Question 12.45

Acidity
Basicity
The Conjugate Seesaw

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Steven Chau 1B
Posts: 21
Joined: Fri Sep 29, 2017 7:07 am

Question 12.45

Postby Steven Chau 1B » Sat Dec 09, 2017 7:49 pm

Arrange the following bases in order of increasing
strength on the basis of the pKa values of their conjugate acids,
which are given in parentheses: (a) ammonia (9.26);
(b) methylamine (10.56); (c) ethylamine (10.81); (d) aniline
(4.63).

After changing the pKa to pKb, how do we determine the strength of the bases? What is the connection between pKb to pOH?

Annie Lieu-1H
Posts: 68
Joined: Fri Sep 29, 2017 7:04 am
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Re: Question 12.45

Postby Annie Lieu-1H » Sat Dec 09, 2017 8:30 pm

The smaller the pKB, the stronger the base is.

Rachel Formaker 1E
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Joined: Fri Sep 29, 2017 7:04 am
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Re: Question 12.45

Postby Rachel Formaker 1E » Sat Dec 09, 2017 8:32 pm

The lower the pKb, the stronger the base.
Because pKa + pKb = 14 (so when pKa increases, pKb decreases), the larger the pKa value for an acid, the stronger the conjugate base.

Danah Albaaj 1I
Posts: 50
Joined: Fri Sep 29, 2017 7:07 am

Re: Question 12.45

Postby Danah Albaaj 1I » Sat Dec 09, 2017 8:34 pm

For this question, you use the given pKb to calculate the pKa by subtracting the pKb from 14. From there you are able to calculate the molecule's Ka using the equation: Ka=10^-pKa The resulting value should then help you determine the strength of each acid and rank them accordingly.

The whole process is very similar to that of finding the concentration of hydroxide/hydronium based on the pOH/pH given to you because the same rules from the autoprotolysis of water apply here.


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