# Why is degree of dissociation used to estimate strength of an acid?

### Problem

Which of the following will give strength of acid?

A) pH
B) Normality
C) Degree of dissociation
D) Dissociation constant

C, D

### Question

If pH is not considered as a metric to assess the strength of an acid because it depends on dilution (even strong acids can have high pH), then why is degree of dissociation a reliable metric for the same? Doesn't degree to dissociation depend on dilution as well?

• It does. Also, some tests look like they were designed by morons. So it goes. Apr 18 at 15:35
• $\ce{HCl 0.001 M}$ and $\ce{CH3COOH 0.055 M}$ have both a pH value equal $3$. Apr 18 at 16:27
• The acid strength could be considered in the absolute way, via dissociation constant, dilution independent, or by the relative way, via dissociation degree, dilution dependent. If at some dilution is for some weak acid dissociation degree approx. 100%, it can be considered at these conditions approx. as strong as strong acids. Apr 22 at 12:22

I would answer only the constant of dissociation $$K_a$$, because it is a thermodynamical constant and you can calculate it from potentiometric measurements without dealing with activities and so on. If you have the $$\Delta G$$ of reaction, which is related with the difference of electric potential of the cell, you can calculate the constant as $$\Delta G = -RT \ln K$$. The pH is not an indicator of the acid strenght, but only its consequence. If you have a very diluite solution of sulfuric acid, its pH will be near to 7, but it does not mean that sulfuric acid is weak. Also the degree of dissociation can give you incorrect information if you are dealing with very diluite or very concentrate solutions.