alleryStart learning →Handbooks list Kb for ammonia as 1.8 x 10^-5 but no Ka for ammonium. How is the missing value found?
ABy subtracting the base value from Kw
BBy taking the square root of Kw over Kb
CBy multiplying the base value by Kw here
DBy dividing Kw by the base value quoted
Answer & Solution
Correct answer: D. By dividing Kw by the base value quoted
1. Ammonium and ammonia form a conjugate pair, so their constants multiply to give Kw.
2. Rearranged, the acid constant equals Kw divided by the base constant.
3. Substitute: 1.0 x 10^-14 divided by 1.8 x 10^-5.
4. That gives 5.6 x 10^-10 for the ammonium ion.
5. The relation holds for any acid and its conjugate base, so it fills any such gap in a table.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 14 "Acid-Base Equilibria", section 14.4 Hydrolysis of Salt Solutions_
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