21. A 0.3729 gm of impure soda ash requires 32.77 mL of a 0.1555 N HCl titrant for complete neutralization. Calculate the % Na₂CO₃ (MW = 106.0) in the sample.

22. You wish to prepare a buffer solution having a pH value of 5.5. You have your choice of the following weak acid/salt combinations:
i) Formic Acid, HCOOH, Ka = 2 × 10 4 and sodium formate, HCOO^- Na⁺
ii) Propionic acid, CH₃CH₂COOH, Ka = 1 × 10 5 and sodium propionate, CH₃CH₂COO^- Na⁺
iii) Pyridinecarboxylic acid, C₅H₄NCOOH, Ka = 5 × 10 6 and sodium pyridinecarboxylate C₅H₄NCOO-  Na+
a) Which weak acid/salt combination would you choose and WHY?
b) If you had 500 mL of a 1.0 M solution of the weak acid you have chosen in part a) above, how much of the corresponding sodium salt, in grams, would you have to add to prepare the buffer described above? (i.e. pH = 5.5) (MW: HCOOH = 46.00; HCOO^- Na+ = 68.00; CH₃CH₂COOH = 74.00; CH₃CH₂COO^-  Na+ = 96.00; C₅H₄NCOOH = 123.0;
C₅H₄NCOO^- Na+ = 145.0) ANS: 1.1x102 g (115)

23. Calculate the pH that results from mixing the following:
a) 20.00 mL of 0.1 M maleic acid (a diprotic acid having formula C₂H₂(COOH)₂ K1 = 1 × 10 2, K2 = 6 × 10 7 and 15.00 mL of 0.1 M NaOH. ANS: 2.6 (quadratic), 2.5 (Auto Assumption)
b) 20.00 mL of 0.1 M maleic acid and 20.00 mL of 0.1 M NaOH. ANS: 4.1
c) What is the pH when 20.00 mL of 0.1 M maleic acid is completely neutralized (i.e. both protons) by 0.1 M
 
 

24. University X, a much-publicized (mostly by itself) center of academic excellence, decided to test the attention span of 6 of its students drawn at random. The results, measured in seconds, are:
7.6, 6.1, 10.8, 10.0, 11.7, and 23.6 = 69.8, y² = 1005.46
(the last student you later learn is a W&L transfer)
a) What is your best estimate of the attention span of students at University X. ANS: mean = 9.2; median = 10.0
b) If locked in a vault somewhere is a statement which says that an acceptable level for attention span is 11.8 sec, what do you conclude about the test you've run? ANS: C.I. = 2.9; Univ X acceptable
c) What is the % error between your experiment and the locked-vault standard? ANS: 22%
d) Offer your suggestion for the identity of University X.

25. A compound routinely used to prepare physiological buffers is tris (hydroxymethyl) aminomethane, NH₂(C₄H₉O₃)
(MW = 121.14, Kb = 1.26 × 10 6) usually called TRIS. TRIS is also a primary standard for use in standardizing acid titrants. What is the concentration of HCl which requires 42.40 mL of this acid solution to titrate a 1.2841 gm sample of primary standard TRIS?

Clueless:

A straightforward % analyte calculation from a titration which could have been performed in the lab.

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Hints:

Set up a skeleton template for the calculation of % analyte, beginning with the titration data.

Your analyte here is Na2CO3.

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Clueless:

This is a buffer determination problem. You are told that it is a 2-component buffer, but you have been given three different choices of weak acid and conjugate combinations. You need to know the criterion by which you decide among the three so that you can use the Henderson-Hasselbalch equation to calculate the ratio of these two components which will fix the pH of the solution to the desired value.

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Hints:

If you know which direction the "attack" on the buffer will come, you can "tailor" your buffer in that direction. For example, if the attack will be from strong acids, a buffer which has a high conc of conjugate is preferred, as more buffering will occur.

If you do not know which direction the attack will come (and there is no indication in this problem of that) then a SYMMETRIC BUFFER is best, as it will offer maximum protection against attack from either strong acid or base. A perfect symmetric buffer is one where the concentrations of conjugate and weak acid are equal. In such a case the pH of the buffered solution will be equal to the pKa of the weak acid used to generate the buffer.

 

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Clueless:

You have a diprotic acid system to which is added a strong base. You need to take care of the major reaction before you can decide how to determine the resulting pH.

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Hints:

Draw the pH line, definitely. Place the formulas (complete with #mmoles) on their places on the pH line. Then carry out the major reaction, readjusting formulas and #mmoles as needed. Only then are you able to decide how to proceed to figure out the resulting pH.

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Clueless:

Another set of measurements which need to be analyzed via statistical techniques in order to extract the most information possible.

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Hints:

This set of data strongly suggests an Outlier test before you can proceed. The rest of the problem is straightforward, once definitions are known so that you can know what is being asked.

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Clueless:

A primary-standard calculation problem. This is used when a prepared solution, here HCl, needs to be standardized against a primary standard, the compound known as TRIS.

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Hints:

Set up the template which is used to calculate the concentration of a solution from titration data for a weighed amount of primary standard. Refer to what you did in your standardization of NaOH vs KHP.

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