The unit of measurement of degree of dissociation ( α ) ?

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Discussion Overview

The discussion revolves around the unit of measurement for the degree of dissociation (α) in the context of electrolytic dissociation. Participants explore the relationship between the dissociation constant (Kd) and the total concentration of the electrolyte (C), examining how these factors influence the calculation of α.

Discussion Character

  • Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • Some participants propose that α is a measure of the fraction of electrolyte molecules that dissociate into ions, suggesting it is a unit-less quantity.
  • Others argue that the formula α=(Kd/C)0.5 implies a need to consider the units of Kd and C, questioning the assumption that Kd is unit-less.
  • A participant mentions that their teacher indicated α is unit-less and that Kd does not have a unit as it is a ratio, leading to the conclusion that α should be measured in Litre/mol.
  • Another participant challenges the notion that Kd is unit-less and requests clarification on the formula for Kd to determine its units.
  • One participant provides a derivation of Kd based on the dissociation reaction, suggesting that Kd can be expressed in terms of α and C, which raises further questions about the validity of the initial formula for α.
  • There is a recognition that the formula for α may be an approximation, and participants express uncertainty about the implications of this approximation on the units of Kd.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the units of α or Kd. There are multiple competing views regarding whether Kd is unit-less and how the formula for α should be interpreted.

Contextual Notes

Participants express uncertainty about the assumptions underlying the formulas discussed, particularly regarding the units of Kd and the implications of approximations used in the derivation of α.

Nader AbdlGhani
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You have to show your attempts, this is a forum policy. Also, all homework like questions should go to homework forum.
What's the unit used to measure α ? if α=(Kd/C)0.5 where Kd is the dissociation constant and C is the total concentration of electrolyte .
 
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Nader AbdlGhani said:
What's the unit used to measure α ? if α=(Kd/C)0.5 where Kd is the dissociation constant and C is the total concentration of electrolyte .

perhaps ..degree of dissociation refers to the
amount of solute dissociated into ions or radicals per mole. its denoted by alpha.

The classical Theory of electrolytic dissociation,is based on the assumption of the incomplete dissociation of the solute,
characterized by the degree of dissociation α, that is,the fraction of the electrolyte molecules that dissociate.
for details ; see <http://encyclopedia2.thefreedictionary.com/Electrolytic+Dissociation>
 
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Why don't you plug units into the formula and see what you get? What is the unit of the concentration? What is the unit of Kd?
 
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Borek said:
Why don't you plug units into the formula and see what you get? What is the unit of the concentration? What is the unit of Kd?
I've already done that , but before I did so , my teacher had told me it's unit-less , he said that according to our school textbook , according to this idea , concentration is measured in mol/Litre and Kd doesn't have a unit as it's a ratio between two constants , therefore the unit of measuring alpha should be Litre/mol ,which sounds a little bit weird
 
Nader AbdlGhani said:
Kd doesn't have a unit

Sure it does. Please write the formula.

Besides, I just realized your formula for α looks odd.
 
Borek said:
Sure it does. Please write the formula.

Besides, I just realized your formula for α looks odd.
I'm sorry but what formula you want me to write ? and what do you mean the formula for alpha looks odd ? thanks .
 
Write the formula for Kd, to see what are Kd units.

α is just the dissociation percentage (fraction), I don't see how it can be calculated using formula you gave in the opening post.
 
Borek said:
Write the formula for Kd, to see what are Kd units.

α is just the dissociation percentage (fraction), I don't see how it can be calculated using formula you gave in the opening post.
If ##\mathbf{AB \leftrightarrow A^+ + B^-}## then ##\mathbf{K_d=\frac{[A^+][B^-]}{[AB]}}##
then ##\mathbf{K_d=\frac{α^2C}{1-α}}## , and ##\mathbf{1-α\approx1}## therefore ##\mathbf{α=\sqrt{\frac{K_d}{C}}}## where C (concentration) is measured in Mol/litre and ##K_d## is unit-less .
 
I see, that's an approximation only.

Why do you think Kd is unit-less? You wrote a formula - what does it tell you?
 

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