1. Avogadro’s constant and the mole

2. Mass on its own is not very useful because when reactions take place they depend on # of atoms not their mass.
Atoms are way to small and numerous to count so you need a way to relate # of atoms to masses that can be measured
In science it is important to measure amounts accurately and conveniently

3. The Mole
People find it easier and useful to work in groups when counting
Pair=2 Dozen=12, Gross=144
To count the number of atoms a dozen or gross isn’t large enough since elements contain a huge # of atoms
Instead chemist use the mole (symbol mol)

4. The Mole
The amount of substance that contains X1023 particles(atoms, molecules, formula units) of that substance
This is called Avogadro Constant
Has the symbol NA
Rarely need 9 sig figs when working with the number so 3 is enough
NA=6.02X1023mol-1

5. Examples One mole of carbon contains 6.02X1023 carbon atoms
One mole of Magnesium Chloride contains 6.02X1023 formula units of MgCl
Etc

6. Application
When someone ask you to buy them 2 dozen eggs, they are also thinking of 24 eggs
2dozen X 12eggs=24eggs
1dozen
You can convert between moles and particles in the same way
2molX6.02X1023atoms=1.20X1024atoms
mol

7. Converting moles to # of particles
Moles are used to help us count atoms and molecules
Relationship can be written using formula below
N=n X NA
# of particles
(atoms, molecules,
formula units)
Multiply by
# of moles
6.02X1023
Where: N=# of particles
n= amount(mols)
NA= Avogadro's constant(mol-1)

8. Converting # of particles to moles
Usually express in moles
To convert # of particles in a substance to number of moles, just rearrange the formula learned on pervious slide
N= n X NA rearranges to
n= N/NA
Since there are far too many particles to work with we usually express the amount of substances in moles

9. Example
How many moles are present in a sample of potassium floride, KF, made up of 5.83X1024 molecules?
Solution: To solve we use Avogadro constant and the number of molecules given and substitute into equation:
n= X1024 molecules KF
6.02X1023 molecules KF/mol KF
n=9.68mol KF
So there are 9.68 moles of KF in the sample

10. Converting from moles to mass
Suppose you were asked to weigh out three moles of a substance
In order to do so you must figure out how much 3 moles is in grams to use the scale
The formula below relates moles and mass by molar mass
Mass=#of moles X molar mass
m= n X M where m=mass
n=# moles
M=molar mass

11. Example
A flask contains mol of carbon dioxide gas. What mass of CO2 gas is in this sample?
First we must find the molar mass of CO2 to use in the equation along with the # of moles that was given
1Carbon + 2Oxygen 12.01g/mol+2(16.00g/mol)=44.01g/mol
Using the equation m=n X M
m= 0.750molX44.01g/mol
m=33g of CO2 in 0.750mol of CO2

12. Converting from mass to moles
Opposite of moles to mass
Use formula below
Mol= mass or n= m
molar mass M

13. Example
How many moles of acetic acid, CH3COOH, are there in a 23.6g sample?
First need to find molar mass:
There are 2C, 2O, and 4H
M=2(12.01g/mol)+(2(16g/mol)+4(1.01g/mol)
M=60.06g/mol
Next substitute values into equation n=m/M
n= g n= 0.393mol of CH3COOH in
60.06g/mol g of CH3COOH

14. Time to practice!