Ka, Kb, calculating concentration of F-

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Discussion Overview

The discussion revolves around calculating the equilibrium concentration of fluoride ions (F-) from a given concentration of hydrofluoric acid (HF) in a chemical equilibrium reaction. Participants explore the relationship between the acid dissociation constant (Ka) and the base dissociation constant (Kb), as well as the proper setup of an ICE (Initial, Change, Equilibrium) table for the reaction.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant attempts to calculate the concentration of F- by using the relationship between Ka and Kb, suggesting that Ka can be derived from Kb using the equation Kw = Ka * Kb.
  • Another participant questions the initial setup of the chemical equation, noting the absence of an equal sign and asserting that HF is a strong acid, which is contested by others who clarify that HF is a weak acid.
  • Concerns are raised about the interpretation of the initial concentration of HF as a formal concentration rather than an equilibrium concentration.
  • One participant provides an ICE table setup but is challenged on the correctness of their approach and whether they have switched the change and equilibrium lines.
  • There is uncertainty regarding whether the provided Kb value is accurate and whether it should actually be Ka.

Areas of Agreement / Disagreement

Participants express differing views on the nature of HF as a weak versus strong acid, the correct interpretation of the equilibrium concentrations, and the validity of the Kb value provided. The discussion remains unresolved with multiple competing perspectives on the calculations and concepts involved.

Contextual Notes

There are limitations regarding the assumptions made about the dissociation of HF, the definitions of concentrations used, and the accuracy of the provided Kb value. The setup of the ICE table is also under scrutiny, with participants questioning the methodology used in the calculations.

ronpaulkid
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Homework Statement



Calculating concentration of F- for .158M of HF?
Calculate the equilibrium concentration of F- for .158M of HF in the reaction below.?
HF+H20=H30+F-

The Kb is 6.66*10-4

Homework Equations



Kw=Ka*Kb
Ka=[H+][A-]/[HA]


The Attempt at a Solution



I figured that you would find the Ka by taking 1*10^-14 over the Kb.
Since Ka=[H+][A-]/[HA], then Ka=x^2/.158M

However, the answer is not 1.54E-6.

I also made an ICE chart and solving for x using the quadratic formula. I must be missing something. Thanks!
 
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hey, I am new here. did i do something wrong?
 
Your equation is ambigious, there is no equal sign in a chemical reaction.

If I remember, HF is a strong acid, it only proceeds in one direction.
 
ronpaulkid said:
hey, I am new here. did i do something wrong?

Yes - this is not physics, this is chemistry, and chemistry - when it comes to homework - classifies as other sciences...

flyingpig said:
Your equation is ambigious, there is no equal sign in a chemical reaction.

This is a perfect shortcut, especially for an equilibrium reaction.

If I remember, HF is a strong acid, it only proceeds in one direction.

HF is a weak acid. Besides, even strong acids are never 100% dissociated (although in most cases that's a perfect approximation).

ronpaulkid said:
Since Ka=[H+][A-]/[HA], then Ka=x^2/.158M

0.158M is a formal concentration of acid, not equilibrium concentration of HF.

I also made an ICE chart and solving for x using the quadratic formula. I must be missing something. Thanks!

Show your ICE table, it should yield a correct result.

Edit: actually it seems like acid is dissociated only slightly, even first approach should give answer reasonably close to reality. Are you sure 6.66*10-4 is Kb, and not Ka?
 
--------HF+H20=F-+H30
I-----.158---0-----0-----0

C-----(-.158)--0---.158---0

E------x-------0------(-x)----0

Ka=x^2/[x-.158]

Kb=[x-.158]/x^2

Is this close to how it looks?
 
Initial is OK, but later you do some strange tricks.

Have you switched C & E lines?

Why do you think ALL HF was dissociated?
 

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