Some Acid Base Questions

[Gandyy]

Hey guys, have some questions
so question 33 of passage 5 in the acids and bases of gen chem tbr
basically it tells you to pick the answer from the reasoning "weakest acid has strongest conjugate base" but that is exactly what they were saying at the beginning of the chapter to be one of the common misconceptions of acid base chemistry. They said thats not always true and now thats the reasoning for this question. I dont get it.

Also, I thought weak acids did not dissociate very well so that [HA} was always higher than [A-]? In another question on the same passage it says that A- = HA concentration for a weak acid.

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[Teleologist]

basically it tells you to pick the answer from the reasoning "weakest acid has strongest conjugate base" but that is exactly what they were saying at the beginning of the chapter to be one of the common misconceptions of acid base chemistry. They said thats not always true and now thats the reasoning for this question. I dont get it.

The weakest acid has the strongest conjugate base - that is axiom. It is stone.

I think what the book might have said is a common misconception is that "a strong acid is a weak base." Bull. Unfortunately I hear this crap spewed all the time. This doesn't really apply well to polyprotic acids, such as monobasic hydrogen phosphate anion. Still a fairly strong weak acid ... and not a bad weak base.

 

[gettheleadout]

The weakest acid has the strongest conjugate base - that is axiom. It is stone.

I think what the book might have said is a common misconception is that "a strong acid is a weak base." Bull. Unfortunately I hear this crap spewed all the time. This doesn't really apply well to polyprotic acids, such as monobasic phosphoric acid. Still a fairly strong weak acid ... and not a bad weak base.

The conjugate of a strong acid is a weak base, given that by "strong" we mean the namesake strong acids. It is also true that the strength of an acid/base is inversely related to the strength of its conjugate.

The actual misconception is that a weak acid/base must have a strong conjugate. This is not true, as is apparent with countless weak reagents (e.g. acetic acid is weak, but acetate is not a strong base and vice versa.) TBR subcategorizes non-strong species into "weak" and "very weak," the latter being the conjugates of actual strong species (e.g. the chloride anion is a very weak base and therefore HCl is a strong acid) and the former describing the aforementioned many, many members of conjugate pairs where both species are weak acid/base reagents.

 

[Gandyy]

ah ok, what about the 2nd question concerning HA and A- concentrations?

 

[gettheleadout]

ah ok, what about the 2nd question concerning HA and A- concentrations?

Unless titrated with base to somewhere near the half-equivalence point, [HA] should always significantly exceed [A-] for weak acids in solution.

What does the statement say in context?


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