Chemistry Unit 7: Moles
C8A- I will define & use the concept of
a mole.
C8B – I will use the mole concept to
calculate the number of atoms, ions, or
molecules in a sample of material.
C8C – I will calculate percent composition
& empirical and molecular formulas.
created by C. Johannesson modified by T.
Howard
I
II
III
IV

A. mole – amount of substance that has particles
equal to the number of atoms in 12g of carbon 12
OR the amount of substance that
has Avogadro’s # of particles --6.02 x 1023 particles
A
large amount!!!!
A. What is the Mole?
1 mole of hockey pucks would equal the
mass of the moon!
1 mole of basketballs would fill a bag the
size of the earth!
1 mole of pennies would cover the Earth
1/4 mile deep!

B. Avogadro’s number – number of
particles in 1 mole of a pure
substance 6.02 x 1023
C. atomic mass number – number of protons +
neutrons; measured in amu’s
---this is NOT on the p-table
D. average atomic mass number – average
weighted mass of an atom/element
---this is on the periodic table
If you change the units from amu to grams you
now have the amount of grams in 1 mole of the
element – something you can actually measure
on a scale!
avg atomic mass calculation
( % isotope #1) ( mass isotope #1) + ( % iso #2) ( mass iso #2) +…
100
C6D Use isotopic composition to calculate average
atomic mass of an element.
The natural abundance for boron isotopes is: 19.9%
10B (10 amu) and 80.1% 11B (11amu). Calculate the
atomic weight of boron.
[(19.9 × 10) + (80.1 × 11)]
100
avg atomic mass is 10.801 amu

E. Molar mass – mass of 1 mole of a
pure substance----units are grams/mole
or g/mol

Turn to p.7 in your packet (Molar Mass Wkst 3)!
Look at right side of page & we will do a couple
of examples…the rest is for you to complete


1. Molar mass is “equal” to the mass
# of an element in amu’s, so if an
element has an mass # of 39.0983
amu’s then the molar mass is
39.0983 g/mol you just change the
unit of measurement!

F. formula mass aka formula weight–
sum of all the mass numbers of the
atoms in a compound

the term formula mass is usually associated with ionic
compounds &
the term molecular mass is usually associated with
covalent compounds
they’re the same basic thing


from the
periodic table
#3 – BeCl2
Be = 1 x 9.012 g =
9.012 g
Cl = 2 x 35.453 g = + 70.906 g
79.918 g

number of atoms
in the formula
formula mass
of BeCl2 aka
molar mass
#7 – Mg(OH)2
Mg = 1 x 24.305 = 24.305 g
O = 2 x 15.999 = 31.998 g
H = 2 x 1.008 =
2.016 g
f.m. of Mg(OH)2 = 58.319 g

You need to do the rest on your own!

G. percent composition- percent by
mass of each element in a compound
mass of element
x 100 = % element in cmpd
mass of total cmpd
What percent by mass of Mg(OH)2 is
oxygen?
 find the formula mass of the Mg(OH)2
Mg = 1 x 24.305 = 24.305 g
O = 2 x 15.999 = 31.998 g
H = 2 x 1.008 =
2.016 g
f.m. of Mg(OH)2 = 58.319 g

mass of element
x 100 = % element in cmpd
mass of total cmpd
31.998g O
x 100 = 54.87% oxygen by mass
58.319g Mg(OH)2
That means more than half of the mass of the Mg(OH)2
(what is the name of the compound?) is due to oxygen!
H. % error= (observed value - actual value) x 100
actual value
Observed value refers to the data you got in the lab while
actual value is the “correct” or “book” value/answer.