# Homework Help: The unit of measurement of degree of dissociation ( α ) ?

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1. Mar 16, 2016

### Nader AbdlGhani

• 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 .

2. Mar 16, 2016

### drvrm

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> [Broken]

Last edited by a moderator: May 7, 2017
3. Mar 16, 2016

### Staff: Mentor

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?

4. Mar 17, 2016

### Nader AbdlGhani

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

5. Mar 17, 2016

### Staff: Mentor

Sure it does. Please write the formula.

Besides, I just realized your formula for α looks odd.

6. Mar 18, 2016

### Nader AbdlGhani

I'm sorry but what formula you want me to write ? and what do you mean the formula for alpha looks odd ? thanks .

7. Mar 18, 2016

### Staff: Mentor

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.

8. Mar 24, 2016

### Nader AbdlGhani

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 .

9. Mar 24, 2016

### Staff: Mentor

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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