, same method of determination
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Percent Composition
Percent composition refers to what percent of the compound is made up of each element. To
calculate the percent composition, divide the mass of the element by the molar mass of the
compound. If there is more than one atom of an element, you must multiply the molar mass by the
number of moles.
Ex: An 8.40 g sample of fluorine completely combines with a 4.90 g sample of sodium. Calculate the
percent composition of the compound that forms.
Ex: Calculate the % composition of copper (II) sulfate.
1) Cu
S
O
2) %Cu
%S
%O
2
Ex: Given 5.0 g of CuSO4, what is the mass of sulfur in the compound?
EX: Given 3629 g of CuSO4, what mass of oxygen is in this compound?
The Mole
•
One mole is defined as the number of atoms in 12 g of __________________________.
•
A mole is a counting number. One mole is a specific number of atoms, molecules, or formula
units. A mole is ALWAYS ____________________________. This number was discovered by a
scientist named ____________________________. So we chemists call this number
Avogadro’s Number.
•
Now we can relate the atomic mass unit to the gram. 1 atom of carbon-12 has a mass of 12.01
amu (atomic mass unit). Avogadro figured out that 6.02x1023 atoms of carbon-12 have a mass
of 12.01 g, therefore, 1 mole of carbon atoms have a mass of 12.01 g. This is why we are now
using g/mol as our unit for MOLAR mass.
The Mole has 3 mathematical definitions:
1) 1 mol = ___________________________________
Where a “particle” can be a ______________________, ______________________,
________________________________________, or ___________________
2) 1 mol = _________________ FOR GAS MOLECULES ONLY
3) 1 mol = _________________ (3 different types already discussed above)
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One-Step Conversions
Use the road map to help you convert between one unit of a chemical quantity to another unit.
This is set up like a metric conversion!
Remember, the mathematical definitions (above) can be written as a fraction for use in a conversion.
Ex: Convert 4.3 g of NaCl to moles.
Given:
Unk:
Road map:
Work:
Ex: Convert 0.00563 mol NH3to grams.
Given:
Unk:
Road map:
Work:
Ex: Convert 3.0x1014 atoms of barium to moles.
Given:
Unk:
Road map:
Work:
Ex: Convert 0.156 mol of carbon dioxide gas to molecules of carbon dioxide.
Given:
Unk:
Road map:
Work:
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Ex: Convert 0.53 L of NO2 gas to mol of NO2 gas.
Given:
Unk:
Road map:
Work:
Ex: Convert 2.3 mol of hydrogen gas to L.
Given:
Unk:
Road map:
Work:
Two-Step Conversions
*** Use and make road maps!!!
Ex:
Given: 5.631 x 1021 atoms Na
Unk: g Na
Road map:
Work:
Ex:
Given: 1.5 x 1022 f.u. Ca(OH)2
Unk: mass Ca(OH)2
Road map:
Work:
5
Ex:
Given: 2.63 L O2
Unk: mass O2
Road map:
Work:
Ex:
Given: 92.1 g Na2SO4
Unk: f.u. Na2SO4
Road map:
Work:
Ex:
Given: 6.3 x 10-3 L SO2 gas
Unk: mlcs SO2
Road map:
Work:
Hydrated Compounds
A hydrated compound is a compound with _______________ molecules attached _________________
to the ionic crystalline structure.
Examples:
CuSO4·5H2O ________________________________________
Calcium sulfate dihydrate _____________________________
BaCl2·9H2O _________________________________________
Water molecules can be removed by ____________________ the compound. This is not
__________________________, therefore not a _____________________ change. It is a
__________________________ reaction.
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Finding the percent composition of Hydrates:
The steps are the same as before, but KEEP ALL WATER MOLECULES TOGETHER! DON’T COUNT O’S
AND H’S SEPARATELY!
Ex: Lead (II) nitrate trihydrate
-count atoms
Pb
N
O
H2O
-calculate %
% Pb
%N
%O
% H2O
Ex: Given a 40 g sample of Pb(NO3)2·3H2O, calculate the mass of water present.
Ex: Given a 100 g sample of Pb(NO3)2·3H2O, calculate the mass of water present.
Ex: Calculate the % of IONIC COMPD and the % WATER in strontium chloride hexahydrate.
Formula:
IONIC
WATER
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Ex: Calculate the mass of water and the mass of strontium chloride in a 0.500 g sample of SrCl2·6H2O
SrCl2:
H2O:
Determining Empirical Formulas
An empirical formula (EF) is the ______________________ whole number ________________ of
elements in a compound.
Rules: 1) From a given _______ or from the _________________ of each element in the compound,
2) Find ________________ of each element in the compound.
Find ________________ of water if compound is _____________________.
3) ________________________________________________ # of moles to get a whole number
4) ________________________________________________ if necessary
(mass mol / small x by whole)
Ex: Calculate the EF (empirical formula) of a compound containing 25.9 % H and 74.1 % O.
1)
2)
3)
4)
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(big hint for EF/MF calculations – carry decimal places way out, like to 0.001 or 0.0001, throughout each step of the process, round at
the end)
Ex: Determine the EF for a compound containing 36.5 g Na, 25.4 g S, and 38.1 g O.
1)
2)
3)
4)
Determining Molecular Formulas
A molecular formula (MF) is not necessarily the simplest whole # ratio between atoms. The MF shows
the _____________________ number of atoms in a compound.
You will need: 1. ________________________________________________________
2. ________________________________________________________
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Ex: Determine the molecular formula (MF) of a compound whose empirical formula is CH2O and the
molecular mass is 120 grams per mol.
1)
2)
3)
This tells you how many of each atom are in the compound.
Ex: Methyl butanoate is a compound that smells like apples (used for artificial flavoring). Its %
composition is 58.8 % C, 9.8 % H, and 31.4 % O. Its molecular mass is 102 grams per mole. Determine
the molecular formula of this compound.
1) Find EF (4 step process, carry decimal places way out)
1.
2.
C:
H:
O:
3.
4.
2) Get Empirical Mass
3) Multiple =
4) MF:
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