Explaining Approximate pH of a 1 M Solution of NH4Br

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Discussion Overview

The discussion revolves around calculating the approximate pH of a 1 M solution of NH4Br, focusing on the relationship between the dissociation constants of NH4+ and NH3, as well as the underlying concepts of acid-base theory.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • One participant attempts to calculate the pH using the dissociation constant of NH4+ derived from the Kb of NH3, leading to a calculated pH of 4.63.
  • Another participant clarifies that the value of 10^-14 refers to Kw, the ion product of water.
  • A question is raised about the definitions of pKa and pKb, indicating a need for clarification on these concepts.
  • A subsequent reply defines pKa as -log(Ka), contributing to the understanding of acid-base relationships.

Areas of Agreement / Disagreement

The discussion includes some agreement on the definitions of pKa and Kw, but there remains uncertainty regarding the initial calculations and the application of Bronsted-Lowry theory to the problem.

Contextual Notes

Participants have not fully resolved the implications of their calculations or the assumptions made regarding the approximations used in the pH calculation.

Who May Find This Useful

This discussion may be useful for students studying acid-base chemistry, particularly those interested in the calculations involving dissociation constants and pH determination.

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Homework Statement



The approximate pH of a 1 M solution of NH4Br [Kb(NH3) = 1.8 x 10-5] is___

Homework Equations





The Attempt at a Solution



Ka(NH4) = 10^-14 / 1.8 x 10^-5 =
=5.5555 x 10^-10

therefore, 5.5555 x 10^-10 = [NH3][H+]/[NH4+]

Since 1 H+ forms for every NH3 that forms and that [NH3] + [NH4+] = 1,
5.5555 x 10^-10 = [H+][H+]/(1-[H+])
Since Ka is small, 1-[H+] approximately = 1
[H+] = 2.357 x 10^-5
ph = 4.63

where did the 10^-14 come from and why are we saying this:
5.5555 x 10^-10 = [NH3][H+]/[NH4+]
 
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10-14 is Kw.

The other part is about Bronsted-Lowry theory. NH4+ is a Bronsted-Lowry acid. Expression that you have listed is its dissociation constant.
 
what exactlly is pKa and pKb?
 
pKa = -log (Ka)
 

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