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  • 1. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 1 Rapid Learning Center Chemistry :: Biology :: Physics :: Math Rapid Learning Center Presents …p g Teach Yourself AP Chemistry Visually in 24 Hours 1/71 http://www.RapidLearningCenter.com A id d BAcids and Bases AP Ch i t R id L i S i Rapid Learning Center www.RapidLearningCenter.com/ © Rapid Learning Inc. All rights reserved. AP Chemistry Rapid Learning Series Wayne Huang, PhD Kelly Deters, PhD Russell Dahl, PhD Elizabeth James, PhD Debbie Bilyen, M.A.
  • 2. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 2 Learning Objectives Several definitions of “acid” and “base” By completing this tutorial you will learn about… Strong versus weak acids and bases Conjugate acids and bases How strong and weak relates to equilibrium How to find the pH of strong and weak acids and bases 3/71 weak acids and bases How salts can be acidic or basic How buffers work Titration curves Concept Map Chemistry Studies Previous content New content and Bases Acids and Bases Can be Matter pH EquilibriumEquilibrium Constants Conjugates Form Form Scale to measure Have Can be 4/71 Strong or Weak Strong or Weak Buffer Common IonCommon Ion Effect Shown with Affected by
  • 3. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 3 Acid and Base Definitions 5/71 Definition - Arrhenius Arrhenius Acid – Produces the hydronium ion in waterhydronium ion in water. Arrhenius Base – Produces the H3O+ 6/71 hydroxide ion in water. OH-
  • 4. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 4 O HH Cl Arrhenius Acids and Bases Arrhenius Acid: HCl + H2O H3O+ + Cl- O HH Cl+1 -1 HH O HHHH O HH Acid Water Hydronium ion Arrhenius Base: NH + H O NH + + OH- 7/71 Arrhenius Base: NH3 + H2O NH4 + + OH- HH N HH HH O HHHH HH N HH HH HH O HH +1 -1 Definition - Brønsted-Lowry Brønsted Lowry AcidBrønsted-Lowry Acid – Donates a proton (H+). Brønsted-Lowry Base – 8/71 Accepts a proton (H+).
  • 5. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 5 Definition - Lewis Lewis Acid – Electron tacceptor. Lewis Base – Electron donor. 9/71 Comparing the Definitions How do the 3 definitions relate? Acid Base Note Produces H3O+ in water Donates a H+ Accepts electrons Produces OH- in water Accepts a H+ Donates electrons Requires water Does not need to be in water Does not need to use “H” Arrhenius Brønsted- Lowry Lewis 10/71 Many Arrhenius acids/bases are also Brønsted-Lowry and Lewis acids/bases.
  • 6. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 6 Properties of Acids and Bases What are some common properties? Acids Bases Taste sour (e.g. citrus) Taste bitter (e.g. soap) React with metals to form H2 gas Have pH levels below 7 Turn Litmus red Feel slippery Have pH levels above 7 Turn Litmus blue 11/71 StStrong versus Weak 12/71
  • 7. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 7 Definition - Strong and Weak Strong – Most of the molecules perform their “duty”.y Strong Acid: Most of the molecules donate their H+. Weak – Very few of the molecules perform their “duty”. 13/71 perform their duty . Weak Acid: Only a very few of the molecules donate their H+. Definition - Concentrated and Dilute Concentrated – Many acid b l l hor base molecules have been added to the system. Dilute – Only a few acid or base molec les ha e been 14/71 base molecules have been added to the system.
  • 8. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 8 Possible Combinations There are 4 combinations of strong, weak, concentrated and dilute: Concentrated Dilute A lot added & almost all do their “duty”. A lot added, but very Not many added, but of what is there, most will do their “duty”. Only a few added and of those, only a Strong W k 15/71 A lot added, but very few do their “duty”. and of those, only a very small % do their “duty”. Weak Common Strong Acids and Bases There are only a few common strong acids and bases to remember - the rest will most likely be weak! HCl, HBr, HI, HNO3, HClO3, HClO4, H2SO4 NaOH, KOH, Ca(OH)2, Ba(OH)2, Sr(OH)2 Strong Acids Strong Bases 16/71
  • 9. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 9 Definition - Polyprotic Acids Polyprotic acid – Acid with more than one proton to donate. Examples … Diprotic: H2CO3 17/71 Triprotic: H3PO4 Strength of Polyprotic Acids Each proton is “weaker” than the one before. A negatively charged ion is An acid donates a proton e.g: H2CO3 The results is a negatively charged ion Becomes HCO3 - less likely to give up another proton and become -2 charged Would be CO3 2- 18/71 H2CO3 is a stronger acid than HCO3 -. Sulfuric acid (H2SO4) is the only common strong polyprotic acid. The 1st hydrogen is “strong” and the 2nd one is “weak”.
  • 10. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 10 Conjugates of Acids and Bases 19/71 Definition - Conjugate Base C j B Wh ’ l f fConjugate Base – What’s left after an acid has donated its hydrogen. HCl + H2O H3O+ + Cl- Conjugate base 20/71 Acid After the proton is donated, it can now act as a base… it could accept a proton.
  • 11. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 11 Definition - Conjugate Acid Conjugate Acid What’s left after aConjugate Acid – What s left after a base has accepted a proton. HCl + H2O H3O+ + Cl- Base Conjugate acid 21/71 After the proton is accepted, it can now act as an acid… it could donate the extra proton. Labeling Species Example: For each of the following, label the acid (A), the base (B), the conjugate acid (CA) and conjugate base (CB). NH3 + H2O NH4 + + OH- 3 2 4 H2SO4 + H2O H3O+ + HSO4 - AB CA CB A B CA CB 22/71 NH3 + H2SO4 NH4 + + HSO4 - AB CA CB
  • 12. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 12 Definition - Amphoteric Amphoteric – A molecule that can actp as an acid or base. HCl + H2O H3O+ + Cl- Base 23/71 NH3 + H2O NH4 + + OH- Acid Acid-Base Equilibrium 24/71
  • 13. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 13 Definition - Ka Acid Dissociation Constant (Ka) – Equilibrium constant for an acid dissociation reaction. General acid dissociation equation: 25/71 HA (aq) H+ (aq) + A- (aq) ][ ]][[ 11 HA AH Ka −+ = Writing Ka Expressions Example: Write the Ka expressions for the following: HCl (aq) H+ (aq) + Cl- (aq) ]][[ 11 ClH −+ H2CO3 (aq) H+ (aq) + HCO3 - (aq) ][ ]][[ HCl ClH Ka = ][ ]][[ 32 1 3 1 COH HCOH Ka −+ = 26/71 HCO3 - (aq) H+ (aq) + CO3 2- (aq) ][ ]][[ 1 3 2 3 1 − −+ = HCO COH Ka
  • 14. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 14 Definition - Kb Base Dissociation Constant (Kb) –( b) Equilibrium constant for a base dissociation reaction. General base dissociation equation: BOH (aq) B+ (aq) + OH- (aq) 27/71 BOH (aq) B (aq) + OH (aq) ][ ]][[ 11 BOH OHB Kb −+ = Writing Kb Expressions Example: Write the Kb expressions for the following: NaOH (aq) Na+ (aq) + OH- (aq) ]][[ 11 OHNa −+ Ca(OH)2 (aq) Ca2+ (aq) + 2 OH- (aq) ][ ]][[ NaOH OHNa Kb = ])([ ]][[ 2 212 OHCa OHCa Kb −+ = 28/71
  • 15. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 15 Definition - Kw Autoionization of Water – Water will spontaneous dissociate into H+ and OH-.p Water Dissociation Constant (Kw) – Equilibrium constant for the autoionization of water. 29/71 Water Dissociation Equation: H2O (l) H+ (aq) + OH- (aq) ]][[ 11 −+ = OHHKw H2O is not included because it’s a pure liquid! Relating Ka, Kb and Kw Ka and Kb of conjugates are related through Kw: baw KKK ×= At 25°C, Kw = 1.0×10-14 Example: If the Ka for CH3COOH is 1.8×10-5, find the Kb for it’s conjugate base CH3COO- at 25°C. bK××=× −− )108.1(100.1 514Ka = 1.8×10-5 Kw = 1.0×10-14 Kb = ? bK= × − − )1081( 100.1 5 14 30/71 Kb = 5.6×10-10 b × − )108.1( 5
  • 16. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 16 Relating Strength and Equilibrium What do the Ka and Kb tell us about strength? A large Ka means the reaction “lies to the right”. There is a large ratio of products : reactants. For acid and base reactions, the products are the dissociated ions. The more dissociated ions, the stronger the acid or base. 31/71 The larger the Ka or Kb, the stronger the acid or base. Strength and Conjugates How does the strength of a species relate to the strength of its conjugate? If it donated the A strong acid donates it’s proton easily. After donating the proton, it then becomes a (conjugate) base. If it donated the proton easily, it will not easily grab and hold onto another one. It will be a weak base. 32/71 Strong Acid Weak Conjugate Base Weak Acid Strong Base Weak Base Strong Conjugate Base Weak Conjugate Acid Strong Conjugate Acid
  • 17. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 17 pH 33/71 Definition - Logarithms Logarithm – The number of times a base must be multiplied by itself to reach a given number. yx blog= # of multiples 34/71 Base # you’re trying to reach
  • 18. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 18 Definition - pH pH scale Logarithmic scalepH scale – Logarithmic scale of the acidity of a solution. The pH scale uses base “10”. ]log[ 1 3 + −= OHpH 35/71 pH OH −+ =10][ 1 3 pH has no units. pH Scale 0 7 14 Some common items and their pH values: 2.0 Stomach Contents Acidic Neutral 6.5 ~8.0 Tap water Basic 36/71 7.4 Blood Contents 3.0 Pop 4.5 Beer 5.5 Bread Milk Tap water
  • 19. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 19 pH of Strong Acids For strong acids, assume there is complete dissociation. Example: Find the pH of a 0.25 M solution of HCl. [H3O+] = 0.25 M pH = ? HCl is a strong acid: HCl + H2O H3O+ + Cl- If [HCl] = 0.25 M, then [H3O+] = 0.25 M ]log[ 1 3 + −= OHpH 37/71 pH ? pH = 0.60 )25.0log( MpH −= pH of Strong Bases When working base problems, you can find [OH-] by the base concentration…but you must find [H3O+] to find pH. Example: Find the pH of a 0.15 M solution of NaOH . NaOH is a strong base: NaOH Na+ + OH- If [NaOH] = 0.15 M, then [OH-] = 0.15 M ]][[ 11 3 −+ = OHOHKw ]10][[1001 114 O+ 38/71 [H3O+] = 6.7×10-14 M pH = ? pH = 13.18 ]log[ 1 3 + −= OHpH )107.6log( 14 MpH − ×−= ]15.0][[100.1 1 3 14 MOH +− =×
  • 20. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 20 pH of Weak Acids - 1 When working with weak acids and bases, you must use the equilibrium ICE chart techniques to find [H3O+]. Example: Find the pH of a 0.25 M solution of CH3COOH if K = 1 8×10-5 CH3COOH H3O+ CH3COO- Initial 0.25 M 0 M 0 M Change Equilibrium -x +x +x 0.25 M - x 0 M + x 0 + x Ka = 1.8×10 5 . CH3COOH (aq) + H2O (l) H3O+ (aq) + CH3COO- (aq) Approximations: 0.25 M 0 M + x 0 M + x 39/71 ][ ]][[ 3 1 3 1 3 COOHCH COOCHOH Ka −+ = )25.0( ))(( 108.1 5 M xx =× − ( )( ) xM =× − 25.0108.1 5 Approximations: 0.25 M 0 M + x 0 M + x Ka is small enough to make approximations. x = 0.0021 M MMOH 0021.00][ 1 3 +=+ [H3O+] = 0.0021 M pH of Weak Acids - 2 When working with weak acids and bases, you must use the equilibrium ICE chart techniques to find [H3O+]. Example: Find the pH of a 0.25 M solution of CH3COOH if K = 1 8×10-5 CH3COOH H3O+ CH3COO- Initial 0.25 M 0 M 0 M Change Equilibrium -x +x +x 0.25 M - x 0 M + x 0 + x Ka = 1.8×10 5 . CH3COOH (aq) + H2O (l) H3O+ (aq) + CH3COO- (aq) Approximations: 0.25 M 0 M + x 0 M + x 40/71 Approximations: 0.25 M 0 M + x 0 M + x x = 0.0021 M [H3O+] = 0.0021 M ]log[ 1 3 + −= OHpH )021.0log( MpH −= pH = 2.67
  • 21. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 21 Acid-Base Properties of Salts 41/71 How Salts Have Acid/Base Properties Understanding conjugates is a key to understanding acid/base properties of salts. A weak acid has a strong conjugate base that can form salts. That strong conjugate base will produce a basic solution. Salts made from the conjugate of a weak acid will have a basic pH. 42/71 The opposite is also true: Salts made from the conjugate of a weak base will be acidic.
  • 22. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 22 The Combinations to Form Salts Cation from… Strong Acid Weak Acid Anion from… Strong Base Weak Base Neutral Salt Acidic Salt Basic Salt Neutral Salt Think of it as the “strong on wins” (e.g. strong acid & weak base = acidic) 43/71 Example: Determine if each salt will be acidic, basic or neutral: NH4Cl NaH2PO4 Weak base & strong acidCame from NH3 and HCl Acidic salt Came from NaOH and H3PO4 Strong base & weak acid Basic salt Buffers 44/71
  • 23. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 23 Definition - Buffer Buffer – Solution containing a weak acid or base and its conjugate that resistsj g change in pH levels. e.g. H3PO4 (weak acid) and PO4 3- (conjugate base) Conjugates are added in the form of a salt 45/71 j g (soluble ionic compound). e.g. To add PO4 3- to a buffer, Na3PO4 salt is added. How Does a Buffer Work? Buffer: Weak Acid and conjugate base How does a buffer resist pH changes? Conjugate base “absorbs” acid and produces more weak acid already in buffer. Strong Acid added Weak acid “absorbs” base and produces more conjugate base already in buffer. Strong Base added Eventually, so much strong acid or base could be added and all of 46/71 the buffer material would be used up. Buffer Capacity – Amount of strong acid or base that can be absorbed without large pH change.
  • 24. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 24 Henderson-Hasselbach Equation This equation is used to find the pH of a buffer system. ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ += ][ ][ log acid base pKapH pKa = -log Ka [acid] & [base] refer to molarity of a weak acid or base and its conjugate. Find the pH of a solution with 0.50 M HC2H3O2 (Ka = 1.8×10-5) and 0.30 M NaC2H3O2. pKa = -log (1.8×10-5) = 4.74 [acid] = 0.50 M HC2H3O2 [base] = 0.30 M C2H3O2 - ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ += M M pH 50.0 30.0 log74.4 Example: 47/71 [base] 0.30 M C2H3O2 pH = ? )22.0(74.4 −+=pH pH = 4.52 Adding Acid to a Buffer - 1 When adding an acid or base to a buffer solution, complete the stoichiometry first. Then, with the remaining components, find the pH. Find the pH of a solution with 0.50 M HC2H3O2 (Ka = 1.8×10-5) and 0.30 M NaC2H3O2 that has had HCl added to equal 0.25 M. Example: HC2H3O2 C2H3O2 - Buffer 0.50 M 0.30 M Acid reacts + 0 25 M - 0 25 M The acid will react with the conjugate base, producing more weak acid. 48/71 Acid reacts + 0.25 M 0.25 M New components 0.75 M 0.05 M pKa = -log (1.8×10-5) = 4.74 [acid] = 0.75 M HC2H3O2 [base] = 0.05 M C2H3O2 - pH = ? ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ += M M pH 75.0 05.0 log74.4 )17.1(74.4 −+=pH pH = 3.57
  • 25. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 25 Adding Acid to a Buffer - 2 When adding an acid or base to a buffer solution, complete the stoichiometry first. Then, with the remaining components, find the pH. Find the pH of a solution with 0.50 M HC2H3O2 (Ka = 1.8×10-5) and 0.30 M NaC2H3O2 that has had HCl added to equal 0.25 M. Example: Although the pH did drop…an earlier example showed adding the same amount of HCl to plain water would drop the pH 49/71 pH = 3.57 p p p from 7.0 to 0.60. The plain water is affected to a much greater degree than the buffer. Adding Base to a Buffer Likewise, adding a base to a buffer will raise the pH, but much less than adding it to water. Find the pH of a solution with 0 50 M HC H O (K =Example: Find the pH of a solution with 0.50 M HC2H3O2 (Ka = 1.8×10-5) and 0.30 M NaC2H3O2 that has had NaOH added to equal 0.15 M. Example: HC2H3O2 C2H3O2 - Buffer 0.50 M 0.30 M Base reacts - 0.15 M + 0.15 M N t 0 35 M 0 45 M The base will react with the weak acid, producing more conjugate base. 50/71 New components 0.35 M 0.45 M pKa = -log (1.8×10-5) = 4.74 [acid] = 0.35 M HC2H3O2 [base] = 0.45 M C2H3O2 - pH = ? ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ += M M pH 35.0 45.0 log74.4 )11.0(74.4 +=pH pH = 4.85
  • 26. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 26 Titrations 51/71 Definition - Titrations Titration Addition of a knownTitration – Addition of a known volume of a known concentration solution to a known volume of unknown concentration solution to determine the concentration. 52/71 determine the concentration.
  • 27. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 27 Definition - End Point End Point (or Stoichiometric Point) – When there is no reactant left over—they h ll b t d d th l tihave all been reacted and the solution contains only products. I di t P li id th t h The end point must be reached in order to use stoichiometry to calculate the unknown solution concentration. 53/71 Indicators – Paper or liquid that change color based on pH level. If the pH of the products is known, the indicator can be chosen to indicate the end point. Definition - Titration Curve Titration Curve – Graph showing pHGraph showing pH of unknown solution verses the volume of the known solution b i dd d 54/71 being added.
  • 28. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 28 Titration Curve of Strong Acids What does a titration of a strong acid look like? Past the end point, there is now pH End Point pH changes dramatically as the last molecule of acid reacts there is now excess base. An indicator is chosen that changes color at this pH pKa 55/71 Volume of base added pH changes very slowly— there is excess acid remaining ½ way to end point Titration Curve of Weak Acids How is the curve of a weak acid different from a strong acid? pH W k Higher starting pH (weak acids are less “acidic”) Less defined end point inflection point End point pH above 7 (basic) 56/71 Volume of base addedStrong Weak acids are less acidic )
  • 29. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 29 Titration Curve of Polyprotic Acids Diprotic acids have 2 protons to titrate. pH 57/71 Volume of base added There are 2 end points - usually one is more defined than the other. Common Ion Effect 58/71
  • 30. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 30 Definition - Common Ion Effect Common Ion Effect – Effect an ion has on the pH of an acidic or basic solution containing the same ion. 59/71 Example: Adding NaCH3COO will affect the pH of a solution of CH3COOH as they have a common ion (CH3COO-). Common Ion and Le Chatelier Le Chatelier’s Principle says that adding a product will push the reaction towards the reactants. CH COOH (aq) + H O (l) H O+ (aq) + CH COO- (aq)CH3COOH (aq) + H2O (l) H3O (aq) + CH3COO (aq) Adding the common ion (CH3COO-) would push the reaction Fewer acid molecules are dissociated, so there are fewer H O+ pH will be higher (less acidic) with this common ion than without 60/71 back to the reactants. fewer H3O ions. than without. Bases would work the opposite - adding the common ion of a base would cause the solution to be “less basic” (have a lower pH).
  • 31. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 31 Solving Common Ion Problems Common Ion problems are like weak acid problems - except you have an initial amount of the conjugate. Example: Find the pH of a 0.25 M solution of CH3COOH (Ka = 1 8×10-5) if it is also 0 10 M NaCH COO CH3COOH H3O+ CH3COO- Initial 0.25 M 0 M 0.10 M Change Equilibrium -x +x +x 0.25 M - x 0 M + x 0.10 + x (Ka = 1.8×10 5) if it is also 0.10 M NaCH3COO. CH3COOH (aq) + H2O (l) H3O+ (aq) + CH3COO- (aq) Approximations: 0.25 M 0 M + x 0 10 M 61/71 ][ ]][[ 3 1 3 1 3 COOHCH COOCHOH Ka −+ = )25.0( )10.0)(( 108.1 5 M x =× − ( )( ) x M M = × − 10.0 25.0108.1 5 Approximations: 0.25 M 0 M + x 0.10 M Ka is small enough to make approximations x = 4.5×10-5 M MMOH 51 3 105.40][ −+ ×+= [H3O+1] = 4.5×10-5 M Solving Common Ion Problems Common Ion problems are like weak acid problems - except you have an initial amount of the conjugate. Example: Find the pH of a 0.25 M solution of CH3COOH (Ka = 1 8×10-5) if it is also 0 10 M NaCH COO CH3COOH H3O+ CH3COO- Initial 0.25 M 0 M 0.10 M Change Equilibrium -x +x +x 0.25 M - x 0 M + x 0.10 + x (Ka = 1.8×10 5) if it is also 0.10 M NaCH3COO. CH3COOH (aq) + H2O (l) H3O+ (aq) + CH3COO- (aq) Approximations: 0.25 M 0 M + x 0 10 M 62/71 Approximations: 0.25 M 0 M + x 0.10 M x = 4.5×10-5 M [H3O+1] = 4.5×10-5 M ]log[ 1 3 + −= OHpH )105.4log( 5 MpH − ×−= pH = 4.35
  • 32. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 32 Acids, Bases & The AP Exam 63/71 Acid/base in the Exam Stoichiometry Common acid/base problems: Titrations pH calculations Ka and Kb 64/71
  • 33. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 33 Multiple Choice Questions Acid/base multiple choice questions tend to be more qualitative. Example: How can 100 mL of a 0.10 M basic solution with pH of 13.00 become a solution with a pH of 12.00? A. Diluting solution with 8 more mL of distilled water B. Diluting solution with 100 mL more of distilled water C. Diluting solution with 900 mL more of distilled water D. Adding 100 mL of 0.10 HA E. Adding 100 mL of 0.10 base 65/71 pH is a log base 10 scale. 1 change in pH = 10 x change in acidity You want the solution to be diluted by 1/10. Making a 100 mL solution into a 1000 mL solution is a 1/10 dilution. Answer: C Free Response Questions An acid-base free response question may contain sub- questions such as: a. Describe the features of the titration curve that indicate the acid is a k idweak acid b. Describe one way in which the acid dissociation constant can be determined from the titration curve c. A 25 mL sample of 0.10 M HA is titrated with 0.35 M NaOH I. What is the volume of NaOH needed to reach the equivalence point? II. Is the pH at this equivalence point above or below 7? 66/71
  • 34. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 34 Answering Free Response Questions a. Describe the features of the titration curve that indicate the acid is a weak acid. Weak acids begin at a higher pH and have a less defined inflection point The stoichiometric point will be above pH b. Describe one way in which the acid dissociation constant can be determined from the titration curve. inflection point. The stoichiometric point will be above pH of 7 67/71 pKa = pH at volume added to get half-way to inflection point. Then pKa = -log Ka Answering Free Response Questions c. A 25 mL sample of 0.10 M HA is titrated with 0.35 M NaOH I. What is the volume of NaOH needed to reach the equivalence point? 0.025 L HA 0.1 mole HA 1 mole NaOH 1 L NaOH = _____ L NaOH 1 L HA 1 mole HA 0.35 mole NaOH 0.0071 L NaOH 68/71 II. Is the pH at this equivalence point above or below 7? Weak acid + strong base = above 7
  • 35. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 35 pH can be determined from pH can be determined from There are several a s to define There are several a s to define Acid/base problems include b ff tit ti Acid/base problems include b ff tit ti Learning Summary Acid and baseAcid and base determined from the concentration of hydronium ions in a solution. determined from the concentration of hydronium ions in a solution. ways to define acids and bases. ways to define acids and bases. buffers, titrations and common ion problems. buffers, titrations and common ion problems. 69/71 Acid and base dissociation reactions have dissociation constants and can be worked with ICE charts. Acid and base dissociation reactions have dissociation constants and can be worked with ICE charts. Some acids and bases are strong, while others are weak. Some acids and bases are strong, while others are weak. Congratulations You have successfully completed the core tutorial Acids and Bases Rapid Learning Center
  • 36. AP Chemistry Rapid Learning Series - 21 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 36 Rapid Learning Center Wh t’ N t Chemistry :: Biology :: Physics :: Math What’s Next … Step 1: Concepts – Core Tutorial (Just Completed) Step 2: Practice – Interactive Problem Drill Step 3: Recap – Super Review Cheat Sheet 71/71 Go for it! http://www.RapidLearningCenter.com