2. Which of these solutions is the least acidic? a) [H 3 O + ] = 4x10-4 M b) [H 3 O + ] = 0.04 M c) ph = 0.04 d) ph = 0.40 e) ph = 4.

Transcription

1 Chem 112, Test 2 -- February 13, page 1 START TIME END TIME Name You are allowed 2 consecutive hours (no cutting classes) for this test. You may use your calculator but NO extra scratch paper. Use the back of pages if necessary. LECTURE: Multiple Choice -- (24 pts) Choose the BEST answer and write the letter in the left margin next to the question. 1. Which of these solutions has the lowest ph? a) 0.50 M NH 4 Cl b) 0.50 M KCl c) 0.50 M HCOOH d) 0.50 M HNO 3 e) M HCl 2. Which of these solutions is the least acidic? a) [H 3 O + ] = 4x10-4 M b) [H 3 O + ] = 0.04 M c) ph = 0.04 d) ph = 0.40 e) ph = In water, what is it that distinguishes a strong acid from a weak acid? a) For a strong acid, there is a total translation to H 3 O + and the appropriate anion b) A strong acid does not have a K a value. c) For a strong acid, the sodium salt of the anion will have a value of ph = 7.00 d) The anion associated with the strong acid has no tendency to fight for a proton e) all of the above f) 3 of the above g) 2 of the above 4. From our presentation in class, what acid/base properties would you ascribe to the species HPO 4 2-? a) It would have a K a b) It would have a K b c) It would have a K a and a K b d) Since the bulk formula for this compound (Na 2 HPO 4 ) is known as a salt, it can t have any Acid/Base properties 5. If you were to see the phrase,... 95% confidence limits are 5.50cm to 6.50cm.., what is it you are controlling with 95% confidence? a) The effect of random error on the true value b) The effect of systematic error on the true value c) The likelihood that systematic error is present in the measurement d) The fact that Gross Error has been reduced to a percentage of at most 5% e) 3 of the above f) 2 of the above 6 You decide you wish to check your balance for the presence of any systematic error. You obtain from the National Institute of Standards and Technology an official weight whose certified value is given as g. You weigh the NIST weight on your balance 6 times, carefully using the prescribed proper set of instructions. How are you going to decide if no systematic error is present? a) If any of your measurement values are , then the balance is OK. b) If the % RSD for your measurements is < 0.4% then the balance is OK c) After a satisfactory Q-test, if a 95% C.I. around the mean of your 6 values excludes the value , then you will conclude the balance is free from systematic error. d) After a satisfactory Q-test, if a 95% C.I. around the mean of your 6 values inludes the value , then you will conclude the balance is free from systematic error. e) You need to get a mean value of in order to conclude that the balance is free from systematic error.

2 Chem 112, Test 2 -- February 13, page 2 7. Ultimately, what type of error or effect is the Gaussian Distribution trying to deal with? a) Random error b) Gross error c) a Real effect d) Systematic error e) The Gaussian Distribution plays some role in all of these 8. Given a single isolated measurement, which of the following could affect the measurement value? a) random error b) systematic error c) gross error d) a Real Effect, as evidenced by a non-zero value e) all of the above f) only 3 of the above 9. The following steps, drawn from your NaOH standardization procedure, describe stages of the process which contain uncertainties. Which of these would NOT be compensated for by the actual standardization titration? a) you had to measure out your two liters of deionized water in more than one container, thereby casting the actual volume of the solution in doubt b) in boiling the water you can lose some through evaporation, or else the bubbling gets so vigorous that some drops can escape from the beaker. c) you did not dry the Primary Standard KHP sufficiently. d) in using a graduate to measure out the concentrated NaOH solution, you had a great deal of difficulty in measuring out the 10.5 ml requested in the instructions. e) You failed to pre-wet your buret with the prepared NaOH solution before filling the buret for purposes of titrating the Primary Standard KHP f) all of the above cannot be compensated for by the standardization process. g) 2 of the above cannot be compensated for by the standardization process. 10. You carry out a determination for %KHP by titrating 5 samples to a %RSD of 0.33%. You obtain and report a mean value of %KHP. A 95% CI for your data is 0.06 %KHP. You later learn that the true value for this sample has a value of %KHP. What are you entitled to conclude? a) There was no gross error present in your work b) There was no systematic error present in your work c) The system was under statistical control in your work d) The % error for your work was 0.11% e) All of the above f) Only 2 of the above 11. When you determine the %KHP in a sample by plotting ph meter measurements on a piece of graph paper, what aspect of using hand plotted graph paper results limits the number of significant figures for your final answer? a) the balance reading of the sample weight taken b) the ph meter readings c) the amount of water needed to cover the electrode if the titration solution is very shallow d) the reading of the buret volume e) the reading of the volume value off the paper f) the actual calibration of the ph meter using the ph standard solutions 12. Many local anesthetics are weak bases. Which of the following is the weakest base? a) phenol, K b = 1.0x10-2 b) benzocaine, pk b = 5.50 c) cocaine, pk b = 5.39 d) lidocaine, K b = 6.3x10-7

3 Chem 112, Test 2 -- February 13, page 3 CONCEPTS FROM LECTURE/LAB 1. (18 pts) The following 3 questions explore the relationship among the following four terms: equivalence point, end point, Range of 0.1% accuracy, and Indicator Window. a) Define each of these 4 terms: equivalence point end point Range of 0.1% Accuracy Indicator Window b) What is the distinction between an end point and an equivalence point? c) Explain, using the above 4 terms as applicable, what must be accomplished in a visual titration in order to avoid a systematic indicator error

4 Chem 112, Test 2 -- February 13, page 4 2. (15 pts) (in Dr. D s terms) ACID WIT AND BASE COMMENTS! Answer with a phrase, formula, or word. NO ESSAYS PLEASE! 1. Which is the stronger acid in water, HCl or HF? 2. Which is the stronger acid in water, HF or NH 3? 3. Which is the stronger acid in water, HClO 4 or HClO 3? 4. Which is the stronger acid in water, H 2 O or H 2 S? 5. What is the structure of the carboxylic acid group? 6. What is a ternary acid? 7. If HX is a binary acid, is X a metal or a non-metal? 8. What is the conjugate base for methane, CH 4? Is it a strong or a weak base? 9. I gave two criteria for determining acid strength from the structure of a molecule. Give me one of them: 10. What is the strongest acid that can exist in a particular solvent? 11. The pk a of an organic acid is 5.2. What can you say about the dominant form of this acid in an aqueous solution at ph = 6.8? 12. The pk a of an organic acid is 5.2. What can you say about the dominant form of this acid in an aqueous solution at ph = 3.8? 13. The pk a of H 2 O is What is it s conjugate base? 14. In a ternary acid, the acidic proton is always bonded to. 15. If a solvent is a leveling solvent for two acids X and Y, then the protonated form of the solvent must be a acid than either X or Y.

5 Chem 112, Test 2 -- February 13, page 5 PROBLEMS. (69 pts) SHOW ALL WORK, METHOD MUST BE CLEAR (NO MAGIC PERMITTED). The following table will prove useful in all of the upcoming Acid/Base characterizations. Use of the ph line is encouraged. Name Formula MW Other Formic Acid HCOOH pk a = 3.77 Acetic Acid HOAc pk a = 4.74 Sulfuric Acid H 2 SO Strong diprotic acid Sodium Chloride NaCl Sodium Hydroxide NaOH Ammonia NH pk b = 4.75 Methyl Amine CH 3 NH pk b = 3.36 Sodium Formate HCOONa Sodium Acetate NaOAc Hydrochloric Acid HCl Methyl Ammonium Formate CH 3 NH 3 HCOO (36 points) Characterize the following Acid/Base systems by determining the final value of ph in all cases. a) The system that arises when ml of M Ammonia is mixed with ml of M Hydrochloric Acid b) The system that arises when ml of M Methyl Amine is diluted to 200 ml with pure distilled water c) The system that arises when 50 ml of M Formic Acid is mixed with 60 ml of M Sodium Hydroxide. d) The system that arises when 50 ml of M Formic Acid is mixed with 60 ml of M Sodium Formate

6 Chem 112, Test 2 -- February 13, page 6 e) The system that arises when ml of M Hydrochloric Acid is diluted to ml in a volumetric flask with a M Sodium Chloride solution. f) The system that arises when ml of M Acetic Acid is diluted to ml in a volumetric flask with a M Sodium Chloride solution 2. (13 pts) In carrying out your KHP determination using the ph meter, you run into a potentially alarming situation when you weigh out your sample. When weighing out your unknown KHP a BIG CLUMP of sample KHP falls into your beaker, making the final weight of sample g instead of the book target weight of 1 gram. a) What is the potential problem which you may face generated by the presence of the BIG CLUMP? b) Your previous work with the same KHP sample has yielded a %KHP value of %KHP. Show by calculation how you can decide beforehand if you are going to encounter the potential problem referred to in question a) above. Your NaOH titrant has a molarity of and the MW of KHP is c) As long as we re here, calculate the ph at the equivalence point if/when you were to carry out this titration, whether the potential problem actually comes to pass or not. Assume that you dissolve the BIG CLUMP in the agreed on value of ml of water before adding the NaOH titrant.

7 Chem 112, Test 2 -- February 13, page 7 3. (20 pts) You carried out your first set of four %KHP determinations, and you wound up with the following percentages for %KHP: 44.30, 44.46, 44.05, and You decided to run a second set and obtained the following results for %KHP: 44.01, 43.97, 44.05, and a) First of all, what was it that made you decide to run the second set in the first place? SHOW BY CALCULATION(S) ACCOMPANIED BY A CLEAR EXPLANATION OF THE REASONING PROCESS YOU USE. (NOTE: if you decide you need statistical decisionmaking that involves a standard deviation, you need to use the official Sum of Squares definition of the Sample Standard Deviation, in ALL such calculations) b) Having run the second set, the logical question is: do you have to do any more determinations? How are you going to decide? SHOW BY CALCULATION(S) ACCOMPANIED BY REASONED EXPLANATION. c) What value do you choose to hand in to be judged/graded by? Why? d) Finally, if/when you learn that the True Value of %KHP in your sample is 44.00, SHOW how you decide whether or not Systematic Error was present in your work. (Recall: you are supposed to get the right answer). Please write and sign the pledge: