Activity 151-9
Introduction to Stoichiometry
Directions: This Guided Learning Activity (GLA) focuses on completing stoichiometric calculations. Part
A discusses the relationships that allow us to convert between chemical quantities, Part B introduces
basic stoichiometric calculations, and Part C discusses limiting reactant problems. The worksheet is
accompanied by instructional videos. See http://www.canyons.edu/Departments/CHEM/GLA/ for
additional materials.
Part A – The Basics
Stoichiometry utilizes quantitative relationships to determine the mass, concentration, volume or amount
of a substance that is produced or consumed during a given chemical process. It can be applied to a wide
range of chemical quantities. For each connection below, give the conversion factor that can be used. An
example conversion for Volume of A to Mass of A is shown.
𝒈
𝒅𝒆𝒏𝒔𝒊𝒕𝒚 𝒐𝒇 𝑨 = ( )
𝒎𝑳
Chemistry Guided Learning Activities
Activity 151 – 9
College of the Canyons
Page 1 of 4
Part B – Mass-to-Mass Conversions
Using the conversion factors from part A, the amount of a product in a chemical reaction can be
calculated.
Example #1a. If 2.54 g of copper reacts with sulfur, how much copper (I) sulfide will be produced?
2𝐶𝑢 (𝑠) + 𝑆(𝑠) → 𝐶𝑢2 𝑆 (𝑠)
Solution:
Mass Cu
Moles Cu2S
Moles Cu
Molar Mass
Cu
Mass Cu2S
Molar Mass
Cu2S
Reaction Coefficients
2.54 𝑔 𝐶𝑢
1 𝑚𝑜𝑙 𝐶𝑢
1 𝑚𝑜𝑙 𝐶𝑢2 𝑆 159.17 𝑔 𝐶𝑢2 𝑆
(
)(
)(
)(
) = 3.18 𝑔 𝐶𝑢2 𝑆 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑
1
63.55 𝑔 𝐶𝑢
2 𝑚𝑜𝑙 𝐶𝑢
1 𝑚𝑜𝑙 𝐶𝑢2 𝑆
Example #1b. In the above problem, how many grams of sulfur were consumed?
Solution:
Mass Cu
Moles Cu
Molar Mass
Cu
(
Moles S
Molar Mass
S
Reaction Coefficients
2.54 𝑔 𝐶𝑢
)(
1
)(
)(
Mass S
)=
𝑔 𝑆 𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑
Example #1c. What volume of copper is needed to produce 58.7 g of Cu2S? (dcu = 8.96 g/cm3)
Solution:
58.7 𝑔 𝐶𝑢2 𝑆
(
)(
1
)(
Chemistry Guided Learning Activities
Activity 151 – 9
)(
)(
)=
𝑐𝑚3 𝐶𝑢 𝑛𝑒𝑒𝑑𝑒𝑑
College of the Canyons
Page 2 of 4
Part C – Limiting Reactants
When two or more reactants are combined, usually one of the reactants will be limiting, meaning it will
run out first. When this happens, no more product will form. To determine the amount of product that
forms, we must first determine which of several reactions will be depleted first, and use that quantity to
calculate the amount of product that could form.
Example #2a. How many grams of ammonia (NH3) can be produced when 50.0 g of hydrogen gas and
25.0 g of nitrogen gas are mixed?
3𝐻2 (𝑔) + 𝑁2 (𝑔) → 2𝑁𝐻3 (𝑔)
Solution:
Step 1. Find how much NH3 could be produced if all of the H2 is used:
Step 2. Find how much NH3 could be produced if all of the N2 is used:
Step 3. Compare the calculated values. The reactant that produces the least amount of product is the
limiting reactant.
The limiting reactant in this reaction is __________________.
The calculated amount of product from the limiting reactant is the theoretical yield. It represents the
maximum amount of product that could be produced by the reaction.
The theoretical yield for NH3 is _______________ g NH3.
Example #2b. If only 23.4 g of NH3 was recovered from the reaction, what was the percent yield?
Solution:
The percent yield for a reaction quantifies the percent of the theoretical yield was achieved.
𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝑌𝑖𝑒𝑙𝑑 =
% 𝑌𝑖𝑒𝑙𝑑 =
Chemistry Guided Learning Activities
Activity 151 – 9
𝐴𝑐𝑡𝑢𝑎𝑙 𝑌𝑖𝑒𝑙𝑑
𝑥 100%
𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑌𝑖𝑒𝑙𝑑
𝑥100% =
%
College of the Canyons
Page 3 of 4
Part D – Extra Practice
1. 9.60 g of sodium metal is tossed into a stream. How many grams of sodium hydroxide are produced?
2𝑁𝑎 (𝑠) + 2𝐻2 𝑂 (𝑙) → 2𝑁𝑎𝑂𝐻 (𝑎𝑞) + 𝐻2 (𝑔)
2. Potassium chlorate is heated to decompose into potassium chloride and oxygen gas. How many grams
of potassium chloride will remain if 391 g of potassium chlorate are heated and the reaction is allowed to
go to completion?
∆
2𝐾𝐶𝑙𝑂3 (𝑠) → 2𝐾𝐶𝑙 (𝑠) + 3𝑂2 (𝑔)
3. Urea is produced from a reaction between ammonia and carbon dioxide. A worker mixes 68.2 kg of
ammonia and 105 kg of carbon dioxide, and produces 87.5 kg of urea. What is the percent yield for this
reaction?
2𝑁𝐻3 (𝑎𝑞) + 𝐶𝑂2 (𝑎𝑞) → (𝑁𝐻2 )2 𝐶𝑂 (𝑎𝑞) + 𝐻2 𝑂(𝑙)
4. Aspirin (C9H8O4) can be synthesized by the following reaction:
𝐶4 𝐻6 𝑂3 (𝑎𝑞) + 𝐶7 𝐻6 𝑂3 (𝑎𝑞) → 𝐶9 𝐻8 𝑂4 (𝑎𝑞) + 𝐶2 𝐻4 𝑂2 (𝑎𝑞)
Aspirin
A typical percent yield for this reaction is 65%. If salicylic acid (C7H6O3) is the limiting reactant, how
much salicylic acid is needed to synthesize 250 mg of aspirin?
Challenge Problems. To complete these stoichiometry problems, first write and balance the relevant
chemical reaction. For extra guidance, refer to the Writing Chemical Reactions GLA.
5. 22.3 g of calcium hydroxide is placed in a beaker containing 1.00 mol of hydrochloric acid. How much
water is produced? Does the final solution contain excess Ca(OH)2 or HCl?
6. You want to recover the silver ions from an aqueous solution. The solution contains 0.1 mole of
AgNO3, and you plan to add enough MgCl2 to the solution to convert all of the AgNO3 to AgCl. How
many grams of MgCl2 should you add?
Chemistry Guided Learning Activities
Activity 151 – 9
College of the Canyons
Page 4 of 4