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IV. Percent Ionization. Instead of characterizing a weak acid by its K a , we can calculate how much it ionizes. The concentration of ionized acid is simply equal to the [H + ] at equilibrium. IV. Sample Problem. Find the \% ionization of a 0.200 M acetic acid solution if its pK a = 4.74.

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iv percent ionization
IV. Percent Ionization
  • Instead of characterizing a weak acid by its Ka, we can calculate how much it ionizes.
  • The concentration of ionized acid is simply equal to the [H+] at equilibrium.
iv sample problem
IV. Sample Problem
  • Find the % ionization of a 0.200 M acetic acid solution if its pKa = 4.74.
iv sample problem1
IV. Sample Problem
  • In a 0.0100 M solution of butyric acid at 20 °C, the acid is 4.0% ionized. Calculate the Ka and pKa of butyric acid at these conditions.
iv mixtures of acids
IV. Mixtures of Acids
  • If there are multiple acids in solution, then there are multiple sources of H3O+.
  • If one of the acids is strong, it will be the major contributor, so much so that we can ignore contributions from others.

HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)

HCOOH(aq) + H2O(l)  H3O+(aq) + HCOO-(aq)

iv mixtures of weak acids
IV. Mixtures of Weak Acids
  • If we have a mixture of weak acids, we need to examine the Ka’s.
    • If the Ka’s differ by more than a factor of several hundred, then we can just calculate based on the strongest acid.
  • If the Ka’s are too close together, we must solve two weak acid equilibria!
    • Start with the strongest weak acid and use that result in the second weak acid equilibrium.
iv sample problem2
IV. Sample Problem
  • Calculate the pH of a solution that is 0.100 M in acetic acid, CH3COOH (Ka = 1.8 x 10-5), and 0.200 M in formic acid, HCOOH (Ka = 1.8 x 10-4).
v basic solutions
V. Basic Solutions
  • Just like acidic solutions, there are strong bases and weak bases.
  • Everything we learned about weak acids applies to weak bases.
  • The two systems are analogous to each other.
v strong bases
V. Strong Bases
  • Strong bases ionize completely.
  • Strong bases are typically ionic compounds containing the hydroxide anion.
v weak bases
V. Weak Bases
  • Weak bases typically do not produce OH- by partially ionizing.
  • Weak bases produce OH- by pulling a proton off water.

B(aq) + H2O(l) BH+(aq) + OH-(aq)

v weak bases1
V. Weak Bases
  • The strength of a weak base depends on its base ionization constant, Kb.
  • For the generic weak base B(aq) + H2O(l) BH+(aq) + OH-(aq):
v weak base structures
V. Weak Base Structures
  • Weak bases tend to have lone pair e-’s that can accept a proton.
v weak base problems
V. Weak Base Problems
  • The method of solving weak base problems is no different than the method of solving weak acid problems!
  • Instead of Ka, you use Kb.
  • To get to pH, remember that you can go through pOH.
v sample problem
V. Sample Problem
  • The pain reliever morphine is a weak base. In a 0.010 M morphine solution, the pH is 10.10. Calculate the Kb and pKb or morphine.
v sample problem1
V. Sample Problem
  • The pKb for pyridine is 8.77. What’s the pH of a 0.010 M aqueous solution of pyridine?
vi ions as weak acids bases
VI. Ions as Weak Acids/Bases
  • Some ions can act as either weak acids or weak bases.
    • e.g. NH4+ and CH3COO-
  • These ions must be introduced into a solution as a salt.
    • e.g. NH4Cl and CH3COONa
  • These ionic salts ionize, and then the weak acid/base sets up its equilibrium.
vi anions as weak bases
VI. Anions as Weak Bases
  • Any anion can be thought of as the conjugate base of an acid.
  • Anions that are conjugate bases of weak acids are themselves weak bases.
  • Anions that are conjugate bases of strong acids are pH neutral.
vi sample problem
VI. Sample Problem
  • What’s the pH of a 0.10 M NaNO2 solution? Note that Ka for nitrous acid is 7.1 x 10-4.
vi cations as weak acids
VI. Cations as Weak Acids
  • When cations go into aqueous solutions,we need to examine whether or not they will set up an equilibrium.
    • Cations of strong bases do nothing and are thus pH neutral.
    • Cations that are conjugate acids of weak bases will establish an acid equilibrium.
    • Small, highly-charged metal cations form weakly acidic solutions.
vi the case of al 3
VI. The Case of Al3+
  • Al3+(aq) will form Al(H2O)63+ which will establish an acid equilibrium.
vi ph of salt solutions
VI. pH of Salt Solutions
  • To determine whether a salt solution will be acidic, basic, or neutral, we need to consider the nature of the cation and anion.
  • There are 4 possibilities:
    • Neither cation nor anion acts as acid or base.
    • Cation acts as acid, anion is neutral.
    • Anion acts as base, cation is neutral.
    • Cation acts as acid, and anion acts as base.
vi ph neutral salt solutions
VI. pH Neutral Salt Solutions
  • Cation comes from a strong base.
  • Anion comes from a strong acid.
vi acidic salt solutions
VI. Acidic Salt Solutions
  • Cation is conjugate acid of weak base or is a small, highly-charged cation.
  • Anion comes from a strong acid.
vi basic salt solutions
VI. Basic Salt Solutions
  • Cation comes from a strong base.
  • Anion is a conjugate base of a weak acid.
vi it depends
VI. “It Depends”
  • Sometimes, cation will be a conjugate acid and the anion will be a conjugate base.
  • In this case, the pH of the salt solution depends on relative values of Ka and Kb.
  • If Ka > Kb, solution will be acidic; if Kb > Ka, solution will be basic.
vi analyzing salt solutions
VI. Analyzing Salt Solutions
  • Use the following steps to determine whether an aqueous solution of a salt will be acidic, basic, or neutral.
    • Break up salt into its cation and anion.
    • Ask yourself whether the cation can donate a proton or whether it is small and highly charged. If so, it is a weak acid.
    • Ask yourself whether the anion can accept a proton. If so, it is a weak base.
    • Consider the combined effect of having the cation and anion in solution.
vi sample problem1
VI. Sample Problem
  • For each compound, predict whether its 0.1 M solution in water will be acidic, basic, or neutral.
    • NaNO2
    • KCl
    • NH4Br
    • Fe(NO3)3
    • NH4CN
vii acids w more than one h
VII. Acids w/ More than one H+
  • Some acids have more than one acidic proton; these are called polyprotic acids.
  • Generally, the Ka of the 2nd proton is much smaller than the 1st, so we generally just solve for the 1st ionization.
    • Exceptions: H2SO4 and when Ka’s are within a few hundred of each other.
    • For exceptions, it’s a double equil. problem!
vii sample problem
VII. Sample Problem
  • What is the pH and [SO42-] of a 0.0075 M sulfuric acid solution if Ka2 = 0.012 for sulfuric acid?
viii acid strength structure
VIII. Acid Strength & Structure
  • Why are some acids strong and some acids weak?
  • Depends on structure and composition of the acid.
  • We examine the factors that contribute to acid strength in binary acids and oxyacids.
viii binary acids
VIII. Binary Acids
  • The strength of a binary acid depends on bond polarity and bond strength.
    • The H must have the partial positive charge.
    • Weaker bond leads to greater acidity.
viii binary group 16 17 acids
VIII. Binary Group 16/17 Acids
  • The combined influence of polarity and bond strength can be seen in the Group 16 and Group 17 binary acids.
viii oxyacids
VIII. Oxyacids
  • An oxyacid (a.k.a. oxoacid) has the general form H-O-Y- in which Y is some atom which may or may not have additional atoms bonded to it.
  • Oxyacid strength depends on the electronegativity of Y and the number of O atoms attached to Y.
viii electronegativity of y
VIII. Electronegativity of Y
  • The more electronegative Y is, the more polar and weaker the O-H bond becomes.
  • If the O kicks off the H as H+, it can claim both electrons in the bond.
viii of o atoms on y
VIII. # of O Atoms on Y
  • More O atoms draw electron density away from Y, which draws electron density from the O-H bond, leading to greater acidity.
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