Molecular Formulas Study Guide

The exam will consist of 20 multiple choice questions with 70% “C” questions, 20% “B” questions, and 10% “A” questions. In general, “C” questions are those involving the most basic concepts, particularly ones that you have used before. “C” calculation problems typically involve one major step with 1-2 more minor steps. (Examples of minor steps would be calculating molecular weights or converting [H3O+] to pH.) “B” questions involve more difficult concepts, particularly ones that are new. “B” calculation questions typically involve at least two major steps with 1-4 minor steps. “A” questions are those involving the most difficult concepts or those that require a thorough understanding in order to answer correctly. Often “A” questions require bringing together different concepts to solve the problem and 2+ calculation steps. Note also that concepts listed under the “C” or “B” categories below can be converted to harder questions, depending on how the question is asked.

Background Concepts [Homework Examples; similar questions in italic that weren’t previously assigned]

  • Know the molecular formulas and charges of common ions. [Common Ion handout]
  • Calculate molar mass, convert between grams and moles using the molar mass. [Problem Set 1]
  • Calculate concentrations in molarity, convert between liters and moles using molarity. [Problem Set 1, Quiz 2]
  • Calculate concentrations after dilution. [Exp. 2]
  • Be able to balance chemical reactions and determine net ionic equations. [Quiz 2]
  • Determine reaction quotient expression for a given reaction. [13.15, Quiz 2]
  • Identify Bronsted acids, bases and conjugate acid-base pairs. [14.7, 14.9, Problem Set 2, Quiz 2]
  • Convert back and forth between [H3O+],[OH ], pH and pOH with correct sig. figs. [14.17, 14.21,

14.25, Exp. 3, Quiz 3]

  • Know the strong bases (OH salts) and the 6 strong acids (HCl, HBr, HI, HNO3, HClO4, H2SO4). [part of 14.28]
  • Calculate pH of strong acid, strong base solutions. [14.19, Problem Set 3, Quiz 3]
  • Calculate pH (or pOH, [H3O+],[OH ]) of a weak acid or base solution knowing Ka or Kb and the initial concentration. [14.69, 14.71, Quiz 3]
  • Recognize molecular weak acids (carboxylic acids and inorganic weak acids) and molecular weak bases (NH3 and amines) [worksheet 1, part of 14.28]
  • Be able to recognize ionic acids and bases. [worksheet 1, part of 14.28, 14.78, 14.79]

“C” Questions [Examples]

  • Convert between Ka and Kb values for conjugate acid-base pairs. [quiz 4’s]
  • Calculate pKa’s. [quiz 4’s]
  • Know what a buffer solution is, what it does (resists changes in pH) and how it works. [quiz 4’s,


  • Calculate pH of a buffer solution using Henderson-Hasselbach equation. [quiz 4’s, 14.97, 14.101a,b]
  • Determine the ratio of acid:base needed for making a buffer solution at a certain pH. [quiz 4’s, 14.99]
  • Know what will be a good acid,base pair to make a buffer solution of a certain pH. [quiz 4’s, 14.103,


  • Know that for a conjugate acid base pair, if pH = pKa then [base] = [acid], if pH > pKa then [base] > [acid], and if pH < pKa then [base] < [acid]. Use these relationships to predict whether the acid or base form of a given conjugate acid/base pair will dominate at a particular pH. [quiz 4’s]
  • Calculate the equivalence point for an acid-base titration. [PS 4, quiz 5’s]
  • Predict what will be in solution at different points during a pH titration.[PS 4, quiz 5’s]
  • Know expected differences in the shape of a pH titration curve for strong acid-strong base, weak acidstrong base, strong acid-weak base.
  • Calculate pH’s at different points on a strong acid/strong base titration curve. [PS 4, quiz 5’s]
  • Write out the Ksp expression for a solubility equilibria [quiz 5’s,15.9]
  • Calculate Ksp from solubility data [old quiz 5’s, 15.11]
  • Calculate solubility from Ksp. [quiz 5’s, 15.13]
  • Calculate the solubility of an ionic compound in the presence of another source of one of the ions. [15.15]
  • Know what complex ions are and be able to write out the formation rxn and Kf expression. [Exp. 5]
  • Understand LeChatelier’s Principle
  • Predict how solubility of an ionic compound changes upon addition of another source of one of the ions (common ion effect), an acid (can react with basic anion), or a ligand that can form a complex ion with the metal cation. [old quiz 6’s and exam 2’s,15.111,15.113]
  • Use the simplified solubility rules presented in lecture to predict if a precipitate will likely form when solutions of two ionic compounds are mixed. [S19Ex2a.11, F19Ex2a.5]
  • Know what enthalpy and entropy are. Which direction of change is favorable for each?
  • What is an exothermic rxn? What is an endothermic rxn? What is the sign of ∆H for each?
  • Know the 3 Laws of Thermodynamics
  • Predict the sign of ΔS for physical processes and chemical reactions. [S20 quiz 6; 16.17]
  • Predict the relative order of S° values based on physical state, size and molecular complexity [16.1 • Calculate ΔS° values from S° values. [S20 quiz 6, 16.29]
  • Calculate ΔH° values from Hf° values.
  • Know how to calculate ΔG° from Gf° values, or ΔHf° and S° values. [16.31, 16.33]
  • Make predictions about how the “favorability” of a reaction will change with temperature based on the signs of ΔH° and ΔS°. [16.31, 16.65, S20Ex2.19]
  • Calculate K from ΔG°, and ΔG° from K. [16.39, 16.41]

“B” Questions [Examples]

  • Calculate pH of a solution of a cationic weak acid or anionic weak base knowing Kb or Ka of their neutral conjugates and the initial concentration. [PS 4]
  • Calculate pH’s at different points on a weak acid/strong base or weak base/strong acid titration curve. [PS 4, quiz 5’s]
  • Determine the pKa of a weak acid from a titration curve. [Exp. 4]
  • Predict whether a precipitate will form using Ksp. [quiz 6’s, 15.31, 15.37, 15.40]
  • Understand the difference between ΔG (determines the direction of spontaneous change at any point in a reaction) and ΔG° (related to the overall favorability of the reaction and K). What has to be true

for a spontaneous process? What has to be true at equilibrium?

Concepts listed under the “C” category can be converted to B questions by combining them and/or requiring a better understanding of the concept in order to be solved.

“A” Questions [ Examples]

  • More complicated questions involving buffers, such as determining how much strong acid or strong base should be added to a solution of a weak base or weak acid to make a buffer solution of a certain pH, or what will be the pH after adding strong acid or base. [S19ex2a.13, F19Ex2a.9, F19ex2b.17,


  • More complicated questions involving solubility equilibria.
  • More complicated problems involving the relationship between pH, pKa and base/acid ratio. [S19ex2a.13; S20ex2.4]
  • Less obvious questions involving the relationship between K and ΔG° [S20ex2.20]

Concepts listed under the “C” or “B” categories can be converted to A questions by combining them and/or asking the question in such a way that the route to the answer is not straightforward and requires a thorough understanding of the concepts.

Information that will be given with exam:

Periodic Table; Henderson-Hasselbach equation; Chap. 16 eqns; Table of Ka and Kb values; Other K’s and thermodynamic values as needed.

Equations you need to know:

[H3O+][OH] = Kw = 1.008×10−14; pH + pOH = 13.9965 pH = −log[H3O+]; pOH = −log[OH]

[H3O+] = 10−pH; [OH] = 10−pOH

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