JOHN J. ALEXANDER University of Cincinnati Cincinnati. 45221
Chemical Predictions from pK, Values A. Hassanali Uniuersity of Dar Es Salaam Dar Es Salaam, Tanzania
2) In order to depratonate (CsH&CH, the eonjugate hase of a weaker acid is required. eNHz in liquid NH3 would be suitable (Ce,Hs).&H
+ eNH2
+ NHs
+ (C6Hs)~Ce
Because water is a strongeracid than (C6Hs)&H, it would readily protanate (CsHs)&Y and is therefore unsuitable as a reaction
Students who understand the meaning of pK, and who grasp the significance of multiple equilibria in the same solution should be able to apply their knowledge by making chemical oredictions. General chemistrv students who have had some exposure to organic molecules and descriptive inorganic chemistry form the intended audience for this aues-
medium.
Freshman Lab Notebooks Wayne L. Felty
Questlon Given the following information, make the requested predictions regarding reactivity.
The Pennsylvania State University Wilkes-Barre Campus Wilkes-Barre, Pennsylvania 18708 A general chemistrv course which involves auantitative Ialwr,ltory work and use of n laboratory nort!hook is the intended lurw tor thisouestim. Its intent i i to test thestudent's knowledge and com~rehensiouof how to keep a proper lahoratory notebook, In addition, performance on this question is some reflection on the student's diligence in actual notebook practice. Having performed the experiment referred to, the student is expected to he familiar with the pertinent lahoratory operations.
1) Which compounds from the above list would you expect to dissolve in dilute (1 M )NaOH? Give a brief explanation of your choice. 2) Mention a suitable hase (or hases) which you would use to generate a relatively large concentration of (C&)3Ce from (CRHrM!H. Would aqueous medium he suitable for . carrying out the reaction? Explain why or why not.
Question Prior to analyzing an unknown oxalate salt by permanganate titration. a student weighed out a samole of nure sodium oxulnte and t~tmrpdit with'k solution of p;,tassi;m permanranate. 'l'he followine is an exceror from the student's laboratory notebook
Satisfactory Answer 1) Water has a pK, of 14 at 2S9, H~O = H+
+ OH-
K,
= 1.0 x io-l4
In 1 M NaOH, [OH-] = 1 and [Hf] = 1X lo-". If any substance which is a stronger acid than water (pK. < 14) is added to the solution, an additional equilibrium is established X-H = H+ + x-
+ OH'
Compounds having pK.
6
< 1 4 are
02N+0H 378 / Jwrnal of Chemical Education
Satisfactory Answer Any six of the following 1) It is not clear whether the known or unknown is heing weighed and
This source of protons will shift the water self-ionizationto the left, consuming both H+ and OH- until [Ht] [OH-] = 1.0 X lo-". In the process, the X-H equilibrium is shifted to the right as its protons are consumed. The net result is X-H
Explain six significant omissions or errors in these entries. Be brief hut specific.
xe + H,O
&OH, and @COOH.
nish weights four decimal places, "25.2" should he reported to three more figures. ~-
~
Exam Question Exchange offersteachers an opportunityto share prize exam questions with others. Guidelines for preparing exam questions for submission were outlined on page 608 of the October 1971 issue. All questions submitted become the property of the Journal of Chemical Education and will not he returned. Questions should be submitted to the column editor.
4) Obliterated data ("25.7011"entry). 5) Significant figure error in subtraction of weights. As is, answer should be rounded off to 0.5. 6) No initial buret reading is recorded. 7) Significant figure error in volume measurement. With the conventional buret. readines can be estimated t o the nearest 0.02 ml.
8) No information is recorded as to how the sample was diiolved and diluted or what other reagents were added. (Thetitrated solution should be acidified; we dissolved the sample in dilute sulfuric acid.) 9) No date is recorded. (This answer is acceptable, although it is possible that the date appeared with data prior to the "EXP.2.. ." entry.)
Volume 55, Number 6, June 1978 1 379
