Stoichiometric Problems I:
Using the chemical equation to relate
moles of chemicals
In stochiometric problems we relate the amount of
one compound in a chemical equation to some other
compound
p
in the same equation.
q
The first, and easiest type of problem, is to relate the
number of moles of one compound to the moles of
another compound. Sometimes we do this to
calculate how much of the other reactant we need to
complete a reaction, or other times we do this to
calculate how much product is formed.
formed This tutorial
starts by introducing the simplest, mole-to-mole
calculations. Other tutorials in the section do
gram-to-gram
gram
to gram calculations,
calculations and them we move into
‘limiting reagents’ and % yield calculations.
Stoichiometric Problems I:
Using the chemical equation to relate
moles of chemicals
Here is a balanced chemical equation:
2 LiOH(s)
( ) + 1 H2SO4((aq)
q) = 1 Li2SO4((aq)
q) + 2 H2O(l)
()
This equation tells you that 2 moles of LiOH will react
with 1 mole of H2SO4. Once the reaction occurs,
occurs 1
mole of Li2SO4 and 2 moles of water (H2O) will be
formed.
Thi links
This
li k the
th molar
l coefficients
ffi i t to
t each
h other
th in
i the
th
following manner:
2 mole
l LiOH
iO = 1 mole
l H2SO4 = 1 mole
l Lii2SO4 = 2 mole
l H2O
Stoichiometric Problems I:
Using the chemical equation to relate
moles of chemicals
Just like in a unit conversion problem where we use the equation 1
inch = 2.54 cm to make a conversion factor between inches and
cm, you can use the molar equalities to make conversion factors.
Y
You
can then
h
use these
h
conversion
i
factors
f
to convert from
f
moles
l
of any one chemical to moles of any other in a specific reaction.
The single
g equality
q
y 2 mole LiOH = 1 mole H2SO4 g
gives y
you 2
conversion factors:
2 mole LiOH/1 mole H2SO4
-and1 mole H2SO4/2 mole LiOH
Since there are several other equations there are lots more
conversions.
Stoichiometric Problems I:
Using the chemical equation to relate
moles of chemicals
Now let’s
N
l ’ use one off these
h
conversions
i
in
i a problem.
bl
Given the balanced chemical equation:
2 LiOH(s) + 1 H2SO4(aq) = 1 Li2SO4(aq) + 2 H2O(l)
If I started with 4.5 moles of LiOH, how many moles
of H2SO4 would I need to complete my reaction?
Start unit: Moles LiOH
End unit : Moles H2SO4
Equation relating units: 2 moles LiOH = 1 mole H2SO4
Equation: Moles LiOH x (1 mole H2SO4/2 moles LiOH)
Solution: 4.5
4 5 mol LiOH x (1 mole H2SO4/2 moles LiOH)
= 2.25 mol H2SO4
Stoichiometric Problems I:
Using the chemical equation to relate
moles of chemicals
T another,
Try
th
given
i
the
th balanced
b l
d equation:
ti
N2(g) + 3H2(g) = 2NH3(g)
How many moles of N2 and H2 will I need to make 5
moles of NH3?
N2: Start Unit: Mole of NH3
End Unit: Mole of N2
C
Conversion
i
equation:
ti
1 mole
l N2 = 2 mole
l NH3
Equation: NH3 x (1mole N2/2mole NH3)
Solution: 5 mole NH3 x (1 mole N2/2 mole NH3)
= 2.5 mole N2
Stoichiometric Problems I:
Using the chemical equation to relate
moles of chemicals
Continuing with H2:
N2(g) + 3H2(g) = 2NH3(g)
How many moles of H2 will I need to make 5 moles of
NH3?
H2: Start Unit: Mole of NH3
End Unit: Mole of H2
C
Conversion
i
equation:
ti
3 mole
l H2 = 2 mole
l NH3
Equation: NH3 x (3mole H2/2mole NH3)
Solution: 5 mole NH3 x (3 mole N2/2 mole NH3)
= 7.5 mole N2
Stoichiometric Problems I:
Using the chemical equation to relate
moles of chemicals
Here is a problem to try on your own:
Given the balanced chemical reaction:
Ca3P2(s) + 6H2O(l) = 3Ca(OH)2(s) + 2 PH3(g)
How many
y moles of Ca3P2((s)) and H2O(l)
( ) do I need
to make 3 moles of PH3(g)
(Answer on next slide. Try it on you own first)
Stoichiometric Problems I:
Using the chemical equation to relate
moles of chemicals
Ca3P2(s) + 6H2O(l) = 3Ca(OH)2(s) + 2 PH3(g)
How many moles of Ca3P2(s) and H2O(l) do I need
to make
k 3 moles
l off PH3(g)
( )
Ca3P2:
3 moles PH3 x (1 mole Ca3P2 / 2 mole PH3) =
= 1.5 mole Ca3P2
H2O:
3 moles PH3 x (6 mole H2O / 2 mole PH3) =
=18 mole of H2O