Unit “5A”: Chemical Quantities –
Name: ______________________________
Understanding the mathematical relationships given by a chemical formula.
I.
The number of atoms of an element in a formula is indicated by the subscript found to the lower right of the
element. When more than 1 molecule or formula unit is specified, multiply the subscript by the given
number of molecules or formula units.
II. Working with a “fistful” of particles so small that they are individually invisible (such as atoms, molecules,
and compounds), requires that we weigh (rather than count) the atoms, molecules, and compounds that
we work with in chemistry.
a. Scientists use a unit, called the “mole”, which is equal to 6.02 x 10 23 representative particles.
b. 1 mole is also referred to as “Avogadro’s number” in honor of Amedeo Avogadro, an early chemist.
c. 1 mole = Avogadro’s number = 6.02 x 10 23 representative particles.
d. A “representative particle” is either an atom, a molecule, a formula unit, or an ion, depending on the
substance it refers to. “Representative particle” refers to the smallest unit of a substance that has the
properties of the whole substance.
i. For metals: the representative particle is an atom.
1. 1 mole Cu = 6.02 x 10 23 atoms of Cu
ii. For molecular substances (both diatomic elements and molecular compounds): the
representative particle is a molecule.
1. 1 mole H2 = 6.02 x 10 23 molecules of H2
2. 1 mole C6H12O6 = 6.02 x 10 23 molecules of C6H12O6
iii. For ionic compounds: the representative particle is a “formula unit” (the whole formula).
1. 1 mole NaCl = 6.02 x 10 23 formula units of NaCl
i.
For solutions of ionic compounds: the representative particle is an ion. No test questions.
Type of substance
Representative Particles
Metals
atoms
Molecular substances
molecules
Ionic compounds
formula units
III. This next set of exercises is focused preparation for the upcoming midterm exam (Jan 16,17, 2014).
a. How to calculate the “the mass of 1 mole”, also referred to as the “molar mass”, of a substance:
-Add together the (molar mass x the subscript) for each element present in the representative unit of
the substance. The molar mass for each element is found within the element’s square on the periodic
table.
-Is there anything to watch out for?
*Many ionic compounds contain polyatomic ions which may be enclosed within parenthesis
and followed by a subscript. In this case, you must understand that the subscript outside the
parenthesis is to be distributed over the atoms within the parenthesis.
*Whenever there is a diatomic element mentioned by name (for example, “nitrogen”,
“oxygen”, etc.), you must understand that it’s symbol would contain a subscript of 2, and that
symbol is the representative unit.
*On the midterm, round the final “molar mass” to the tenths position.
Example 1- What is the molar mass of copper, or, what is the mass of 1 mole of copper?
Find copper on the periodic table; rounded to tenths:
63.6 g
Example 2- What is the molar mass of hydrogen gas, or, what is the mass of 1 mole of hydrogen gas?
Since hydrogen is a diatomic element, “hydrogen” in the question means: H2
Find hydrogen on the periodic table:
2 (1.0) = 2.0 g
Example 3- What is the molar mass of Al(NO3)3, or, what is the mass of 1 mole of Al(NO3)3?
Find Al, N, and O on the periodic table:
Remember to distribute the “3” over the (NO3); calculate the sum.
1 Al
molar mass =
= 27.0
1x3= 3N
molar mass = 3 (14.01)
=
3x3= 9O
molar mass = 9 (16.00)
= 144.0
The final sum is:
42.0
= 213.0 g
Example 4 - What is the molar mass of BaCl2, or what is the mass of 1 mole of BaCl2?
Find Ba and Cl on the periodic table; calculate the sum of the molar masses.
137.3 + 2 (35.5) =
208.3 g
Example 5- Find the molar mass of (NH4)2Cl
1Nx2=
2N
molar mass = 2 (14.0)
=
28.0
4Hx2=
8H
molar mass = 8 (1.0)
=
8.0
1 Cl
The final sum is:
molar mass = 35.5
35.5
=
71.5 g
b. How to calculate the “mass” of a substance, when given a number of moles of the substance.
Step 1: Calculate the molar mass of the substance. This will be used as a factor within a factor label problem.
Step 2: Start your setup with the amount of the substance given.
Step 3: Multiply by the molar mass to convert to mass.
Example 6: What is the mass in grams of 8.215 moles of N2O4?
Calculate the molar mass of N2O4:
2N=
molar mass = 2 (14.0)
=
28.0
4O=
molar mass = 4 (16.0)
=
64.0
=
92.0 g
The final sum is:
Setup:
8.215 moles N 2 O4  92.0 grams N 2 O4 
  756 grams N 2 O4
 
1
mole
N
O
2 4


c. How to calculate the “mass” of a substance, when given a number of molecules of the substance:
Step 1: Calculate the molar mass of the substance. This will be used as a factor within a factor label problem.
Step 2: Start your setup with the number of molecules given.
Step 3: Divide by 6.02 x 10 23 molecules to convert to moles.
Step 4: multiply by the molar mass to convert to mass.
Example 7: What is the mass of 2.25 x10 23 molecules of CH4?
2.25 x10 23 moleculesC H 4
d.

  16.0 g CH 4 
1moleCH 4
  
  6.0 g CH 4
 
23
 6.02 x10 molecules CH 4   1moleCH 4 
How to calculate the volume of a gas at STP (standard conditions), when given the number of moles.
Step 1: Start your setup with the number of moles given.
Step 2: Multiply by the “molar volume” to convert to liters.At STP, 1 mole of any gas has a volume equal to 22.4 L.
Example 8: What is the volume of 3.50 moles of CO2 at STP?
3.50 molesCO2
e.
 22.4 LCO2 
  78.4 LCO2
 
 1moleCO2 
How to calculate the volume of a gas at STP when given its mass.
Step 1: Calculate the molar mass of the gas. This will be used as a factor within a factor label problem
Step 2: Start your setup with the mass given.
Step 3: Divide by the molar mass to convert to moles.
Step 4: Multiply by the molar volume to convert to liters.
Example 9: What is the volume of 9.34 g of O2 at STP?
9.34 g O2
 1moleO2   22.4 LO2 
  
  6.54 LO2
 
 32.0 g O2   1moleO2 