What is the pH of a mixture made with ammonia hydroxide and HCl?

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    Ammonia Hcl Ph
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Discussion Overview

The discussion revolves around calculating the pH of a mixture created by combining ammonia hydroxide and hydrochloric acid. Participants explore the chemical reactions involved, the relevant dissociation constants, and the stoichiometry of the reactants.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a specific homework problem involving the addition of 25 ml of 0.100M HCl to 100ml of 0.025M ammonia hydroxide and seeks to calculate the resulting pH.
  • Another participant questions the meaning of Kd and suggests it may refer to a dissociation constant.
  • There is a suggestion that Kd could be related to the dissociation of NH4OH, although this is not confirmed.
  • One participant emphasizes the importance of determining which reactant is in excess (ammonium hydroxide or HCl) to proceed with the pH calculation.
  • Another participant notes that the value of Kd provided is close to the dissociation constant for ammonia in water and suggests using stoichiometry to find the concentration of excess HCl to calculate [H+] and pH.
  • There is a mention of the need to use the dissociation constant expression for weak bases, indicating a potential misunderstanding of the problem's requirements.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of Kd and its application in the context of the problem. There is no consensus on the correct approach to calculating the pH, as some participants propose different methods based on their interpretations of the reactions and constants involved.

Contextual Notes

Limitations include unclear definitions of Kd and its specific application in the context of the reaction. There are unresolved steps in the calculations presented, particularly regarding the determination of concentrations after the reaction and the subsequent pH calculation.

tyrant
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Homework Statement


A mixture is made by adding 25 ml of 0.100M HCl to 100ml of 0.025M ammonia hydroxide. Kd = 1.76 x 10^-5 . My task is to calculate the pH.


Homework Equations


I think this is the reaction that follows:
NH4OH + HCl <--> NH4 + H2O + Cl



The Attempt at a Solution


I have calculated the concentrations in the mixture:
c(NH4OH) = (0.025mol/l x 0.100l) / ( 0.100l + 0.025l) = 0.02 mol/l
c(HCl) = (0.100 mol/l x 0.025l) / ( 0.100l + 0.025l) = 0.02 mol/l

NH4OH + HCl <---> NH4 + H2O + Cl
Initial C 0.02M 0.02M 0
final C 0 0 0.02M

After this I'm not sure what happens, I have tried calculating the concentration of hydrogen ions formed by this reaction NH4 <---> NH3 + H ( I guess Kd is for this reaction) and calculating the pH then but it was wrong.
My teacher refuses to help me solve this and won't even tell me where I go wrong.
 
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tyrant said:
I have tried calculating the concentration of hydrogen ions formed by this reaction NH4 <---> NH3 + H

That's the correct approach, question is - what is Kd?
 
I think Kd is a dissociation constant, the task was written as I wrote it to the problem statement. Kd is just given without specifically telling for what reaction it is.
Strangely I can't get the correct answer ( I don't know what the answer should be). I have double checked my calculations and there shouldn't be any errors in them. When I got my calculation back from my teacher there was only one mark and it said "wrong anwer and wrong solution". When I asked where exactly I had gone wrong he just mumbled something like check my lectures and I did but still no idea what seems to be the problem.
 
What if I tell you Kd is for NH4OH dissociation?
 
The Kd being 1.76 x 10^-5 is close enough to the dissociation constant for ammonia in water.

First task is use stoichiometry to determine which is in excess: ammonium hydroxide or HCl? If HCl is in excess, then use simple molarity of the excess HCl to find [H+] and pH. Otherwise, use dissociation constant expression formula to find pH.

Please note that we usually indicate dissociation constant of a weak base using symbol, Kb.
 

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