Comparing Strength of Acids (1 Viewer)

[fl0]

The question I found was:
Acid A: 0.01M with pH 2
Acid B: 0.57M with pH 2
(assuming equal volumes since the drawing that is supplied is of identical reagent bottles of the acid)

with options:
(A) the [H+] is greater in solution of acid A
(B) the [H+] is greater in solution of acid B
(C) the acids are of equal strength
(D) A is the stronger acid

So like why is a 0.01M acid with pH 2 a STRONGER acid than a 0.57M acid with the same pH of 2 (i.e. Ans: (D) from the solutions page)?

I thought the strength of an acid was only determined by its degree of ionisation, which for this problem would refer to the pH.. since pH is the same, wouldn't that mean they are of the same strength?

The concentration difference between the acids should then indicate whether the acids are concentrated or dilute solutions right? so i thought the answer to this problem from a 2004 Scots Trial exam was (C) - the acids are of equal strength..... just that the 0.01M acid is a dilute version of the 0.57M acid right? But then I guess option (B) could also be an answer... because a higher concentration of an acid would produce more H+ ions, which, I guess I could then see my dilemma and maybe go with (D) because I know (A) is NOT right.

Could someone please explain to me why the hell the 0.01M acid is a stronger acid thought? Thanks for those who are willing to help me understand this.

 

[jet]

pH isn't a measure of ionisation of the acid, it's a measure of the concentration of protons in the solution (i.e. [H+]).

When we talk about the strength of an acid, it's the degree to which it releases protons in water. If you get complete ionisation of all molecules, you have a strong acid. If you only get partial dissociation, then you have a weak acid.

So then, with the above, you have two different acids dissolved at different concentrations which produce the same pH. [Acid A] is 0.01 M and [Acid B] is 0.57 M.

Since [Acid A] is lower, it takes less of Acid A to release the same amount of protons as 0.57 M of Acid B does. This means that Acid A dissociates to a greater extent then acid B, which makes it stronger.

(If you actually do the calculation, acid A releases all of it's protons, whilst acid B doesn't)

 

[fl0]

pH isn't a measure of ionisation of the acid, it's a measure of the concentration of protons in the solution (i.e. [H+]).

When we talk about the strength of an acid, it's the degree to which it releases protons in water. If you get complete ionisation of all molecules, you have a strong acid. If you only get partial dissociation, then you have a weak acid.

So then, with the above, you have two different acids dissolved at different concentrations which produce the same pH. [Acid A] is 0.01 M and [Acid B] is 0.57 M.

Since [Acid A] is lower, it takes less of Acid A to release the same amount of protons as 0.57 M of Acid B does. This means that Acid A dissociates to a greater extent then acid B, which makes it stronger.

(If you actually do the calculation, acid A releases all of it's protons, whilst acid B doesn't)

Ahhhhhhhh I see! Thanks a bunch!