The Mole’s Unit NOTES
Name_____________________________ Period_____
The Mole
The mole (also abbreviated as mol) is the SI unit of measure for the amount of a substance. It is based on
the number of representative particles in 12 g of C-12.
A representative particle is any kind of particle, atoms, molecules, formula units,
electrons or ions.
The number 6.02 x 1023 is called Avogadro’s number. This is the number of particles
in one mole.
So if you had one mole of M&M’s that would be equal to 6.02 x 1023 M&M’s!
602,000,000,000,000,000,000,000.0 pieces of anything
Molar Mass
>The mass in grams of one mole of any pure substance is called its molar mass.
>The molar mass of any element is equal to its atomic mass and has the unit of g/mol.
*Remember, find atomic mass (average atomic mass) by looking elements up on the periodic table!
Example:
The molar mass of Cu is 63.54 g/mol
one mole of Cu = 63.54 g
The molar mass of Au is 196.967 g/mol
one mole of Au = 196.967g
The molar mass of He is 4.003 g/mol
one mole of He = 4.003g
Molar Mass of Compounds
To determine the molar mass of a compound, you need to calculate the molar mass of each individual
element in the compound, and then add them all together.
The units for molar mass are always g/mol (grams / mole)
*Remember that the molar mass of each element is its atomic mass on the periodic table.
To find mass of each element in the compound
Element 1 = (# of moles) x (molar mass) = mass in grams
Element 2 = (# of moles) x (molar mass) = mass in grams
Then you add up all of your answers.
(Element 1 mass in g) + (Element 2 mass in g) = TOTAL Molar Mass for the Compound
Examples: H2O (Water)
Mass of H= (2 moles) x (1.008g) = 2.016g
Mass of O= (1 mole) x (15.999) = 15.999g
TOTAL Molar Mass of H2O = 2.016g + 15.999g =
NOTES PAGE 1
18.015g/mol
Find the molar mass of the following compound:
Calcium Phosphide (Ca3P2)
Mass of Ca= (3 moles) x (40.078g) = 120.234g
Mass of P = (2 moles) x ( 30.974g) = 60.148g
TOTAL Molar Mass of Ca3P2= 120.234 + 60.148g = 180.382g/mol
Aluminum Sulfite [Al2(SO4)3]
Mass of Al = (2 moles) x (26.982g) = 53.964g
Mass of S = (3 moles) x (32.066g) = 96.198g
Mass of 0 = (12 moles) x (15.999g) = 191.988g
TOTAL Molar Mass of Al2(SO4)3 = 53.964g + 96.198g + 191.988g = 342.150 g/mol
*Remember, you are still responsible for knowing how to write formulas of ionic compounds
See your Ionic Compounds Unit Homework Packet for how to write formulas!!
Questions can and will just give you the name of the compound and will require you to find the formula
before you are able to calculate the molar mass (just like the previous examples).
Put the following elements & compounds in order of INCREASING molar mass
(Smallest)
mass is…
___________H__________
Neon (Ne) _____________
__________Ne___________
Oxygen (O2) _____________
__________O2___________
Hydrogen (H2) _____________
______Table Salt_________
Table Salt (NaCl) _____________
_________Acetate________
Copper (Cu) _____________
_________Copper_________
Acetate (C2H3O2) _____________
(Biggest)
NOTES PAGE 2
Percent Composition
When we talk about compounds it can be useful to know what elements are in the compound, and
relatively how much of each element there is in the compound.
We talk about “how much” of each element in terms of percentages.
The percent composition then tells what percent of the whole compound is element 1,
what percent of the whole compound is element 2, and what percent of the whole compound is element 3.
If these 3 elements are the ONLY things in the compound, then the percentages should add up to 100%.
To calculate the % Composition:
% composition = (total mass of element/ mass of compound) x 100%
**Usually you have to find the mass of each element, and the
Total mass of the compound before you can calculate the % composition
Let’s find the % composition of the elements in water H2O :
Mass of H2
= 1.008 x 2 moles = 2.016g
Mass of O
= 15.999 x 1 mole = 15.999g
Mass of H2O = 2.016g + 15.999g = 18.015 g/mol of H2O
Mass of H2 = 2.016g
% H = (2.016 /18.015) x 100% = 11.19 % H
Mass of O = 15.999g
% O = (15.999/18.015) x 100% = 88.81% O
Total = 11.19% + 88.81% = 100% water
Practice:
Aluminum Oxide: find the % composition of Al and of O in this compound
Find the formula: Al2O3
Mass of Al2: 26.982 x 2 moles = 53.964g
Mass of O3: 15.999 x 3 moles = 47.997g
Molar Mass of Al2O3: 53.964g + 47.997g = 101.961g/mol of Al2O3
% Al: (53.964/101.961) x 100% =
0.52926 x 100% = 52.93%
% O: (47.997/101.961) x 100% =
0.4707 x 100% = 47.07%
Total = 52.93% + 47.07%= 100%
NOTES PAGE 3
Moles to Grams & Grams to Moles
We are able to convert between number of moles and the mass (in grams) of a substance easily by
using the molar mass of the atom or compound!
We just use dimensional analysis to solve these!
To convert moles of a compound to mass:
simply multiply the number of moles of the compound by the molar mass of the compound.
For example 8.55 moles of K2CrO4 would be calculated as follows:
2 mol K x 39.10 g/ mol = 78.20 g
1 mol Cr x 52.00 g/ mol = 52.00 g
4 mol O x 16.00 g/ mol = 64.00 g
Molar mass K2CrO4 = 194.20 g/mol
8.55 mols of K2CrO4 x 194.20 g = 1660.41 g
1
1 mole
Examples:
1.50 mol Potassium
= __________________ g
0.750 mol Aluminum
= __________________ g
3.850 mol Lithium Fluoride
= __________________ g
Going from mass to moles is the same process in reverse! It is still just dimensional analysis.
Examples:
148 g Argon
78.3 g Rubidium Fluoride
= __________________ mol
= __________________ mol
NOTES PAGE 4
Moles to Particles & Particles to Moles
**Particle is generic way of saying atom, formula units, or molecule
Different ways of saying the same thing:
Atom: We say atom when it is just one pure element with no bonds (Ex. Al , Cu, C, Ne…1 element)
Formula Unit: when we are talking about ionic compounds (metal bonded with nonmetals) Ex. NaCl
Molecule: is for covalent compounds (two or more nonmetals bonded together) Ex. CO2
It’s nice and easy when we talk about 1 mole at a time. But sadly, not always the case.
Let’s try thinking about eggs first, since they are easier to visualize and we are more comfortable with “a
dozen eggs” than we are “a mole of atoms”
It’s dimensional Analysis!
If you have 1 dozen eggs you have = 12 eggs
If you have 1.75 dozen eggs you have…
1.75 x 12 = 21 = 21 eggs
Moles to Atoms Calculations
If you have 1 mole of Copper you have = 6.02 x 1023 atoms of Cu
If you have 1.75 moles of Copper you have = 1.0535 x 1024 atoms of Cu
1.75 x (6.02 x 1023) = (1.0535 x 1024)atoms of Cu
It’s dimensional Analysis!
(what you are given) (what you are looking for)
“Disco” your units down
Examples:
3.25 mol AgNO3
= __________________ formula units AgNO3
11.5 mol H2O
= __________________ molecules H2O
0.4 mol of Au
= ___________________ atoms Au
NOTES PAGE 5
Going from particles to moles is the same process in reverse! But it’s still just dimensional analysis.
If you have 12 eggs you have = 1 dozen
If you have 27 eggs you have = 2.25 dozen
27 eggs / 12 per dozen = 2.25 dozen
Atoms to Moles Calculations :
If you have 6.02 x 1023 atoms of Lithium you have = 1 mol Li
If you have 2.0468 x 1024 atoms of Lithium you have = 3.4 mol Li
Examples:
2.58 x 1024 atoms of Calcium
= __________________ mol Ca
3.6 x 1023 molecules of CH4 = __________________ mol CH4
2.04 x 1024 formula units of KBr
= __________________ mol KBr
Grams to Particles & Particles to Grams
We are able to go directly from mass (in grams) to number of particles (atoms, formula units, or
molecules) by using Avogadro’s number and the molar mass.
Examples:
29 g Helium
= __________________ atoms He
36.08 g CO2
(44.009 g is molar mass)
= __________________ molecules CO2
0.50 g ZnS
(97.386 is molar mass)
= __________________ formula units ZnS
Going from particles to mass is the same process in reverse! But it is still just dimensional
analysis
Examples:
1.98 x 1023 atoms Titanium
= ________________ g Ti
8.500 x 1023 atoms Xe
= ________________ g Xe
1.187 x 1024 molecules Cl2
= ________________ g Cl2
NOTES PAGE 6