THE MOLE
A MOLE IS:
23
6.02 x 10 of anything.
Na = Avogadro’s Number = 6.02 x 1023
In chemistry, we talk about moles of representative particles.
? ? ? So what’s a representative particle ? ? ?
Substance Examples Representative Particle
Element C, Mg, Fe Atoms
Diatomic element H2, O2, Br2, F2, I2, N2, Cl2 Molecules
Molecular compound H2O, CH3OH Molecules
Ionic compound NaCl, MgBr2 Formula Units
SO . . .
One mole of carbon contains 6.02 x 1023 atoms of carbon.
One mole of nitrogen contains 6.02 x 1023 molecules of nitrogen
On mole of NaI contains 6.02 x 1023 formula units of sodium iodide.
The mole is important because:
The mole concept allows us to weigh atoms in the lab.
The mole concept allows us to compare amounts of atoms in chemical reactions.
MOLAR MASS
the mass of a mole of any element or compound in grams
6
The mass of one mole of C
carbon atoms Carbon
12.011
HOW TO CALCULATE THE MOLAR MASS OF A COMPOUND:
1. You need to know how many atoms of each element are in a substance in order to calculate its
molecular weight.
For example, H2O has two atoms of hydrogen and one atom of oxygen.
2. You need to know the molar mass of each element in order to calculate the molar mass of the
compound. You can find these values on your periodic table.
3. Multiply each element's molar mass by how many atoms are present in the formula, then add
the answers.
To calculate the molar mass of H2O:
Hydrogen: two atoms, molar mass = 1.0079g/mol
Oxygen: one atom, molar mass = 15.9994g/mol
(2 x 1.0079g) + (1 x 15.9994g) = 18.0152g/mol
Here’s another example: Calculate the molar mass of Al2(SO4)3:
two atoms of aluminum and the molar mass of Al is 26.98g/mol
three atoms of sulfur and the molar mass of S is 32.06g/mol
twelve atoms of oxygen and the molar mass of O is 15.9994g/mol
(2 x 26.98g/mol) + (3 x 32.06g/mol) + (12 x 15.9994g/mol) = 342.1328g/mol
Calculate the molar mass of:
a) (NH4)2S
2) Fe2O3
3) KclO4
4) SF6
5) (NH4)2SO4
BASIC MOLE CALCULATIONS
Now we’re ready to do some basic calculations. Before we begin, let’s review the process we use to
convert from one unit to another in chemistry, called the factor-label method.
In this process, numbers and units are equally important!
Conversion factors are relationships that convert one quantity to another.
Multiply what is given by a conversion factor set up as a fraction. Flip the conversion factor
fraction if necessary to eliminate the units you’re trying to get rid of.
Get into the habit of putting a line through the units as they “divide out”. The only units
that are not “canceled” should be the correct units of the final answer. When all units
have been canceled except those needed for the final answer, you are ready to pick up
your calculator and find the number.
Example:
Convert 1000 grams to pounds. It is known that one pound is equal to 454 grams. This
becomes the conversion factor needed to work the problem.
Factor-label tips:
Setting up the factor-label problem is just as important as the answer
Form the habit of working neatly, canceling units, and circling the answer
Remember, units are just as important as numbers in the answer
CONVERSIONS HIGHWAY
Number of particles:
MASS MOLES Single atoms
Molecules
Formula Units
ATOMS
We can convert between MOLES and MASS by means of the MOLAR MASS
a. What is the mass of 3 moles of H2O?
b. How many moles of O2 are in 4.00 g of O2?
We can convert between MOLES and NUMBER OF PARTICLES by means of
AVOGADRO’S NUMBER
a. How many atoms are in 5 x 10-7 mol of gold, Au?
b. If we have 5 x 108 molecules of O2, how many moles of O2 do we have?
We can convert between MOLECULES/FORMULA UNITS and ATOMS by
counting the number of atoms.
a. How many atoms are in 1.00 molecule of CO2?
b. How many atoms are in 7.8 moles of CO2?
c. How many atoms are in 50.0 g of O2?
PRACTICE PROBLEMS
1. How many moles are present in the following samples?
a. 25.0 g of carbon
b. 0.353 g of MgO
c. 400.2 g of water
2. What is the mass in grams of the following:
a. 0.00100 mol of K2CO3
b. 3.50 mol of iron (II) chloride
c. 5 x 10-4 mol of hydrogen sulfide
3. Make the following conversions:
a. 1.00 mol of ammonium chloride to formula units
b. 2.5 mol of O3 to molecules
c. 0.003 mol of cadmium to atoms
4. Make the following conversions:
a. 200 x 1023 formula units of AgCl to moles
b. 6.02 x 1025 atoms of nitrogen to moles
c. 120.2 x 1015 molecules of H2 to moles
5. How many atoms are contained in one molecule of:
a. HC2H3O2
b. Ammonium chloride
c. (CH3)2CO
d. ammonium sulfate
6. Make the following conversions:
a. 3.655 mol of K2CO3 to atoms
b. 2.5 mol of O2 to atoms
c. 1.35 mol of ammonium chloride
7. Make the following conversions
a. 5.0 g of KClO3 to moles
b. 2.5 mol of HC2H3O2 to grams
c. 4.2 mol of NaCl to formula units
d. 1.2 x 1022 formula units of KI to moles
e. 7.8 mol of CH3OH to atoms