6. The weak acid or the weak base of the below species is given along with eithe

Question

6.   The weak acid or the weak base of the below species is given along with either the pKa or the pKb. Determine the unknown information for each species from the information given. Then determine the chemical formula of the predominant species that occurs at the given pH listed below for each species.

a. HCOO-   pKb = 10.25                     Kb = _____________

                                                Ka = _____________ pKa = _____________

The predominant species at pH 4.00 _____________________________________

b.   (CH3)2AsO2-    pKb = 7.73            Kb = _____________

                                                Ka = _____________ pKa = _____________

The predominant species at pH 7.00 ___________________________________

c. (CH3)2NH2+    pKa = 10.8             Ka = ________

                                                            Kb = _____________ pKb = _____________

The predominant species at pH 11.00 _______________________________________

Explanation / Answer

a) HCOO- Given pKb= 10.25

Thus kb = antilog (-10.25) =5.623x10-11

andpka = 14 - 10.25 = 3.75

thus ka = antilog (-3.75) = 1.78 x10-4

the predominant species at pH= 4.00 (closer to pKa) is HCOOH , the conjugate acid of HCOO-

b) For the give base pKb = 7.73 and thus pKa = 14- 7.73 =6.27

Thus Kb = antilog (-7.73) and Ka = antilog (-6.27)

Kb = 1.862x10-8  and Ka = 5.37 x10-7

At pH = 7.0 (greater than pKa) the base (CH3)2ASO2- is predominant.

c) for the give acid pKa = 10.8 and pKb for the base (CH3)2NH = 14-10.8 = 3.2

Thus ka = antilog of -10.8 = 1.585 x10-11

  and Kb = antilog (-3.2) = 6.31 x10-4

The species that is predominant at pH = 11.0 , which is much more than the pKa of the acid, is the conjugate base (CH3)2 NH.

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