Chem 121 Module 4: Chemical Reactions And Stoichiometry Questions And Answers 2026/2027 Update || 100% Guaranteed Pass

CHEM 121 MODULE 4: CHEMICAL
REACTIONS AND STOICHIOMETRY
QUESTIONS AND ANSWERS 2026/2027
UPDATE || 100% GUARANTEED PASS


1. Balance the following combustion reaction: C3H8 + O2 -> CO2 + H2O. What is the

coefficient for O2?

A. 3


B. 4


C. 5


D. 10


Answer: C


Conceptual Explanation: Balancing the equation gives C3H8 + 5O2 -> 3CO2 + 4H2O. There

are 10 oxygen atoms on the right, requiring 5 O2 molecules.


2. In the reaction 2Al + 3Cl2 -> 2AlCl3, how many grams of AlCl3 are produced from 5.4g of Al?

(Al=27, Cl=35.5)

A. 13.35g


B. 40.2g


C. 20.1g

,D. 26.7g


Answer: D


Conceptual Explanation: Moles of Al = 5.4/27 = 0.2 mol. Ratio Al to AlCl3 is 1:1, so 0.2 mol

AlCl3 is produced. Molar mass of AlCl3 = 133.5. 0.2 * 133.5 = 26.7g.


3. Which type of reaction is 2KClO3 -> 2KCl + 3O2?

A. Decomposition


B. Single Displacement


C. Synthesis


D. Double Displacement


Answer: A


Conceptual Explanation: A single reactant breaking down into two or more products is a

decomposition reaction.


4. What is the oxidation state of Manganese in KMnO4?

A. +7


B. +4


C. +2


D. +6


Answer: A

, Conceptual Explanation: K is +1, O is -2. 1 + Mn + 4(-2) = 0. Mn - 7 = 0, so Mn = +7.


5. Which of the following compounds is insoluble in water according to solubility rules?

A. AgCl


B. KNO3


C. Na2SO4


D. CaCl2


Answer: A


Conceptual Explanation: Most chlorides are soluble except for Silver, Mercury(I), and

Lead(II).


6. Calculate the percent yield if the theoretical yield is 50g and the actual yield is 42g.

A. 75%


B. 84%


C. 92%


D. 119%


Answer: B


Conceptual Explanation: Percent yield = (Actual / Theoretical) * 100. (42/50)*100 =

84%.