Titration An experimental method used to determine the concentration of an unknown solution

Transcription

1 Titration An experimental method used to determine the concentration of an unknown solution Acid-Base Titration Curve Make a plot of titrant delivered vs ph of solution Shape is characteristic of reagents used in titration Strong acid titrated with strong base OR Weak acid titrated with a strong base OR Weak base titrated with strong acid Titration of Strong Acid with Strong Base ex: ml 0.10 M HCl titrated with 0.10 M NaOH. Neutralization reaction: HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 1 of 14

2 Four Stages on titration curve: Stage 0 (prior to addition of base): Stage I (prior to equivalence point): Stage II (at equivalence point): Stage III (after equivalence point): The general steps to titration problems are 1. Classify the acid (or base) as strong or weak 2. Determine the amount of acid (or base) initially present 3. Determine the amount of base (or acid) added Assume the neutralization reaction proceeds to completion 4. Determine the stage of your titration on the titration curve (what region 0, I, II, or III) 5. Decide the type of calculation to perform based on what is in solution strong acid strong base weak acid weak base buffer 6. Do the calculation (allow for new solution volume when calculating concentrations) 7. Sanity check Ask yourself if the answer makes physical sense We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 2 of 14

3 Strong Acid-Strong Base Example Consider a ml volume of 0.10 M HCl that is titrated with 0.10 M NaOH. Write the neutralization reaction and characterize the resulting salt as acidic, neutral or basic Calculate the ph of the solution after addition of 0.00 ml of 0.10 M NaOH (start of titration) Calculate the ph of the solution after addition of ml of 0.10 M NaOH 1.00 Calculate the ph of the solution after addition of ml of 0.10 M NaOH 1.48 Calculate the ph of the solution after addition of ml of 0.10 M NaOH We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 3 of 14

4 Titration of a Weak Acid with a Strong Base Example: ml 0.10 M propanoic acid (KA = ) titrated with 0.10 M NaOH Neutralization reaction: Four Stages on titration curve: Stage 0 (prior to addition of base): Stage I (prior to equivalence point): Stage II (at equivalence point): Stage III (after equivalence point): We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 4 of 14

5 Weak Acid-Strong Base Example Consider the titration of ml 0.10 M propanoic acid (KA = ) is titrated with 0.10 M NaOH. Represent propanoic acid as HA and its conjugate base as A Calculate the ph of the solution after the addition of 0.00 ml of 0.10 M NaOH (start of titration) Calculate the ph of the solution after the addition of ml of 0.10 M NaOH 2.95 Calculate the ph of the solution after the addition of ml of 0.10 M NaOH 4.89 Calculate the ph of the solution after the addition of ml of 0.10 M NaOH We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 5 of 14

6 Calculate the ph of the solution after addition of ml of 0.10 M NaOH In titration of HCN by KOH (KA = ) What type of salt is present at the equivalence point? What is the ph half-way to the equivalence? Concept Check 25 ml of 0.20 M NaOH was added to 50. ml of 0.12 M HNO2. At this stage of the titration the solution will be and the calculation involves. a. acidic; a buffer problem b. basic; a buffer problem c. acidic; a hydrolysis/ weak acid problem d. basic; a hydrolysis/ weak base problem e. neutral; no problem Concept Check 40.0 ml of 0.10 M NH3 was added to 50.0 ml of M HCl. At this stage of the titration the solution will be and the calculation involves. a. acidic; buffer problem b. basic; a buffer problem c. acidic; a hydrolysis/ weak acid problem d. basic; a hydrolysis/ weak base problem e. acidic; a strong acid problem We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 6 of 14

7 Titration of a Weak Base with a Strong Acid Example: ml M NH3 (KB = ) titrated with M HCl Neutralization reaction: Four Stages on titration curve: Stage 0 (prior to addition of acid): Stage I (prior to equivalence point): Stage II (at equivalence point): Stage III (after equivalence point): We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 7 of 14

8 Strong Acid-Weak Base Example Consider the titration of ml 0.10 M piperidine (KB = ) is titrated with 0.10 M HCl. Represent piperidine as B and its conjugate acid as BH + Calculate the ph of the solution after the addition of 0.00 ml of 0.10 M NaOH (start of titration) Calculate the ph of the solution after the addition of ml of 0.10 M HCl Calculate the ph of the solution after the addition of ml of 0.10 M HCl 9.11 Calculate the ph of the solution after addition of ml of 0.10 M HCl We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 8 of 14

9 In titration of NH3 by strong acid HCl What type of salt is present at the equivalence point? acidic Consider the titration of 15 ml of M HF with M Ca(OH)2. KA of HF is What volume of Ca(OH)2 is needed to reach equivalence point? What is the ph at the equivalence point? 45 ml ph = 8.02 Titration of polyprotic acids We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 9 of 14

10 Equivalence Point Indicator End Point Determined by stoichiometry just enough titrant added to neutralize the titrate Organic weak acid or base appearance depends on solution ph ex: Phenolphthalein is colorless in acidic solution and pink when ph > 8 Determined by indicator occurs when indicator changes color choose indicator so that end point and equivalence point coincide Acid-Base Indicators a weak organic acid or base whose color differs from that of its conjugate example: methyl-red (2-(N,N-Dimethyl-4-aminophenyl)azobenzenecarboxylic acid) H 3 C CH 3 N N COOH H 3 C N N COO- CH 3 + H + The color of an indicator changes over a narrow ph range, when ph pka,in ± 2 ph units We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 10 of 14

11 Consider a titration of a weak base by a strong acid where the ph at equivalence is 5.2 Choose an appropriate indicator What experimental complication would occur if Crystal violet was chosen as an indicator Methyl red color would be blue at start and never change What experimental complication would occur if phenolphthalein was chosen as an indicator color change would occur before equivalence point (too little titrant added) We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 11 of 14

12 Extra Practice Problems Consider the titration of 40.0 ml of M a weak acid (Ka = ) with M Ca(OH)2. How many milliliters of base are required to reach the equivalence point? Calculate the ph After addition of 0.0 ml of base 50.0 ml Calculate the ph After addition of 10.0 ml of base 2.04 Calculate the ph half-way to the equivalence point 2.85 Calculate the ph At the equivalence point 3.46 Calculate the ph after addition of 80.0 ml of base We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 12 of 14

13 Consider the titration of 40.0 ml of M NH3 (Kb = ) with 0.10 M HCl. Is the ph of the equivalence point acidic basic or neutral? Calculate the ph after addition of 0.0 ml of M HCl acidic Calculate the ph after addition of 10.0 ml of M HCl Calculate the ph half-way to the equivalence point 9.95 Calculate the ph at the equivalence point 9.26 Calculate the ph after addition of 80.0 ml of M HCl We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 13 of 14

14 Extra Challenge Problem A 24.0 ml volume of M H2SO4 is titrated with M NaOH. Calculate the ph of the solution after addition of the following volume of base: Calculate the ph of the solution after addition of 0.00 ml of M NaOH Calculate the ph of the solution after addition of ml of M NaOH Calculate the ph of the solution after addition of ml of M NaOH Calculate the ph of the solution after addition of 46.0 ml of M NaOH We are what we repeatedly do. Excellence, then, is not an act, but a habit. Aristotle Page 14 of 14