Conjugate Seesaw and Kw

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Conjugate Seesaw and Kw

Postby KNguyen_1I » Wed Jan 15, 2020 12:14 pm

Can someone explain again the quantitative reasoning Prof. Lavelle presented today in class?
Why do we add the two expressions together? In real life application, how would we model this?
Why do we relate everything to Kw?
My understanding is that this is essentially a neutralization reaction between an acid and its conj base and vice versa (the terminology is flexible) and so when all is said and done and the moles of the NH3 and NH4+ cancel out we are left with only concentrations w H30 and OH in the water. And, because we know water has its own equilibrium constant relating H30 and OH it and the H3O and the OH produced in the initial neutralization reaction then interacts with the autoprotolysis reaction in water, which is why we can relate everything to Kw in the end because Kw is a constant.

Anisha Chandra 1K
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Re: Conjugate Seesaw and Kw

Postby Anisha Chandra 1K » Wed Jan 15, 2020 3:22 pm

We add the two expressions together so that we are left with the equation for the autoprotolysis of water. Prof. Lavelle used NH3 and NH4+ to demonstrate the relationship between Ka and Kb for a base and its conjugate acid, and ultimately he derived the equation that relates Ka, Kb, and Kw. This is so any time we have a Ka or Kb, we can find the other since we know Kw is 10^-14.

Hui Qiao Wu 1I
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Re: Conjugate Seesaw and Kw

Postby Hui Qiao Wu 1I » Wed Jan 15, 2020 3:34 pm

Rather than thinking everything is about Kw, why not try thinking that it's all these useful information we can get out of Kw. We can know about Ka/Kb, [H30]/[OH], or even the pH/pOH of an acid/base. In a way, all these values are connected. Kw=Ka x Kb and Ka and Kb connects back to the [H30] and [OH].

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Re: Conjugate Seesaw and Kw

Postby LBacker_2E » Wed Jan 15, 2020 4:28 pm

We relate everything to Kw because it is equal to the concentration of H3O+ times the concentration of OH-. These can then be used to determine the pH and pOH, or the Ka/Kb and pKa/pKa using logarithmic functions. So it is the other side of the equation that makes Kw so useful.

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