JOHN J. ALEXANDER University of Cincinnati Cincinnati, 45221
Chemical Predictions from pKa Values
2)
In order to deprotonate (CflH^CH, the conjugate base of a weaker acid is required. eNH2 in liquid NH;{ would be suitable
A. Hassanali
(C6H5)3CH + eNH2
University of Dar Es Salaam Dar Es Salaam, Tanzania
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Question
Given the following information, make the requested predictions regarding reactivity.
Acid
Conjugate Base
PKa (25°)
NHri
34
0
(C{>H5);,CH
31
(C,;H.,).,Ce
(CH;l);{COH
19
(CH;i)3COe
nh2
10
OH
0
7.2
a
Freshman Lab Notebooks Wayne L. Felty The Pennsylvania State University Wilkes-Barre Campus Wilkes-Barre, Pennsylvania 18708 A general chemistry course which involves quantitative laboratory work and use of a laboratory notebook is the intended locus for this question. Its intent is to test the student’s knowledge and comprehension of how to keep a proper laboratory 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 be familiar with the pertinent laboratory operations.
©
4,2
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 base (or bases) which you would use to generate a relatively large concentration of (CeHgJsC0 from (CeHjOsCH, Would aqueous medium be suitable for carrying out the reaction? Explain why or why not. 1)
Question Prior to analyzing an unknown oxalate salt by permanganate titration, a student weighed out a sample of pure sodium oxalate and titrated it with a solution of potassium permanganate. The following is an excerpt from the student’s laboratory notebook EXR3.— KMnOif TuXnA&rvv 35.10m! I 'Us&ftfctxi,: 3 5.3___ 0.50 !
Xcr
Satisfactory Answer 1)
Water has a pKa of
H20
—
(C6H5)3Ce + NH3
stronger acid than (CeHsJaCH, it would readily protonate (CeHs^C0 and is therefore unsuitable as a reaction medium. Because water is
Students who understand the meaning of pKa and who grasp the significance of multiple equilibria in the same solution should be able to apply their knowledge by making chemical predictions. General chemistry students who have had some exposure to organic molecules and descriptive inorganic chemistry form the intended audience for this question.
—
14
at 25°.
H+ + OH-
Ka
=
1.0 X
10-“
In 1 M NaOH, [OH"| = 1 and [H+] 1 X lO"14. If any substance which is a stronger acid than water (pKa < 14) is added to the solution, an additional equilibrium is established
Explain six significant omissions or brief but specific.
me. errors
in these entries. Be
=
X-H
—
H+ + X“ =
4-
OH0
Compounds having pK(l
