Unit 6 - Chapter 12
The Mole
The Mole – a word that means a #
Other words that mean #s: One dozen =
12
One gross = 144
One score = 20
One ream = 500
One MOLE = 6.02 x 1023
602000000000000000000000
Atoms are too small/light
Atoms are too small and too light to count or mass as individuals– One Carbon-12 atom = 1.99 x 10-23 grams
We use this as the basis for atomic mass units
1 Carbon atom is 12 amu.– 6 protons + 6 neutrons = 12 “things” that make up
the mass of a carbon atom
Atomic Mass & Molecular Mass
Atomic mass - the mass of 1 mole of atoms (also known as “molar mass”)
Molecular Mass – the masses of all of the atoms in a chemical compound. – Ex: CH4
– Ex: CaCl2
Molecular Mass – Example
So, the formula weight of calcium chloride, CaCl2, would be
Ca: 1(40.1 amu)
+ Cl: 2(35.5 amu)
111.1 amu
To convert to molar mass, change amus to g/mol. 1 amu = 1 g/mol
Molecular Mass – Example
For the molecule methane, CH4, the molecular weight would be
C: 1(12.0 amu)
16.0 amu+ H: 4(1.0 amu)
The molar mass of methane would be 16.0 g/mol
1 mole - different physical states
Moles provide a bridge from the molecular scale to the real-world scale.
Examples of Molar Mass
List the molar masses of the following:– Zn
– CuSO4
– NaCl
– H2O
– Al2(SO4)3
Avogadro’s Number
Avogadro’s Number - 6.02 x 1023 particles in 1 mole 1 mole of 12C has a mass of 12 g.
Mole Conversions
Moles tell you how many atoms you have, AND how much mass in grams you have.
Today we will convert from moles to grams.
Mole Conversion Factors
Today there are 2 possible conversion factors for mole conversions:– Show your work -use a conversion factor every time
Mole Diagram
Mole Examples
If you have 8.64g of Mg, how many moles do you have?
If you have 1.23 moles of Phosphorous, how many grams do you have?
Mole Diagram
Using your calculator
To Enter 6.02 x 1023 into your calculator, use the EE button, so it would appear as 6.02E23. Practice with the two following examples
Examples:
If you have 4.25 x 1022 atoms of Al, how many moles do you have?
If you have 0.37 moles of Al, how many atoms do you have?
Mole Relationships
One mole of:– atoms– ions– molecules
contains Avogadro’s number of those particles.
One mole of:– molecules or formula units
contains Avogadro’s number times the number of atoms or ions of each element in the compound.
Formula mass and % composition
Percent Composition =
%100)(
)(x
gthingwholetheofmass
gparttheofmass
All of the parts should add up to 100% !!
Example: What is the mass percent of each element in carvone (C10H14O) ?
C10H14O
1. To find % composition you must 1st find the mass of the whole thing!
2. Then find the mass of each part (you kind of already did that) and divide it by the mass of the whole thing x 100 to find % mass of each part.
Empirical and Molecular Formulas
Unit 7 - 2009
Empirical and Molecular Formulas
An empirical formula shows the simplest whole number ratio of atoms of each element in a compound
A molecular formula shows the actual number of atoms of each element in a compound
Example: Glucose
Glucose has a molecular formula of:– C6H12O6
It has an empirical formula of:– CH2O
Formulas cont’d
A molecular formula is always a multiple of empirical formula
A compound has ONE molecular formula
Many compounds have the same empirical formula
Example
0.18 grams of magnesium react with 0.12 grams of oxygen to form magnesium oxide
Convert all grams to moles Divide all numbers of moles by the smallest # present
(do a ratio!) Multiply all numbers by the smallest # that will make
them WHOLE NUMBERS
Example
An oxide of aluminum is formed with 4.151 g of Aluminum and 3.692 g of Oxygen. Calculate the empirical formula of the compound:
1. Convert all grams to moles
2. Divide all numbers of moles by the smallest # present (do a ratio!)
3. Multiply all numbers by the smallest # that will make them WHOLE NUMBERS
Find molecular formulas from empirical formulas
A white powder is analyzed and found to have an empirical formula of P2O5. It has a molar mass of 283.88 g/mol. What is the molecular formula for this compound?
massmolarempirical
massolarmolecularmn
Hydrated compounds
Sometimes, compounds contain water molecules within their crystal structure
If a compound contains water molecules, we write it like this:
MgSO4•7H2O
Hydrates
MgSO4•7H2O
The little dot between the MgSO4 and the H2O is NOT a multiplication symbol – the H2O is just “attached”
How many atoms of each element are in the compound above?
How about in Na3PO4•6H2O?
Lab 9: Percent Comp of a Hydrate
If you know the mass of hydrate (what you start with) and you know the mass after you cook it, what have you lost?
That is the mass of the H2O you lost
If you know mass H2O and you know mass whole thing….you can calculate % of H2O in the compound!!
