Chemical Quantities Moles
Chapter 10
Counting Particles


Particles are counted in moles
Types of representative particles
• Atoms- smallest unit of an element
• Molecule- smallest unit of a molecular
•
compound
Formula Unit- smallest unit of an ionic
compound
Why?


Atoms are
inconceivably
small
Avogadro’s
Number- 6.02 x
1023 is the number
of carbon-12
atoms in 12g
Understanding Reactions



N2
+ 3H2

2NH3
1 molecule of nitrogen and 3 molecules of
hydrogen produce 2 molecules of nitrogen
trihydride
1 mole of nitrogen molecules and 3 moles of
hydrogen molecules produce 2 moles of
nitrogen trihydride molecules
Counting Particles

Moles → Particles (multiply by
Avogadro’s number)
• Example: How many atoms are there in 3
moles of aluminum?

Particles → Moles (divide by AN)
• Example: How many moles are there in
5.71x1027 formula units of calcium chloride?
Mass and the Mole


MM (molar mass)- mass of 1 mole of
representative particles in a compound or
element
Converting Between Moles and Grams
•
•
Grams → Moles (divide by molar mass)
• Example: How many moles are there in 135.2 g of
water?
Moles → Grams (multiply by MM)
• Example: How many grams are there in 3.46 moles
of calcium carbonate?
Volume and the Mole

Under a specific set of conditions, namely STP
(standard temperature and pressure, 0oC and 1
atm)
1 mole = 22.4 L

Converting Between Moles and Volume
•
•
Volume → Moles (divide by 22.4 L)
• Example: How many moles are there in 3.8L of SO3 at STP?
Moles → Volume (multiply by 22.4 L)
• Example: What volume, in liters, does 0.5 moles of Cl2 occupy
at STP?
Density, Volume and Moles

If provided with the density of a gas at
STP, then using the volume of 22.4L will
allow you to calculate the molar mass of
the gas.
The density of a gas containing carbon and oxygen at
STP is 1.25g/L. What is the molar mass of this gas?
What is its identity?
All Roads Go Through Moles

Calculations between mass/particles,
particles/volume, and mass/volume are possible
by first calculating the number of moles.
@STP
Ultimately………..
Percent by Mass (Percent
Composition)


Percentage of each element in the mass
of a compound.
What is the percent composition of each
element in sodium phosphate?
Practice


What is the percent composition of
Mg(OH)2?
What is the mass of Mg in 97.4g
Mg(OH)2?
Determining an Empirical
Formula

Empirical Formula- lowest whole number
ratio of elements in a compound
•
•
•
•
•
Use % composition and assume 100g
Divide by MM of each element to get moles
Divide all by lowest number of moles
Maybe Multiply
Record empirical formula
Example: a compound is determined to be 36.4%
nitrogen and 63.16% oxygen, what is the empirical
formula?
Practice
Find the empirical formula of a
compound that is 72.4% Fe & 27.6% O.
Determining a Molecular Formula

Molecular Formula- multiple of empirical
formula that accurately shows number of
each atom
•
•
•
•
Calculate mass of empirical formula (EFM)
Divide MM of the compound by the EFM
Multiply empirical formula by result
Record molecular formula
Example: A compound has a molar mass of 110.0 g/mol.
What is the molecular formula if it is 65.45% C, 5.45%
H, and 29.09% O?
Practice

Find the molecular formula of a
compound that has an empirical formula
of C3H5O2 and a gfm = 146 g/mol.
TEST


Multiple Choice and....
Elegant Problem Solving