Grade 11 Chemistry
Quantities in Chemical Reactions
The Avogadro Constant & Mole
(the chemist’s dozen)
For convenience, we group items into smaller more measurable amounts.
Below are some common examples.
Item
Gloves
Quantity
Pair
Amount
2
Soft-Drinks
6 pack
6
Eggs
Dozen
12
Pens
Gross
144
Paper
Ream
500
Atoms/molecules
Mole
6.02 x 1023
How many gloves are there in one pair of gloves? _________________
2 Gloves x 1 Pair = 2 Gloves
Pair
How many soft-drinks are there in 2 6-packs of coke? ______________
How many eggs are there in 5 dozens of eggs? ______________
12 eggs x 5 Dozen = 60 eggs
Dozen
How many pens are there in ½ a gross of pens? ______________
How many sheets of paper are there in 10 reams of paper? ______________
How many oxygen molecules are there in 2 moles of Oxygen gas? ______________
6.02 x 1023 molecules x 2 Mole = 12.04 x 1023 molecules of Oxygen
Mole
Chemistry: The importance of counting atoms and molecules
In order for medicine to work, it must have the correct amount of active ingredient.
Atoms are so tiny and there are so many. Counting is time consuming and does not
make sense. Chemists group large numbers of atoms into measurable amounts.
The Definition of the Mole
One mole (1 mole) of a substance contains 6.02 x 1023 particles of that
substance.
Avogadro constant: 6.02 x 1023 particles = NA
Mole
Grade 11 Chemistry
Quantities in Chemical Reactions
Putting the Mole into perspective (Just like a dozen, the mole can be used for
any object):
If Avogadro’s number of sheets of paper were divided into a million equal piles, each
pile would be so tall that it would stretch from Earth to the sun and beyond.
If one mole of pennies were distributed equally among the 6 billion people on Earth,
then each person would have 1 x 1014 pennies, or 1 x 1012 dollars. In other words,
every person would be a trillionaire.
-Note: A mole of atoms is a lot of atoms yet does not take up a lot of space. Why is
that? Because atoms are super small….
Basic Mole Calculations
a) Converting moles to number of particles (molecules,
atoms, formula units) (dozens  # of eggs).
b) Converting number of particles to moles (# of eggs 
dozens).
a) Converting moles to number of particles.
When looking for number of particles or atoms: N = n x NA
NA = Avogadro constant (units = molecules) = 6.02 x 1023 molecules
Mole
Mole
Mole = n (units = mole)
Number of particles = N (units = particles)
Sample problem #1:
A sample contains 1.25 mol of nitrogen dioxide, NO2.
a) How many molecules are in the sample?
b) How many atoms are in the sample (# of molecules x # atoms found in the
compound)?
a) Step 1: Write the formula down: since you are looking for # of molecules, the formula is N =
n x NA
Step 2 write down you givens and your unknowns (what you are looking for):
Given: n = 1.25 mole, NA = 6.02 x 1023 molecules/mole,
unknown: N = ?
Step 3: plug the given information into the equation from step 1:
N = 1.25 mole x (6.02 x 1023) molecules = 7.53 x 1025 molecules
mole
Step 4: Statement: there are 7.53 x 1025 molecules of NO2 in 1.25 mole of NO2.
b) To find the atoms we have in the sample:
-In 1 molecule of NO2 we have 3 atoms (1 N and 2 O)
-In 2 molecules of NO2 we have 3 atoms x 2 molecules = 6 atoms
Molecule
-In this sample we have 7.53 x 1025 molecules of NO2
In 7.53 x 1025 molecules of NO2 we have:
(7.53 x 1025) molecule x 3 atoms = 2.25 x 1024 atoms
Molecule
Grade 11 Chemistry
Quantities in Chemical Reactions
Converting number of particles to moles
When looking for number of moles: n = N/ NA
NA = Avogadro constant (units = molecules)
Mole
Mole = n (units = mole)
Number of particles = N (units = particles)
Sample Problem #2:
How many moles are present in a sample of CO2, made up of 5.83 x 1024
molecules?
Step 1: Write the formula down: since you know you are looking for moles, the
formula will be n = N/ NA
Step 2: write your unknown and givens:
Unknown: n = ?
Given: N = 5.83 x 1024 molecules
Given: NA = 6.02 x 1023 molecules
mole
Step 3: plug the given information into the equation you wrote in step 1.
n = N/NA = 5.83 x 1024 molecules / 6.02 x 1023 molecules = 9.68 moles
mol
or
5. 83 x 1024 molecules x 1 mol
6.02 x 1023 molecules
Step 4: Statement: There are 9.68 moles of CO2 in 5.83 x 1024 molecules of CO2.
N = n x NA and n = N/ NA Sample Problems
MOST SCIENTISTS USE FACTOR LABEL METHOD (crossing out so that units
work out)
Grade 11 Chemistry
Quantities in Chemical Reactions
Introductory Mole Calculations Exercises
1. What is a mole?
2. What are the units of Avogadro’s Number?
3. How many atoms are there in 1.38 mol of B?
4. (a) How many molecules are there in 2.75 mol of CH4?
(b)How many atoms of hydrogen are there in part (a)?
5. How many moles are there in 4.70 x 1026 atoms?
6. How many formula units (molecules) are in 5.33 mol of CuCl2?
7. (a) How many copper atoms are in 5.33 mol of CuCl2?
(b) How many moles of Cl atoms are in 5.33 mol of CuCl2?
8. How many moles are present in 2.45 x 1023 molecules of CH4?
9. A sample of zinc oxide, ZnO, contains 3.28 x 1024 molecules of zinc oxide. A
sample of zinc metal contains 2.78 mol of zinc atoms. Which sample contains more
zinc? The compound or the element? Show your reasoning mathematically.)