[offlinedoctor]

Hello  ,

I've just got one question that's been bugging me  ,

For a question such as, '0.1M of Hpoidodous Acid has a pH of 5.8, determine the Ka value',
for the numerators, [H3O] and [OI], how come we would assume that they are the same concentration
and not use the 10^-14 rule as we would for a question such as,
'Perchloric Acid has a concentration of 0.01M determine the concentration of [OH]?

Thanks.

[Borek]

Please elaborate, I don't see how these things are related.

For a weak acid HA solution and the dissociation reaction

HA H⁺ + A^-

[H⁺]=[A^-] is just a result of the reaction stoichiometry. We do assume [H⁺] from the acid dissociation is high enough so that we can ignore water autodissociation.

[ramboacid]

Perchloric acid is a strong acid, and thus it will dissociate completely in water. Therefore you don't need to use Ka to calculate [H⁺]. For hypoiodous acid, a weak acid, the Ka determines the extent to which the acid ionizes. Here you use the given pH to calculate [H⁺], and using an ICE table you can calculate the concentrations of the other species in the solution.

The 10^-14 rule only concerns the relationship between an acid's and conjugate base's Ka and Kb values. To use the 10^-14 rule, you'd be concerned with [HIO] and [IO^-], or [H⁺] and [OH^-]. You can't mix and match acids and conjugate bases there.

[offlinedoctor]

Ahhh, makes sense now.

Is there anyway to tell whether an acid is 'very strong' or not? Or should it become second-nature after exposure to wide-range of questions?

[Borek]

List of strong acids is pretty short and worth of memorizing - then you know everything not on the list is weak.

[offlinedoctor]

Thanks again, I've just got one more query in regards to determining pH.

For example, 'What volume of water must be added to 100mL of 0.001M NaOH, to change its pH to 10?'.

I've been looking for resources that discuss changing pH levels but there doesn't seem to be any, is there any logical way in approaching these types of questions?

Thanks.

[Borek]

For example, 'What volume of water must be added to 100mL of 0.001M NaOH, to change its pH to 10?'.

This one is pretty simple dilution problem. What is the concentration of NaOH in the solution that has pH 10? What is the initial concentration of NaOH?