acid and base strength

[joonkimdds]

In H2O, which of the following is the weakest base?
a) HClO4
b) NH3
c) NH2 -
d) OH -
e) ClO4 -

If NaBr was in solution, we separate it into Na and Br then Na+OH, Br+H and we get NaOH, HBr as the final product. NaOH is strong base and HBr is a strong acid so "NaBr is neutral in solution."

If I apply the same method,
HClO4 --> H + OH and H+ClO4 --> H2O and HClO4
NH3 --> NH4 +
NH2 --> NH3
OH- --> H2O
ClO4 - --> HClO4

HClO4 is a strong acid. Conjugate base of strong acid can be weak base. But the result in solution shows that
the final product is HClO4 which is not weak base.
I am confused.

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[SupahShyGuy]

In H2O, which of the following is the weakest base?
a) HClO4
b) NH3
c) NH2 -
d) OH -
e) ClO4 -

If NaBr was in solution, we separate it into Na and Br then Na+OH, Br+H and we get NaOH, HBr as the final product. NaOH is strong base and HBr is a strong acid so "NaBr is neutral in solution."

If I apply the same method,
HClO4 --> H + OH and H+ClO4 --> H2O and HClO4
NH3 --> NH4 +
NH2 --> NH3
OH- --> H2O
ClO4 - --> HClO4

HClO4 is a strong acid but the answer says E is the answer.
How can E be the answer?

I like to think of it as "the strongest acid/base is the weakest conjugate acid/base; respectively that is". Thats the best mantra for acid-base chemistry I've heard. I believe the question stated that the substances are in H20 as a solvent, that means the ions will have dissociated. Since perchloric acid (HClO4 is the strongest acid among the answer choices), then write out the chemical equation in water. HClO4 + H2O -> H3O+ + ClO4-.

Try to get a hold of a list that has a list of strong acids and bases; you don't have to remember pKas or too many acids / bases, but rather know the more widely used acids and bases.

 

[SupahShyGuy]

this link is a pretty nice list, not long at all and covers some widely used acids and bases.

http://www.chemistry.pomona.edu/Che...06/powerptpresentations/5Anions/acidbaseT.htm

 

[joonkimdds]

If the question asked me which is the weakest base, then I know that ClO4 - is the answer because that's the conjugate base of the strong acid.

But it said in water, and if it's in water, we have to think about dissociating it first and then adding H and OH to each ion and whatever we get is the final result.

For example, NaBr becomes NaOH and HBr and they are strong base and strong acid so we call it neutral.

If we have ClO4 - in water, it will become HClO4 which is strong acid, not weak base but the answer is saying
that ClO4 - --> HClO4 = weak base.

 

[Kneecoal]

i remember this question, and i thought it was weird too, but as per the answer, if i remember correctly, they're asking straight up which is the weakest base. and then it's obviously ClO4-, the conjugate base of a really strong acid.

 

[joonkimdds]

acid and base strength question can be asked in two ways.
The way it is vs. when it is in water or in solution.
For example, if we look at KBr, it's same as Br- and that's conjugate base of strong acid, HBr so we call it weak base.
But if KBr is in water, we dissociate it and add H and OH thus we end up having KOH and HBr. KOH is strong base and HBr is a strong acid so KBr in this case is neutral instead of weak base.

In this original question, ClO4 - is weak conjugate base of strong acid if the question just asked which of the following is weak base. However, the question said it's in H2O, so I think we should add H to ClO4 to make HClO4 and we can't call this product a weak base since it's a strong acid.

This is where I got confused.