7th Ed 6D.15

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Henry Krasner 1C
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Joined: Fri Sep 28, 2018 12:15 am

7th Ed 6D.15

Postby Henry Krasner 1C » Sun Jan 20, 2019 4:24 pm

How does one find the Ka, when the only information given is that you are trying to find the pH of 0.19 M NH4Cl (aq).
The soln manual says that you take the Kw constant and divide by 1.8 x 10^(-5), the Kb. But where does this value even come from?

Rachana Jayaraman 1H
Posts: 61
Joined: Fri Sep 28, 2018 12:26 am

Re: 7th Ed 6D.15

Postby Rachana Jayaraman 1H » Sun Jan 20, 2019 6:11 pm

The book gives a table of Ka and Kb values for common weak acids and bases. In this case, you have to separate NH4Cl into its ion, NH4+ and Cl-. Cl- is a neutral ion since it is the conjugate base of HCl, which is a strong acid. However, NH4+ is the conjugate acid of NH3, which is a weak base, so it's reaction with water will be at equilibrium. To find the Ka value of this reaction, you need to use the table in the book to obtain the Kb value for NH3 and then use the Ka x Kb = Kw relation to get Ka.

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