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# IV. Percent Ionization - PowerPoint PPT Presentation

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

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### 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

• Find the % ionization of a 0.200 M acetic acid solution if its pKa = 4.74.

### 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

• 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

• 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 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

• 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

• Strong bases ionize completely.

• Strong bases are typically ionic compounds containing the hydroxide anion.

### 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 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

• Weak bases tend to have lone pair e-’s that can accept a proton.

### 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

• 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 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

• 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

• 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

• 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

• 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 Al3+

• Al3+(aq) will form Al(H2O)63+ which will establish an acid equilibrium.

### 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

• Cation comes from a strong base.

• Anion comes from a strong acid.

### 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

• Cation comes from a strong base.

• Anion is a conjugate base of a weak acid.

### 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

• 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 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+

• 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

• 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

• 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

• 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

• The combined influence of polarity and bond strength can be seen in the Group 16 and Group 17 binary acids.

### 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

• 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

• More O atoms draw electron density away from Y, which draws electron density from the O-H bond, leading to greater acidity.