Hi guys,
We did this example during lecture:
What is the ph of .15 M NH4Cl? KB for NH3=1.8 x 10^5
For this question, how did we know that we needed to use the kA value rather than the kB value?
Thanks
Calculating the pH of a salt solution (lecture example)
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Re: Calculating the pH of a salt solution (lecture example)
Hey! We know we need to use the Ka rather than the Kb, because when NH4Cl dissociates, it turns into NH4+ and Cl-. We can forget about Cl- in this example because it is a spectator ion, so then we have NH4+ and that will react with H20, to form H3O+ and NH3. Thus, because we are making H30+, we need to use the Ka rather than the Kb. If NH4+ was our product and we add OH- as the other product, then you would use Kb. Hope this helps !
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Re: Calculating the pH of a salt solution (lecture example)
Hey,
Ka and Kb can be relatively ambiguous as to what species they refer to, and I understand your confusion. Like if we receive Kb (NH3), it would be easy to confuse this as to refer to NH3 or NH2- or even ka(NH3) to refer to NH3 or NH4+. To best evaluate which value it refers to, you can assess the magnitude of the constant. More stables species will be preferred.
In answering your question, we know that NH4+ (dissociation of the salt generated NH4+ and Cl-) will function as an acid much more readily than a base (wants to remove additional charge), generating NH3 and H+. Kb refers to an equilibrium constant of a base, while ka refers to the equilbrium constant of an acid deprotonating. To calculate kb from ka, see the autoionization of water formula.
Ka and Kb can be relatively ambiguous as to what species they refer to, and I understand your confusion. Like if we receive Kb (NH3), it would be easy to confuse this as to refer to NH3 or NH2- or even ka(NH3) to refer to NH3 or NH4+. To best evaluate which value it refers to, you can assess the magnitude of the constant. More stables species will be preferred.
In answering your question, we know that NH4+ (dissociation of the salt generated NH4+ and Cl-) will function as an acid much more readily than a base (wants to remove additional charge), generating NH3 and H+. Kb refers to an equilibrium constant of a base, while ka refers to the equilbrium constant of an acid deprotonating. To calculate kb from ka, see the autoionization of water formula.
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