A Brief about Stoichiometry

Contributed by:

Jonathan James Mon, Jan 17, 2022 08:46 PM UTC

The highlights are:
1. Stoichiometry
2. Stoichiometry questions
3. Moving along the stoichiometry path
4. Converting moles to grams

2. “stochio” = Greek for element
“metry” = measurement
Stoichiometry is about measuring the amounts of
elements and compounds involved in a reaction.
Consider the chemical equation:
4NH3 + 5O2  6H2O + 4NO

3. Stoichiometry 4NH3 + 5O2  6H2O + 4NO
Recall that Chemical formulas are balanced with coefficients
4 X NH3 = 4 nitrogen + 12 hydrogen
5 X O2 = 10 oxygen
6 X H2O = 12 hydrogen + 6 oxygen
4 X NO = 4 nitrogen + 4 oxygen

4. 4NH3 + 5O2  6H2O + 4NO
With Stoichiometry we find out that
Coefficients like
4:5:6:4
convey the ratio of substances need for a reaction to occur, in terms of moles
( mole ratio)
4:5:6:4

5. 4NH3 + 5O2  6H2O + 4NO
4:5:6:4
Can mean either:
4 molecules of NH3 react with 5 molecules of O2
to produce 6 molecules of H2O and 4 molecules of NO
OR
4 moles of NH3 react with 5 moles of O2
to produce 6 moles of H2O and 4 moles of NO

6. Stoichiometry Question (1)
4NH3 + 5O2  6H2O + 4NO
 Howmany moles of H2O are produced if 2.00
moles of O2 are used?
6 mol H2O
2.00 mol O2 = 2.40 mol H2O
5 mol O2
Notice that a correctly balanced equation is
essential to get the right answer

7. Stoichiometry Question (2)
4 NH3 + 5 O2  6 H2O + 4 NO
How many moles of NO are produced in the
reaction if 15 mol of H2O are also produced?
4 mol NO
15 mol H2O = 10. mol NO
6 mol H2O

8. Stoichiometry Question (3)
4 NH3 + 5 O2  6 H2O + 4 NO
 How many grams of H2O are produced if 2.2 mol
of NH3 are combined with excess oxygen?
2.2 mol NH3 6 mol H2O 18.02 g H2O 59 g
=
4 mol NH3 1 mol H2O H2O

9. Stoichiometry Question (4)
4 NH3 + 5 O2  6 H2O + 4 NO
 How many grams of O2 are required to produce
0.3 mol of H2O?
0.3 mol H2O 5 mol O2 32 g O2
= 8 g O2
6 mol H2O 1 mol O2

10. Stoichiometry Question (5)
4 NH3 + 5 O2  6 H2O + 4 NO
 How many grams of NO is produced if 12 g of O2
is combined with excess ammonia?
12 g O2 x 1 mol O2 4 mol NO 30.01 g NO
x x
32 g O2 5 mol O2 1 mol NO
= 9.0 g NO

11. Moving along the stoichiometry path
 Wealways use the same type of information to
make the jumps between steps:
Molar mass of x Molar mass of y
grams (x)  moles (x)  moles (y)  grams (y)
Mole ratio from
balanced equation

12. Converting grams to grams
Many stoichiometry problems follow a pattern:
grams(x)  moles(x)  moles(y)  grams(y)
We can start anywhere along this path
depending on the question we want to answer
Notice that we cannot directly convert from
grams of one compound to grams of another.
Instead we have to go through moles.

13. Have we learned it yet?
Try these on your own - 4 NH3 + 5 O2  6 H2O + 4 NO
a) How many moles of H2O can be made using 1.6 mol NH3?
b) what mass of NH3 is needed to make 0.75 mol NO?
c) how many grams of NO can be made from 47 g of NH3?

14. 4 NH3 + 5 O2  6 H2O + 4 NO
a) 6 mol H2O
1.6 mol NH3 x = 2.4 mol
4 mol NH3 H2O
4 mol NH3 x 17.04 g NH3
0.75 mol NO x = 13 g
4 mol NO 1 mol NH3 NH3
47 g NH3 x 1 mol NH3 x 4 mol NO
x
30.01 g NO
17.04 g NH3 4 mol NH3 1 mol NO
= 83 g NO

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