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Problem

Which of the following will give strength of acid?

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

Answer

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?

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    $\begingroup$ It does. Also, some tests look like they were designed by morons. So it goes. $\endgroup$
    – Ivan Neretin
    Commented Apr 18, 2021 at 15:35
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    $\begingroup$ $\ce{HCl 0.001 M}$ and $\ce{CH3COOH 0.055 M}$ have both a pH value equal $ 3$. $\endgroup$
    – Maurice
    Commented Apr 18, 2021 at 16:27
  • $\begingroup$ 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. $\endgroup$
    – Poutnik
    Commented Apr 22, 2021 at 12:22

2 Answers 2

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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.

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Ivan may be right that the question was designed by morons, but an argument can be made that the strength of an acid governs the pH in a solution with a given concentration, rather than the pH governing the strength. The question apparently asked what properties of the acid can be used to assess its strength and thus the predict the pH you get at a given concentration.

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    $\begingroup$ Then again, pH (option A) is a fairly good predictor of itself. $\endgroup$
    – Ivan Neretin
    Commented Apr 18, 2021 at 16:05
  • $\begingroup$ Ka can be calculated from deg. of diss. and conc. $\endgroup$
    – Mithoron
    Commented Apr 18, 2021 at 18:04

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