How Much Acetic Acid Converts to Dimer at Equilibrium?

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Discussion Overview

The discussion revolves around the equilibrium between monomer and dimer forms of acetic acid in the gas phase, specifically focusing on calculating the percentage of acetic acid that converts to dimer at equilibrium. The context includes a homework problem involving equilibrium constants and concentration calculations.

Discussion Character

  • Homework-related, Mathematical reasoning, Debate/contested

Main Points Raised

  • One participant presents a homework problem involving the equilibrium constant for the dimer-monomer equilibrium of acetic acid and attempts to calculate the percentage of dimer formed.
  • Another participant questions the initial setup of the equilibrium expression and suggests that the equilibrium constant must have units, implying that the provided value may be incorrect or misinterpreted.
  • A later reply indicates that the initial calculations made by the first participant may contain errors, particularly in the application of the quadratic formula.
  • There is a discussion about the correct formulation of the equilibrium expression, with some participants expressing differing views on the setup and results of the calculations.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correctness of the equilibrium constant value or the calculations performed. Multiple competing views and interpretations of the problem remain unresolved.

Contextual Notes

There are limitations regarding the assumptions made in the equilibrium expression and the potential need for units in the equilibrium constant. The discussion also highlights unresolved steps in the mathematical calculations presented.

George3
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URGENT! Dimer-Monomer Percentage

Homework Statement


In the gas phase acetic acid exists as an equilibrium of monomer and dimer molecules.(The dimer consists of two molecules linked through hydrogen bonds.) The equilibrium constant for the dimer-monomer equilibrium (CH3CO2H)2 <=> 2 CH3CO2H has been determined to be 17 at a particular temperature. Assume that acetic acid is present initially at a concentration of .30mol/L at 25 C and that no dimer is present initially. What percentage of acetic acid is converted to dimer?


Homework Equations





The Attempt at a Solution


I did an ice table and found that:
I 0 .3

C x -x

E x .3-x


Then I went 17 = ((.3-x)^2)/x and got the quadratic .09 - 17.6x + x^2 after doing the
quadratic formula I found x equal to .005115 or 17.59. I figured that .005115 was the only answer that made sense. I divided that concentration by the concentration give and got the percent to be 1.7% dimer but that is incorrect. Can someone point me in the right direction?
 
Last edited:
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Anything guys?
 
Ratio of dimer/monomer is 17, right?

So, 17 = [x]/[0.3-x]2 and you get 0 = x2 - 17.6x + 0.09 from that?

That's not what I get.
 
Anyway please realize it is not a pedantry to say that that equilibrium constant cannot be 17. It has to have UNITS or it's meaningless.

If you quote them they will at least tell you if you have got your equation the right way up.
 
epenguin said:
...If you quote them they will at least tell you if you have got your equation the right way up.

Yep, I've got mine wrong side up. Sorry.

George3, this part is wrong...
...after doing the
quadratic formula I found x equal to .005115 or 17.59.
 
Last edited:

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