Stoichiometric Problems II:
Using the chemical equation to relate
grams of chemicals
The next level of complexity is to start with a gram
amount of some chemical in a reaction and then
calculate grams amounts of other chemicals in the
reaction.
The problem really isn’t that much harder, you just
need to take three steps: convert from grams to
moles,
l
then
th
do
d the
th mole
l to
t mole
l conversion,
i
then
th
do
d
a conversion from mole back to grams. Let’s try this
using the same examples we used in the last section.
Stoichiometric Problems II:
Using the chemical equation to relate
grams of chemicals
Given the balanced chemical equation:
2 LiOH(s)
( ) + 1 H2SO4((aq)
q) = 1 Li2SO4((aq)
q) + 2 H2O(l)
()
If I started with 25 grams of LiOH, how many grams
of H2SO4 would I need to complete my reaction?
Start unit: grams LiOH
End unit : grams H2SO4
1st conversion:
i
grams LiOH tto moles
l LiOH
2nd conversion: moles LiOH to moles H2SO4
3rd conversion: moles H2SO4 to grams H2SO4
Stoichiometric Problems II:
Using the chemical equation to relate
grams of chemicals
Given the balanced chemical equation:
2 LiOH(s)
( ) + 1 H2SO4((aq)
q) = 1 Li2SO4((aq)
q) + 2 H2O(l)
()
If I started with 25 grams of LiOH, how many grams
of H2SO4 would I need to complete my reaction?
Equation used in
1st conversion: 23.949 grams LiOH = 1 moles LiOH
2nd conversion:
i
2 moles
l LiOH =1
1 mole
l H2SO4
3rd conversion: 1 moles H2SO4 = 98.086 grams
H2SO4
Stoichiometric Problems II:
Using the chemical equation to relate
grams of chemicals
Given the balanced chemical equation:
2 LiOH(s)
( ) + 1 H2SO4((aq)
q) = 1 Li2SO4((aq)
q) + 2 H2O(l)
()
If I started with 25 grams of LiOH, how many grams
of H2SO4 would I need to complete my reaction?
Equation:
mole LiOH Mole H 2 SO4 gram H 2 SO4
g LiOH ×
×
×
=
gram LiOH
Mole LiOH mole H 2 SO4
Stoichiometric Problems II:
Using the chemical equation to relate
grams of chemicals
Given the balanced chemical equation:
2 LiOH(s)
( ) + 1 H2SO4((aq)
q) = 1 Li2SO4((aq)
q) + 2 H2O(l)
()
If I started with 25 grams of LiOH, how many grams
of H2SO4 would I need to complete my reaction?
Plugging in the numbers:
1 mole LiOH
iO
1 Mole
ole H 2 SO4 98.086 g
gram
am H 2 SO4
25 g LiOH ×
×
×
=
23.949 gram LiOH 2 Mole LiOH
1 mole H 2 SO4
= 51 g H2SO4
Stoichiometric Problems II:
Using the chemical equation to relate
grams of chemicals
Try another, given the balanced equation:
N2(g) + 3H2(g) = 2NH3(g)
How many grams of N2 and H2 will I need to make 15
grams of NH3?
N2:
Start Unit: grams of NH3
End Unit: grams of N2
1st conversion: grams of NH3 to moles of NH3
2nd conversion: moles of NH3 to moles of N2
3rd conversion moles of N2 to grams of N2
Stoichiometric Problems II:
Using the chemical equation to relate
grams of chemicals
Try another, given the balanced equation:
N2(g) + 3H2(g) = 2NH3(g)
How many grams of N2 and H2 will I need to make 15
grams of NH3?
Conversion equations:
1st conversion 17.034 g NH3 = 1 mole NH3
2nd conversion 1 mole N2 = 2 mole NH3
3rd conversion 1 mole N2 = 28.02 g N2
Stoichiometric Problems II:
Using the chemical equation to relate
grams of chemicals
Try another, given the balanced equation:
N2(g) + 3H2(g) = 2NH3(g)
How many grams of N2 and H2 will I need to make 15
grams of NH3?
Conversion equation:
mole
l NH 3
M l N2
Mole
gram N 2
g NH 3 ×
×
×
gram NH 3 Mole NH 3 mole N 2
Stoichiometric Problems II:
Using the chemical equation to relate
grams of chemicals
Try another, given the balanced equation:
N2(g) + 3H2(g) = 2NH3(g)
How many grams of N2 and H2 will I need to make 15
grams of NH3?
Conversion equation:
1 mole
l NH 3
1 Mole
M l N2
28.02 gram N 2
15g NH 3 ×
×
×
17.034 gram NH 3 2 Mole NH 3
1 mole N 2
= 12 g N2
Stoichiometric Problems II:
Using the chemical equation to relate
grams of chemicals
Try another, given the balanced equation:
N2(g) + 3H2(g) = 2NH3(g)
How many grams of N2 and H2 will I need to make 15
grams of NH3?
H 2:
Start Unit: grams of NH3
End Unit: grams of H2
1st conversion: grams of NH3 to moles of NH3
2nd conversion: moles of NH3 to moles of H2
3rd conversion moles of H2 to grams of H2
Stoichiometric Problems II:
Using the chemical equation to relate
grams of chemicals
Try another, given the balanced equation:
N2(g) + 3H2(g) = 2NH3(g)
How many grams of N2 and H2 will I need to make 15
grams of NH3?
Conversion equations:
1st conversion 17.034 g NH3 = 1 mole NH3
2nd conversion 3 mole H2 = 2 mole NH3
3rd conversion 1 mole H2 = 2.016 g H2
Stoichiometric Problems II:
Using the chemical equation to relate
grams of chemicals
Try another, given the balanced equation:
N2(g) + 3H2(g) = 2NH3(g)
How many grams of N2 and H2 will I need to make 15
grams of NH3?
Conversion equation:
mole
l NH 3
M l H2
Mole
gram H 2
g NH 3 ×
×
×
gram NH 3 Mole NH 3 mole H 2
Stoichiometric Problems II:
Using the chemical equation to relate
grams of chemicals
Try another, given the balanced equation:
N2(g) + 3H2(g) = 2NH3(g)
How many grams of N2 and H2 will I need to make 15
grams of NH3?
Conversion equation:
1 mole
l NH 3
3 Mole
M l H2
2.016 gram H 2
15g NH 3 ×
×
×
17.034 gram NH 3 2 Mole NH 3
1 mole H 2
= 2.7g H2
Stoichiometric Problems II:
Using the chemical equation to relate
grams of chemicals
Try another, given the balanced equation:
N2(g) + 3H2(g) = 2NH3(g)
How many grams of N2 and H2 will I need to make
15 grams of NH3?
In this particular problem we have a built in check.
Since matter cannot appear or disappear in a
chemical reaction
reaction, the sum of the masses of the
reactant should equal the mass of the product. Our
two reactant masses, 2.7 g of H2 and 12 g of N2
d
does
add
dd up to 15 g off NH3
Stoichiometric Problems II:
Using the chemical equation to relate
grams 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 grams
g
of Ca3P2((s)) and H2O(l)
( ) do I need
to make 30 grams of PH3(g)
(Answer on next slide. Try it on you own first)
Stoichiometric Problems II:
Using the chemical equation to relate
grams 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 grams
g
of Ca3P2((s)) and H2O(l)
( ) do I need
to make 30 grams of PH3(g)
Ca3P2(s) = 80 g
1 mole Ca 3 P2 182.18 g Ca 3 P2
1 mole PH 3
30 g PH 3 ×
×
×
33.994 g PH 3 2 mole PH 3
1 mole Ca 3 P2
Stoichiometric Problems II:
Using the chemical equation to relate
grams 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 grams
g
of Ca3P2((s)) and H2O(l)
( ) do I need
to make 30 grams of PH3(g)
H2O(l) = 47.7 g
6 mole H 2 O 18.016 g H 2 O
1 mole PH 3
30 g PH 3 ×
×
×
33.994 g PH 3 2 mole PH 3
1 mole H 2 O