Numbers And Units Worksheet With Answer Key Page 4
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We can use the same trick to make sure we get the right equation when we want to work out the
mass from the molar mass and the number of moles. There are three ways in which M and n can
be combined:
M
n
M × n,
and
and it can be easy to pick the wrong ones.
n
M
A quick dimensional analysis check helps:
–1
M × n has units of g mol
× mol = g
−
1
M
g mol
–2
has units of
= g mol
n
mol
n
mol
–1
2
has units of
= g
mol
−
1
M
g mol
We are trying to calculate a mass (unit = “g”). This means that our formula must be:
m = M × n
The mass is the molar mass times the number of moles.
A convenient way to help remember these relationships is to use the ‘triangle method’:
Use a finger to cover up the quantity you want to calculate.
The other two values show you how to do the calculation.
To calculate the mass (m), cover it up and you are left with
“n M”, hence m = n × M.
To calculate the molar mass (M), cover it up and you are left with
m
m
"
"
, hence M =
.
n
n
Concentration (c), volume (V) and the number of moles (n)
–1
The units of concentration are “mol L
”. This is read as “moles per litre” and means moles
divided by litres. Hence:
moles
–1
concentration units = mol L
=
litres
From the units, we have worked out that the equation must be:
number of moles
concentration =
volume
n
c =
V
The concentration is the number of moles divided by the volume.
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