How Do You Calculate pH and [OH-] for a Mixed NH3 and NH4Cl Solution?

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SUMMARY

The discussion focuses on calculating the pH and hydroxide ion concentration ([OH-]) for a mixed solution of 0.20M NH3 and 0.20M NH4Cl. The weak base NH3 has a dissociation constant (Kb) of 1.8 x 10-5. Two methods were employed: the Henderson-Hasselbalch equation, yielding a pH of 9.25, and a direct calculation method resulting in a pH of 5.13. The discrepancy between the two methods highlights the importance of correctly applying the Henderson-Hasselbalch equation and understanding the equilibrium dynamics in buffer solutions.

PREREQUISITES
  • Understanding of weak base and weak acid equilibria
  • Familiarity with the Henderson-Hasselbalch equation
  • Basic knowledge of molarity and solution preparation
  • Ability to perform ICE (Initial, Change, Equilibrium) calculations
NEXT STEPS
  • Study the Henderson-Hasselbalch equation in detail
  • Learn how to derive pH from Kb values for weak bases
  • Explore buffer solution calculations and their applications
  • Practice solving ICE tables for various acid-base equilibria
USEFUL FOR

Chemistry students, educators, and professionals involved in analytical chemistry or chemical education, particularly those focusing on acid-base equilibria and buffer solutions.

Alex Santos
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Homework Statement

[/B]
NH3(aq) is a weak base with Kb = 1.8*10-5, Calculate [OH-] in a solution that is made by mixing togeather 500ml of 0.20M NH3(aq) and 500ml of 0.20M NH4CL(aq), what is the value of Ph?

Homework Equations


Henderson Hasselbalch: ph = Pka + log([conj.base/acid])
As well as another approach on the example, the regular one.

The Attempt at a Solution


To start with I am always a bit confused on how to set up the example and giving me the right inital values, for example I have NH4Cl---->NH3 + Cl + H. Now can I use my initial values, as NH3 in terms of moles
moles NH4Cl: 0.5L*0.2M = 0.1 moles
moles NH3: 0.5*0.2M = 0.1moles

using the " normal method" I get PH = 5.13 and by using the Hendersone Hasselbalch I get PH = 9.25.

I will send an attachment of my calculation because I write everything on an Ipad, so you can see clearly what I am doing.
Please correct me and tell me what is wrong and which method is wrong in my case.
Thank you
 

Attachments

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Solve full quadratic for the ICE table method.
 

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