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Unit 9: Acids and Bases

What makes a solution acidic or basic, how the pH scale measures it, and how a titration finds an unknown concentration one drop at a time.

9 topics · 27 questions

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9.1 Properties of Acids and Bases

  1. How it worksHow does an acid's or a base's chemical behavior explain the properties you can actually taste or feel?
  2. Common mistakeWhy isn't every sour-tasting substance automatically safe to identify as an acid by taste?
  3. Going furtherWhat happens to litmus paper when it's dipped into an acidic versus a basic solution?
Your turn1 of 2
Biting into a lemon delivers an unmistakably sour taste, while rubbing a bar of soap between wet fingers gives a distinctly slippery feel, two very different sensations tied to two different classes of chemicals. Why does lemon juice taste sour while soap feels slippery on your skin?

9.2 The Arrhenius and Bronsted-Lowry Definitions

  1. How it worksHow does the Arrhenius definition classify acids and bases by the ions they release in water?
  2. How it worksHow does the Bronsted-Lowry definition classify acids and bases by which one donates a proton?
  3. Common mistakeWhy isn't every base able to release hydroxide ions the way the Arrhenius definition requires?
Your turn1 of 2
Ammonia acts as a base in water even though its own formula, NH3, contains no hydroxide ion to release, a behavior the original Arrhenius definition of a base can't fully explain. Why do chemists need more than one definition to describe what makes something an acid?

9.3 Strong versus Weak Acids and Bases

  1. How it worksHow does a strong acid dissociating completely in water make it more reactive than a weak acid?
  2. Common mistakeWhy isn't a strong acid automatically more concentrated than a weak acid?
  3. Going furtherWhat happens to the concentration of ions in solution when a weak acid only partially dissociates?
Your turn1 of 2
A weak acid with an initial concentration of 1.41 M is 9.91 percent dissociated in water. Find the resulting concentration of H+ ions in solution.

9.4 The pH Scale

  1. How it worksHow does the pH scale compress a huge range of hydrogen ion concentrations into numbers from 0 to 14?
  2. Common mistakeWhy doesn't a pH of exactly 7 mean a solution has no ions in it at all?
  3. Going furtherWhat happens to a solution's pH as its hydrogen ion concentration increases by a factor of ten?
Your turn1 of 2
A solution's hydrogen ion concentration increases by a factor of 10 raised to the 6 power, where 6 is a whole number of factor-of-ten steps. By how many pH units does the solution's pH decrease?

9.5 Calculating pH from Hydrogen Ion Concentration

  1. How it worksHow does taking the negative log of the hydrogen ion concentration produce a solution's pH?
  2. Worked exampleFind the pH of a solution with a hydrogen ion concentration of 1 times 10 to the negative 4 mol/L.
  3. Common mistakeWhy doesn't doubling the hydrogen ion concentration simply double the pH value?
Your turn1 of 2
Find the pH of a solution with a hydrogen ion concentration of 7.2 times 10 to the -5 power mol/L.

9.6 Neutralization Reactions

  1. How it worksHow do hydrogen ions from an acid and hydroxide ions from a base combine to form neutral water?
  2. Worked exampleFind the products formed when hydrochloric acid reacts with sodium hydroxide in a neutralization reaction.
  3. Common mistakeWhy doesn't neutralizing an acid with a base always produce a solution with a pH of exactly 7?
Your turn1 of 2
Find the volume, in mL, of a 2.8 M base solution needed to exactly neutralize 41 mL of a 2.12 M acid solution, assuming a 1 to 1 mole ratio between acid and base.

9.7 Titration Basics

  1. How it worksHow does a titration use a known concentration and volume of one solution to find the unknown concentration of another?
  2. Worked exampleFind the concentration of an unknown HCl solution if 25 mL of it is exactly neutralized by 30 mL of 0.5 M NaOH.
  3. Common mistakeWhy doesn't reaching the endpoint color change guarantee you've stopped at the exact equivalence point?
Your turn1 of 2
Find the concentration of an unknown acid solution if 43 mL of it is exactly neutralized by 54 mL of a 2.8 M base solution, assuming a 1 to 1 mole ratio between the acid and base.

9.8 Reading a Titration Curve

  1. How it worksHow does plotting pH against volume of titrant added trace out a titration curve as the base is added drop by drop?
  2. Common mistakeWhy doesn't the equivalence point of a titration curve always sit at exactly pH 7?
  3. Going furtherWhat happens to the shape of a titration curve when a weak acid is titrated instead of a strong one?
Your turn1 of 2
Before the equivalence point of a titration, 3.5 millimoles of excess acid (H+) remain in a total solution volume of 137 mL. Find the pH of the solution at this point.

9.9 Acid-Base Indicators

  1. How it worksHow does an indicator molecule change its own structure and color depending on the pH around it?
  2. Common mistakeWhy doesn't every indicator change color at exactly pH 7?
  3. Going furtherWhat happens to the color of an indicator as a solution's pH crosses from below its range to above it?
Your turn1 of 2
Phenolphthalein stays completely colorless when added to an acidic solution, then suddenly turns bright pink the moment enough base has been added to push the pH above its particular threshold. Why does phenolphthalein stay clear in an acid and suddenly turn pink in a base?