AP Chemistry: Chapter 15 Student Notes

Objectives

15.1: The Common Ion EffectSometimes equilibrium solutions can have more than one ion—common ions:

NaF(s) __________ + ____________ (dissociation)

This goes into the reaction:HF(aq) ↔ H+(aq) + F-(aq)

Adding NaF does _________________

Example 1: What is the percent dissociation of:

a. A 0.10-M solution of acetic acid: CH3COOH?

b. A mixture that contains 0.10-M acetic acid and 0.10-M sodium acetate?

Bergmann Page 1 9/11/2023

15.1: The Common Ion Effect 15.2: What are Buffers 15.3: Buffer Capacity 15.4: Titrations and pH Curves 15.5: Acid-Base Indicators 15.6: Solubility Equilbria and Ksp 15.7: Ksp and Qualitative Analysis

15.2: What are BuffersWhat is a buffer

A buffer ____________ a change in pH

A buffer must have

Example 1What is the pH of a solution where 50. mL of 0.50M NaC2H3O2 is mixed with 25mL of 0.25M HC2H3O2

A cool Equation: The Henderson-Hasselbach Equation

Bergmann Page 2 9/11/2023

Example 2: Calculate the pH of the following solution: 25 mL of a 0.150M solution of hypochlorous acid (HOCl) and 32mL of a 0.45M potassium hypochlorite (KOCl)

This can be simplified if you work in mmol throughout the problem

Working in mmol only works if the solution is _______________________

Example 3: Calculate the pH when 25.0 mL of 0.50 M methylammonium nitrate is mixed with 75 mL of 0.30 M methylamine.

Bergmann Page 3 9/11/2023

Adding Strong Acids and Bases to solutions:

Equivalence Point: When the __________ of ____________ = the ____________ of ___________

How do you know why you have reached the equivalence point?

1.

2.

Diagram for # 2 above

Description of this experiment

Bergmann Page 4 9/11/2023

Example 4:[Strong Acid + Salt of Weak Base] What is the pH when 20mL of 0.25M HCl is reacted with 20 mL of 0.35M sodium nitrite?

Example 5: [Strong Acid + Buffer Solution] What is the pH when 15 mL of 0.20 M HNO3 is added to a buffer that contains 50.0 mL of a 0.25 M HCO2H and 0.30 M NaCO2H?

Example 6: [Strong Base + Buffer Solution] What is the pH when 15 mL of 0.20 M NaOH is added to a buffer that contains 50.0 mL of a 0.25 M HCO2H and 0.30 M NaCO2H ?

Bergmann Page 5 9/11/2023

Example 7: [Strong Base + Weak Base Buffer System]: What is the pH when 40 mL of 0.25M NaOH is added to a buffer that contains 100 mL of 0.40M Ethalmine (C2H5NH2) and 0.40M ethylammonium chloride (C2H5NH3Cl)?

Example 8: [Strong Base + Weak Acid—ep]What is the pH when 40 mL of 0.25 M NaOH is mixed with 20 mL of 0.50M propanoic acid (HC3H5O2)

Example 9: [Strong Base + Weak Acid—beyep]What is the pH when 42 mL of 0.25 M NaOH is mixed with 20 mL of 0.50M propanoic acid (HC3H5O2)

Bergmann Page 6 9/11/2023

Example 10: [Strong Acid + Weak Base]What is the pH when 20 mL of 0.20 M methylamine(CH3NH2) is mixed with 10 mL of 0.20 M HNO3.

Example 11: [Strong Acid + Weak Base]What is the pH when 20 mL of 0.20 M methylamine(CH3NH2) is mixed with 20 mL of 0.20 M HNO3.

Example 11: [Strong Acid + Weak Base]What is the pH when 20 mL of 0.20 M methylamine(CH3NH2) is mixed with 24 mL of 0.20 M HNO3.

Example 12: [Ratio Problem]The ratio of NH3 to NH4

+ in a buffered solution is 3.2: What is the pH?

Bergmann Page 7 9/11/2023

15.4: Titrations and pH CurvesStrong Acid with Strong Base

Strong Base Strong Acid

Bergmann Page 8 9/11/2023

Weak Acid-Strong Base

Strong Acid-Weak Acid with Strong Base Comparison

Bergmann Page 9 9/11/2023

Ka and Titration Curves

Weak Base with Strong Acid

Bergmann Page 10 9/11/2023

A Special Point: when the pH = pKa

Bergmann Page 11 9/11/2023

15.5: Acid-Base Indicators

Equivalence Point is determined by1. Colorometrically:

2. Using a pH meter:

How do you pick an indicator—for colorometric analysis?

1. Determine the pH at the equivalence point either from a calculation or using a pH meter

2. Choose the appropriate indicator from the chart below:

Indicator Low pH color Transition pH range High pH color

Gentian violet (Methyl violet) yellow 0.0–2.0 blue-violet

Leucomalachite green (first transition) yellow 0.0–2.0 green

Leucomalachite green (second transition) green 11.6–14 colorless

Thymol blue (first transition) red 1.2–2.8 yellow

Thymol blue (second transition) yellow 8.0–9.6 blue

Methyl yellow red 2.9–4.0 yellow

Bromophenol blue yellow 3.0–4.6 purple

Congo red blue-violet 3.0–5.0 red

Bergmann Page 12 9/11/2023

Methyl orange red 3.1–4.4 orange

Bromocresol green yellow 3.8–5.4 blue-green

Methyl red red 4.4–6.2 yellow

Methyl red / Bromocresol green red 4.5–5.2 green

Azolitmin red 4.5–8.3 blue

Bromocresol purple yellow 5.2–6.8 purple

Bromothymol blue yellow 6.0–7.6 blue

Phenol red yellow 6.8–8.4 purple

Neutral red red 6.8–8.0 yellow

Naphtholphthalein colorless to reddish 7.3–8.7 greenish to

blue

Cresol Red yellow 7.2–8.8 reddish-purple

Phenolphthalein colorless 8.3–10.0 fuchsia

Thymolphthalein colorless 9.3–10.5 blue

Alizarine Yellow R yellow 10.2–12.0 red

Bergmann Page 13 9/11/2023

Which indicator would you pick?

Bergmann Page 14 9/11/2023

How do indicators work?All indicators are ____________________Their “partners” have color and change when the equilibrium is shifted.

HIn ↔ H+ + In-

Blue greenOr some such derivation

Each indicator has a Ka and the color change occurs at approximately the –log of the Ka.

Example 1: The equivalence point of a titration is 4.3: Which of the following indicators would be a good choice? Explain your choice.

Indicator KaThymol Blue 2.3 x 10-2

Eriochrome Black T 5.4 x 10-5

Alizarin 6.6 x 10-6

m-Nitrophenol 8.3 x 10-8

Thymolphthalien 2.5 x 10-9

Alizarin Yellow R 4.3 x 10-11

15.6: Solubility Equilbria and Ksp

Ksp: the study of the solubility’s of ____________ ionic ________________

Writing Solubility Product Expressions

CaF2

Mg3(PO4)2

BaSO4

Ksp values are _______________

Bergmann Page 15 9/11/2023

Ksp ValuesBromides  PbBr2 6.3 x 10-6 Chlorides PbCl2 1.7 x 10-5

 AgBr  3.3 x 10-13 AgCl  1.8 x 10-10

Carbonates  BaCO3 8.1 x 10-9 Chromates   BaCrO4 2.0 x 10-10

CaCO3 3.8 x 10-9 CaCrO4 7.1 x 10-4

CoCO3 8.0 x 10-13 PbCrO4 1.8 x 10-14

CuCO3 2.5 x 10-10 Ag2CrO4 9.0 x 10-12

FeCO3 3.5 x 10-11 Cyanides Ni(CN)2 3.0 x 10-23

PbCO3 1.5 x 10-13 AgCN 1.2 x 10-16

MgCO3 4.0 x 10-5 Zn(CN)2 8.0 x 10-12

MnCO3 1.8 x 10-11 Fluorides  BaF2 1.7 x 10-6

NiCO3 6.6 x 10-9 CaF2 3.9 x 10-11

Ag2CO3 8.1 x 10-12 PbF2 3.7 x 10-8

ZnCO3 1.5 x 10-11 MgF2 6.4 x 10-9

Hydroxides  AgOH 2.0 x 10-8 Sulfates  BaS04 1.1 x 10-10

Al(OH)3 1.9 x 10-33 CaS04 2.4 x 10-5

Ca(OH)2 7.9 x 10-6 PbS04 1.8 x 10-8

Cr(OH)3 6.7 x 10-31 Ag2S04 1.7 x 10-5

Co(OH)2 2.5 x 10-16 Sulfides  CaS  8 x 10-6

Cu(OH)2 1.6 x 10-19 CoS  5.9 x 10-21

Fe(OH)2 7.9 x 10-15 CuS  7.9 x 10-37

Fe(OH)3 6.3 x 10-38 FeS 4.9 x 10-18

Pb(OH)2  2.8 x 10-16 Fe2S3 1.4 x 10-88

Mg(OH)2  1.5 x 10-11 PbS  3.2 x 10-28

Mn(OH)2 4.6 x 10-14 MnS 5.1 x 10-15

Ni(OH)2 2.8 x 10-16 NiS 3.0 x 10-21

Zn(OH)2 4.5 x 10-17 Ag2S  1.0 x 10-49

Iodides  PbI2 8.7 x 10-9 ZnS  2.0 x 10-25

AgI 1.5 x 10-16 Sulfites BaS03 8.0 x 10-7

Oxalates  BaC2O4 1.1 x 10-7 CaS03 1.3 x 10-8

CaC2O4 2.3 x 10-9 Ag2S03 1.5 x 10-14

MgC2O4 8.6 x 10-5

Phosphates AlP04 1.3 x 10-20

Ba3(P04)2 1.3 x 10-29

Ca3(P04)2 1.0 x 10-25

CrP04 2.4 x 10-23

Pb3(P04)2 3.0 x 10-44

Ag3P04 1.3 x 10-20

Zn3(P04)2 9.1 x 10-33

Bergmann Page 16 9/11/2023

Example 1: The solubility of copper I bromide is 2.0 x 10-4M. What is the value of Ksp

Example 2: What is the molar solubility of Silver Sulfide?

Example 3: What is the molar solubility of Bismuth III Sulifde. The Ksp = 1.1 x 10-73

Example 4: What is the molar solubility of Iron III hydroxideSilly Hydroxides

Bergmann Page 17 9/11/2023

Will a ppt form:Compare the Ksp to the QspQsp> Ksp ________________

Qsp < Ksp ________________

Example 5: Will a ppt form50.0 mL of 0.00025 M Na3PO4 is mixed with 50.0 mL of 0.0025 M BaCl2. Will a ppt form? Show all calculations to support your answer.

Example 6: Competing pptSodium chloride is added to a 50 mL beaker that contains a mixture or 0.00015 M lead II nitrate and 0.00035 M silver nitrate. What ppt will form first. Show all work.

Bergmann Page 18 9/11/2023

Example 7: Competing ppt and how much remains.At 25˚C the solubility product constant, Ksp, for strontium sulfate, SrSO4, is 7.610-7. The solubility product constant for strontium fluoride, SrF2, is 7.910-10.(a) What is the molar solubility of SrSO4 in pure water at 25˚C?

(b) What is the molar solubility of SrF2 in pure water at 25˚C?

(c) An aqueous solution of Sr(NO3)2 is added slowly to 1.0 litre of a well-stirred solution containing 0.020 mole F- and 0.10 mole SO4

2- at 25˚C. (You may assume that the added Sr(NO3)2 solution does not materially affect the total volume of the system.)1. Which salt precipitates first?

2. What is the concentration of strontium ion, Sr2+, in the solution when the first precipitate begins to form?

(d) As more Sr(NO3)2 is added to the mixture in (c) a second precipitate begins to form. At that stage, what percent of the anion of

Bergmann Page 19 9/11/2023

AP Chemistry Chapter 15 Practice Exam

1. A buffer solution contains 0.40 mole of formic acid, HCOOH, and 0.60 mole of sodium formate, HCOONa, in 1.00 liter of solution. The ionization constant, Ka, of formic acid is 1.8x10-4.(a) Calculate the pH of this solution.(b) If 100. milliliters of this buffer solution is diluted to a volume of 1.00 liter with pure

water, the pH does not change. Discuss why the pH remains constant on dilution.(c) A 5.00 milliliter sample of 1.00 molar HCl is added to 100. milliliters of the original

buffer solution. Calculate the [H3O+] of the resulting solution.(d) A 800.-milliliter sample of 2.00-molar formic acid is mixed with 200. milliliters of

4.80-molar NaOH. Calculate the [H3O+] of the resulting solution.

Bergmann Page 20 9/11/2023

2. In water, hydrazoic acid, HN3, is a weak acid that has an equilibrium constant, Ka, equal to 2.8x10-5 at 25ºC. A 0.300 liter sample of a 0.050 molar solution of the acid is prepared.(a) Write the expression for the equilibrium constant, Ka, for hydrazoic acid.(b) Calculate the pH of this solution at 25ºC.(c) To 0.150 liter of this solution, 0.80 gram of sodium azide, NaN3, is added. The salt

dissolved completely. Calculate the pH of the resulting solution at 25ºC if the volume of the solution remains unchanged.

(d) To the remaining 0.150 litre of the original solution, 0.075 liter of 0.100 molar NaOH solution is added. Calculate the [OH-] for the resulting solution at 25ºC.

Bergmann Page 21 9/11/2023

3. The equations and constants for the dissociation of three different acids are given below.

HCO3- <=> H+ + CO3

2- Ka = 4.2 x 10-7

H2PO4- <=> H+ + HPO4

2- Ka = 6.2 x 10-8

HSO4- <=> H+ + SO4

2- Ka = 1.3 x 10-2

(a) From the systems above, identify the conjugate pair that is best for preparing a buffer with a pH of 7.2. Explain your choice.

(b) Explain briefly how you would prepare the buffer solution described in (a) with the conjugate pair you have chosen.

(c) If the concentrations of both the acid and the conjugate base you have chosen were doubled, how would the pH be affected? Explain how the capacity of the buffer is affected by this change in concentrations of acid and base.

(d) Explain briefly how you could prepare the buffer solution in (a) if you had available the solid salt of the only one member of the conjugate pair and solution of a strong acid and a strong base.

Bergmann Page 22 9/11/2023

4. The solubility of iron(II) hydroxide, Fe(OH)2, is 1.43x10-3-gram per litre at 25oC.(a) Write a balanced equation for the solubility equilibrium.(b) Write the expression for the solubility product constant, Ksp, and calculate its value.(c) Calculate the pH of a saturated solution of Fe(OH)2 at 25oC.(d) A 50.0-millilitre sample of 3.00x10-3 molar FeSO4 solution is added to 50.0-

millilitres of 4.00x10-6-molar NaOH solution. Does a precipitate of Fe(OH)2 form? Explain and show calculations to support your answer.

Bergmann Page 23 9/11/2023

5. Solve the following problem related to the solubility equilibria of some metal hydroxides in aqueous solution.(a) The solubility of Cu(OH)2(s) is 1.72x10-6-gram per 100. milliliters of solution at 25°C.

(i) Write the balanced chemical equation for the dissociation of Cu(OH)2(s) in aqueous solution.

(ii) Calculate the solubility (in moles per liter) of Cu(OH)2 at 25oC.(iii) Calculate the value of the solubility-product constant, Ksp, for Cu(OH)2 at 25oC.

(b) The value of the solubility-product constant, Ksp, for Zn(OH)2 is 7.7x10-17 at 25oC.(i) Calculate the solubility (in moles per liter) of Zn(OH)2 at 25oC in a solution with

a pH of 9.35.(ii) At 25oC, 50.0-milliliters of 0.100-molar Zn(NO3)2 is mixed with 50.0-milliliters

of 0.300-molar NaOH. Calculate the molar concentration of Zn2+(aq) in the resulting solution once equilibrium has been established. Assume that volumes are additive.

Bergmann Page 24 9/11/2023