# STOICHIOMETRY – MOLE RELATIONSHIPS IN CHEMICAL REACTIONS by hcj

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Experiment: Mole Relationships in Chemical Reactions (pg. 235)

Purpose:
To investigate the relationship between the moles of products and reactants in a chemical
reaction. Then compare the relationships with the balanced chemical equation. The balanced
chemical reaction for NaHCO3 reacting with HCl is:

Procedure:
Follow the procedure steps on page 235, with the following changes:
 When you add the NaHCO3 to the microplate, put a small scoop (not 1g).
 Use 10 M HCl instead of 8M HCl.
 When you’re adding the HCl to the NaHCO3, be careful not to add too much at once. Add
it drop-by-drop, so that it doesn’t overflow (if it does overflow, that will obviously affect your
mass measurements).
 You can use the dropper of HCl to stir the mixture to ensure all the NaHCO3 reacts.

Observations:
Mass of NaHCO3
Mass of microplate and NaHCO3
Mass of microplate, NaHCO3 and HCl before reaction
Mass of microplate, NaHCO3 and HCl after reaction is done
Calculate mass difference (which is mass of CO2 produced)

Analysis:
STOICHIOMETRY – MOLE RELATIONSHIPS IN CHEMICAL REACTIONS                                           2

Mole Calculations using the Balanced Chemical Equation
Reference: 7.1 pages 243 - 249

Stoichiometry is the study of the relative quantities of reactants and products in chemical
reactions. Based on the results of moles relationship experiment, what is the relationship
between moles of reactants and products compared to the balanced chemical equation?

Using this information we can take it a step further and use the mass of any reactant or product
to predict he mass of another reactant or product using the following process:

Mass of          Moles of           Use mole ratio in        Moles of        Mass of
known             known           chemical equation to       unknown         unknown
material         material         determine number of        material        material
moles of unknown
material

Sample problem – Mole Ratios
Solving problems using the mole ratios given by the coefficients in the reaction follow a few
simple steps:

1. Write a balanced chemical equation for the reaction (if not already given).
2. Identify the known moles and the unknown moles.
3. Identify the ratio of the unknown to known moles given by the equation.

Example 1: Vanadium reacts with oxygen to form divanadium pentoxide. Determine the
number of moles of vanadium needed to produce 7.47 mol of divanadium pentoxide.
4V + 5O2  2V2O5

4V       +            5O2                                                V2O5

______ mol V                                                              7.47mol V2O5

* notice how the moles of known units cancel out in this calculation!*
STOICHIOMETRY – MOLE RELATIONSHIPS IN CHEMICAL REACTIONS                                        3

Example 2: Now you try it!
If 2.0 mol of iron (steel wool) are reacted with enough copper(I) chloride, how many moles of
copper metal should be produced?

Fe + CuCl  FeCl2 + Cu

Practice Questions pg 238 # 4, 5, 6, 7; pg 240 #9, 10
STOICHIOMETRY – MOLE RELATIONSHIPS IN CHEMICAL REACTIONS                                     4

Sample problem – Mass to Mass and Mass to Particle Calculations
Solving this type of problem involves five steps that should be carried out in the correct
sequence:
1. Write a balanced chemical equation.
2. Read the question and identify the known and unknown materials in the equation.
3. Calculate the number of moles of the known or given material.
4. Use the mole ratio from the balanced chemical equation to determine the number of
moles of unknown material.
5. Convert the number of moles of unknown material to whatever is required in the question
(mass or molecules).
Key Point: To use the ratio of reactants and products in the balanced equation, you MUST
HAVE THE quantities in __________________. So if the question gives you mass, you must
convert to moles first.

Example 2: Calculate the mass of oxygen produced by the decomposition of 12.26g of
potassium chlorate into potassium chloride and oxygen.
2KClO3  2 KCl + 3O2

2KClO3                            2 KCl                +             3O2

________ mol KClO3                                                           ______mol O2

12.26 g KClO3                                                           ? g O2
STOICHIOMETRY – MOLE RELATIONSHIPS IN CHEMICAL REACTIONS                                5

Example 3: Now You Try It!
What mass of sodium would have to be burned in an excess of oxygen to produce 6.2g of
sodium oxide?
4Na + O2  2Na2O

4Na                       +     O2                              2Na2O

? g Na                                                               6.2 g Na2O

Example 4: Nitrogen gas and sodium metal are generated by the decomposition of sodium
azide, NaN3. How many atoms of Na are produced when 80.0g of N2 are generated in this
reaction?
2NaN3  3N2 + 2Na

2NaN3              3N2                +           2Na

`                        80.0g N2                        ? atoms Na

Exercise:
1. Questions 11, 12, 13, 14; page 244
2. Questions 16, 17, 18; page 246
3. Questions 20, 21, 22; page 248-249
STOICHIOMETRY – MOLE RELATIONSHIPS IN CHEMICAL REACTIONS                                            6

Experiment: Limiting Reactant in a Chemical Reaction

Complete Thoughtlab: The Limiting Item (page 252) and/or Investigation 7-A (page 255).

Calculations Involving the Limiting Reactant
Reference: Section 7.2 pages 251-257

These problems are only slightly different from mass-mass problems. The key difference is that
information will be given for                                            and you will always be
asked for the amount of                                              .

Example: Zinc and sulfur react to form zinc sulphide. If 6.00g of zinc and 4.00g of sulfur are
available for reaction, determine the limiting reactant and the mass of zinc sulfide produced
8Zn + S8  8ZnS

8Zn         +        S8                                               8ZnS

______ mol Zn                                                           ______ mol ZnS

OR

______ mol S8                                    ______ mol ZnS

6.00 g Zn              4.00g S                             ______g ZnS

Practice:
1. Questions 24 page 254.
2. Questions 27, 28, 30; page 258.
STOICHIOMETRY – MOLE RELATIONSHIPS IN CHEMICAL REACTIONS                                              7

Percentage Yield

Reference: Section 7.3 pages 260 – 270

When calculating the mass of products using the process given, it is a theoretical yield
that is being calculated. The theoretical yield is the amount that would be obtained if everything
in the reaction (experiment) was perfect. Both in the school laboratory and in industry things are
not 100% perfect in any chemical production. This is the number you have been calculating in
the mass-mass and limiting reactant problems. The amount of product that you actually obtain in
an experiment is called the actual yield. To the percentage yield from an experiment, you divide
the actual yield by the theoretical yield and multiply by 100%.

Example:      The actual yield of an experiment is 12.6 g when the calculated value of the
theoretical yield was 15.0g. The percentage yield would be calculated as follows:

percentage yield =       actual yield     x 100 = 12.6 g      x 100 = 84.0% yield
theoretical yield           15.0 g

Exercise:
1. Question 32, 33 page 262.
2. Questions 35, 27 page 264.

UNIT REVIEW

Chapter 5
Terms: average atomic mass, isotopic abundance, the mole, Avogadro’s number, molar mass
(of an element or a compound), the mole equation, the mole/mass equation
Pg. 170 # 5, pg. 193 # 9, 13(a), pg. 194 # 14(a), 16(d), 19, 25

Chapter 6
Terms: Law of Definite Proportions, Mass Percent, Percentage composition of a compound,
Simplest formula vs. Molecular formula, Hydrates and Anhydrous compounds
Pg. 229 # 6 (a) and (b), pg. 211, #14 and 16, pg 218 # 20, pg 230 # 18 (a)(b),
pg 225 #23 to 25;
Plus Problem Set

Chapter 7
Terms: mole ratios, coefficients, stoichiometry, limiting reactant, excess reactant, actual yield,
theoretical yield, percentage yield
Pg. 271 #1, 3, 5, 7, 9(a)(b), 15(a)(b)

*Pay attention to SIG DIGS and UNITS in your review, because they will count on the test!

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