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Ariel Davydov 1C
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Postby Ariel Davydov 1C » Mon Dec 02, 2019 9:53 am

Draw the Lewis structure of boric acid, B(OH)3. (a) Is resonance important for its description? (b) The proton transfer equilibrium for boric acid is given in a footnote to Table 6C.1. In that reaction does boric acid act as a Lewis acid, a Lewis base, or neither? Justify your answer by using Lewis structures of boric acid and its conjugate base.

Does anyone have any suggestion as to how to approach this problem? I'm slightly confused as to why B(OH)3 acts like an acid due to its pKa value, which is over 7 (9.13)

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Re: 6.13

Postby DarrenKim_1H » Mon Dec 02, 2019 10:00 am

So I would start the problem by drawing out B(OH)3's Lewis Structure. From there, you notice that the structure has no delocalization of electrons so therefore no relevant resonance. If you draw out the structure for the reaction

B(OH)3 + 2 H2O -> H3o+ + B(OH)4^-. You can see that since Boric acid donates a proton to form an H3O+ molecule, or in other words, accept a electron pair to form B(OH)4^-, it is a lewis acid

Ruby Tang 2J
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Re: 6.13

Postby Ruby Tang 2J » Mon Dec 02, 2019 7:20 pm

pKa is not the same as the pH scale. While pKa is also an indicator of the strength of the acid, it does not follow the 0-14 pH scale. pKa values can be as low as -5 and as high as 50, so knowing that the pKa value is greater than 7 has no significance (you can't know whether a compound is an acid or a base just from this fact).

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