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Unit 14: Acid-Base Equilibria

How a proton changing hands defines an acid and a base, and why a solution's pH depends on far more than just how concentrated the acid is.

12 topics · 44 questions

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14.1 Bronsted-Lowry Acids and Bases

  1. How it worksHow does a Bronsted-Lowry acid donating a proton and a base accepting one replace the older idea of acids releasing H+ into water?
  2. Common mistakeWhy isn't ammonia, NH3, disqualified from being a base just because it contains no hydroxide ion?
  3. Going furtherWhat happens to water's role in a reaction with a substance that is stronger at donating protons than water itself?
Your turn1 of 2
Ammonia, NH3, is classified as a Bronsted-Lowry base even though its formula contains no hydroxide ion at all. Why isn't that a problem for calling it a base?

14.1 Conjugate Acid-Base Pairs

  1. How it worksHow does removing a proton from an acid leave behind its conjugate base, ready to accept that same proton back?
  2. Worked exampleFind the conjugate base of HSO4- and the conjugate acid of NH3.
  3. Common mistakeWhy isn't a strong acid's conjugate base also a strong base?
  4. Going furtherWhat happens to the strength of a conjugate base as the strength of its parent acid increases?
Your turn1 of 2
HCl is a strong acid that ionizes completely in water. Why isn't its conjugate base, Cl-, also a strong base?

14.2 The Autoionization of Water and Kw

  1. How it worksHow does water reacting with itself to form H3O+ and OH- set a fixed product, Kw, that holds in every aqueous solution?
  2. Worked exampleFind the concentration of OH- in a solution where the H3O+ concentration is 1.0 times 10−3 M, using Kw.
  3. Common mistakeWhy doesn't an acidic solution have zero OH- ions floating around in it?
Your turn1 of 2
Find the concentration of OH- in a solution where the H3O+ concentration is 7.2 times 10−3 M, using Kw = 1.0 times 10−14.

14.2 The pH and pOH Scales

  1. How it worksHow does taking the negative log of hydrogen ion concentration compress an enormous range of concentrations onto one simple scale?
  2. Worked exampleFind the pH of a solution with an H3O+ concentration of 2.5 times 10−5 M.
  3. Worked exampleFind the pOH and then the pH of a solution given that pH plus pOH equals 14 at 25 degrees C.
  4. Common mistakeWhy isn't a pH of 0 the lowest possible pH value a solution can actually have?
Your turn1 of 2
Find the pH of a solution with an H3O+ concentration of 7.2 times 10−5 M.

14.3 Strong Acids and Strong Bases

  1. How it worksHow does a strong acid ionizing completely mean its H3O+ concentration is just equal to its starting concentration?
  2. Worked exampleFind the pH of a 0.025 M solution of HCl.
  3. Common mistakeWhy isn't a strong acid the same thing as a concentrated acid?
  4. Going furtherWhat happens to the pH calculation once you compare a strong diprotic acid like H2SO4 to a strong monoprotic acid like HCl?
Your turn1 of 2
Find the pH of a 0.349 M solution of HCl (a strong acid that fully dissociates).

14.3 Weak Acid Equilibria and Ka

  1. How it worksHow does a small Ka value mean a weak acid only partially ionizes, leaving most of it as the intact molecule?
  2. Worked exampleFind the pH of a 0.10 M solution of acetic acid given a Ka of 1.8 times 10−5.
  3. Common mistakeWhy can't you assume a weak acid's H3O+ concentration equals its starting concentration the way you can for a strong acid?
  4. Going furtherWhat happens to a weak acid's percent ionization as its solution is diluted with more water?
Your turn1 of 2
Find the pH of a 0.351 M solution of acetic acid given a Ka of 1.8 times 10−5 (assume the change x is negligible compared to 0.351).

14.3 Weak Base Equilibria and Kb

  1. How it worksHow does a small Kb value mean a weak base only partially reacts with water to form hydroxide?
  2. Worked exampleFind the pH of a 0.15 M solution of ammonia given a Kb of 1.8 times 10−5.
  3. Common mistakeWhy isn't every base required to already contain hydroxide in its own formula?
Your turn1 of 2
Find the pH of a 0.351 M solution of ammonia given a Kb of 1.8 times 10−5 (assume x is negligible compared to 0.351).

14.3 Percent Ionization

  1. How it worksHow does dividing the concentration of ionized acid by the initial concentration turn equilibrium math into one comparable percentage?
  2. Worked exampleFind the percent ionization of a 0.050 M solution of a weak acid with a Ka of 6.3 times 10−4.
  3. Common mistakeWhy doesn't a higher percent ionization always mean a stronger acid when comparing two different concentrations?
Your turn1 of 2
Find the percent ionization of a 0.351 M solution of a weak acid with a Ka of 6 times 10−4.

14.3 Relating Ka and Kb for a Conjugate Pair

  1. How it worksHow does multiplying Ka by Kb for a conjugate acid-base pair always give back Kw?
  2. Worked exampleFind the Kb of the acetate ion given that acetic acid has a Ka of 1.8 times 10−5.
  3. Common mistakeWhy doesn't a weak acid's conjugate base automatically count as a strong base just because the acid itself is weak?
  4. Ties togetherHow does the Ka times Kb equals Kw relationship connect back to water's own autoionization constant?
Your turn1 of 2
Find the Kb of a conjugate base given that its parent acid has a Ka of 7 times 10−5 (Ka times Kb equals Kw = 1.0 times 10−14).

14.4 Polyprotic Acids

  1. How it worksHow does each successive ionization step of a polyprotic acid have its own, progressively smaller, equilibrium constant?
  2. Worked exampleFind the pH of a 0.10 M solution of H2CO3 using only its first ionization constant, Ka1.
  3. Common mistakeWhy can the second ionization step of most polyprotic acids usually be ignored when calculating a solution's pH?
  4. Going furtherWhat happens to the concentration of the fully deprotonated form of a triprotic acid like H3PO4 in a moderately acidic solution?
Your turn1 of 2
Find the pH of a 0.363 M solution of a diprotic acid using only its first ionization constant, Ka1 = 6 times 10−7 (assume x is negligible compared to 0.363, and ignore the second ionization).

14.5 Acid-Base Properties of Salts

  1. How it worksHow does a salt's cation or anion react with water, through hydrolysis, to shift the solution's pH away from neutral?
  2. Worked exampleFind whether a solution of sodium acetate is acidic, basic, or neutral, and explain using the acetate ion's reaction with water.
  3. Common mistakeWhy isn't every salt solution automatically neutral just because it came from neutralizing an acid with a base?
  4. Going furtherWhat happens to the pH of a solution made from a salt formed by a weak acid and a weak base, once both Ka and Kb are compared?
Your turn1 of 2
Find the pH of a 0.328 M solution of a salt NaA, where A- is the conjugate base of a weak acid with Ka = 7 times 10−5 (Kb of A- equals Kw divided by Ka; assume x is negligible compared to 0.328).

14.6 Lewis Acids and Bases

  1. How it worksHow does defining an acid as an electron pair acceptor and a base as an electron pair donor cover reactions Bronsted-Lowry theory can't?
  2. Worked exampleFind the Lewis acid and Lewis base in the reaction between BF3 and NH3, and identify the new bond that forms.
  3. Common mistakeWhy isn't every Lewis acid also a Bronsted-Lowry acid?
  4. Ties togetherHow does the Lewis definition of acids and bases connect back to the electron-pair donation that also forms a coordinate covalent bond in a complex ion?
Your turn1 of 2
In the reaction between BF3 and NH3, which species is the Lewis acid, which is the Lewis base, and what bond forms between them?