Avogadro’s number and the Mole
6,020,000,000,000,000,000,000,000
uses for a mole
What is a Mole?
A Mole is a burrowing rodent
A mole is a blemish on the skin - sometimes
dangerous!
A Mole is a VERY large
number of atoms or
molecules
How much does an atom weigh?
The mass of a single atom is way too small to determine by weighing
The relative atomic mass m
agiven in the periodic table gives us a comparison of the average masses of
different atom types
This allows us to compare the number of atoms in a given mass of substance
There are as many atoms in 1 gram of hydrogen as there are in 12 grams of carbon
Atomic mass
Proton Neutron Electron
Hydrogen Helium Carbon
Mass = 4 Mass = 12
Mass = 1
} Nucleons
How many atoms?
So if we know 1 gram of hydrogen contains the exact same amount of atoms as 12 grams of carbon, we would only have to go through the effort to count them out once
The number of atoms in one gram of hydrogen, or 12 grams of carbon, was determined by
Amadeo Avogadro
Avogadro’s Number
The number of atoms in one gram of hydrogen is:
602214085700000000000000
This number is known as the Avogadro constant (symbol N
A)
This is quite tedious to write, which is why the scientific notation is often used:
6,02 x 10
23Amadeo Avogadro
1776-1856
What does the mole say?
Because Avogadro‘s number is quite tedious to even say, chemists use the equivalent term mole (dt. Mol) to express amounts of atoms or molecules
Using a word to express a specific number of things is not that uncommon by the way!
Examples
A dozen = 12
A baker‘s dozen = 13
Une grosse (fr.) = 12x12 = 144 [daher dt. en gros]
A mole = 6,02 x 10
23…
Just to get an idea…
There are roughly
10.000.000.000.000.000 ants on earth (10 x 10
15).
How many moles (6,02 x 10
23) of ants is that?
How many molecules are there in 2
moles of water?
Amounts of substance
A substance amount (dt. Stoffmenge) is
usually given in multiples of moles
To convert a number of atoms to a substance amount, it is divided by N
A The symbol for the
substance amount is n,
its unit mol
Molar mass
relative atomic mass m
a(in u)
molar mass M (in )
By definition, the atomic
masses in the periodic table are exactly equal to the
mass of one mole (= molar mass, dt. molare Masse) of
any given atom type
The symbol for the molar
mass of an atom or molecule is M, its unit
Since we combine atoms into molecules, the molar mass of any molecule is the sum of its molar masses!
Molar mass
The molar mass of water (H2O) is M (H2O) = 2 x M(H) + M(O)
= 2 x 1 + 16
= 18
Determine the molar mass M for oxygen (O2), methane (CH4) and chloroform (CHCl3)
By the way: When doing calculations with molar masses, it‘s enough to only use the first decimal
BONUS: How many units (u) are there to a gram (g)?
Molar concentrations
Molar concentrations tell us
how many moles of a substance are dissolved in 1 liter of a
solution
The symbol for the molar concentration is c, it‘s unit
Since the volume of a dilute solution barely changes when the substance is dissolved, we can calculate the concentration of a solution we make in
advance
Molar concentration
The molar mass of NaOH is 40
If 40 g of NaOH are dissolved in 1
litre of solution, the concentration of the resulting solution is c = 1
Determine the concentration of 1 litre of solution containing 117 g of sodium
chloride (NaCl)
Determine the mass of sodium hydroxide (NaOH) in 500 ml of a solution with c = 4
Tasks
Work through the rests of the tasks on
worksheet 19
EXTENSION:
What is the molarity of water?
Water is H
2O, water has a molar mass of
There are of water in a liter
How many moles of
water are there in one liter?
EXTENSION 2:
The Avogadro constant’s unit
If you take a look at german year 9 chemistry books, things can get a bit confusing. The number 6,02 x 10
23is referred to as „Loschmidtzahl“ N
L The Avogadro constant N
Aon the other hand is used to convert a
number of atoms or molecules (Teilchenzahl N) to substance amounts n via the equation
Because of this the actual Avogadro constant has a unit. Can you figure out what it is?
The grand finale:
Mole Traffic lights
You have to decide, whether the statements are true or false
False Sit down
Don‘t know Stand up
True Stand up with hand in the air
The grand finale:
Mole Traffic lights
The mole is a unit
The mole is a mass
One mole of water (H2O) and one mole of glucose (C6H12O6) have the same mass
One mole of H2 contains the same number of molecules as one mole of CO2
One mole of H2 contains the same number of atoms as one mole of CO2
There are more atoms in 12 g of carbon than in 12 g of copper
The grand finale:
Mole Traffic lights
A mole is a molecule
The mole is a number
One mole of a substance contains Avogadro‘s number of atoms or molecules
One mole of a substance contains 6,02 x 1023 atoms or molecules
The amount of a substance is measured in kilograms
The amount of substance is a volume
I understand what a mole is
