Solubility (or acid/base?) question

[lex1489]

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Got another one for you guys (I hope it's ok to post this question. I think the wording is causing most of my confusion).

Q: If 20 mL of 0.012 M solution of Ca(OH)2 is added to 48 mL of HBr, what is the concentration of the HBr?

The answer key uses N1V1=N2V2 to solve straight up:
N1(48) = (0.024)(20)
N = molarity in HBr, so the answer is 0.01M HBr.

Conceptually, I don't really understand this problem. My first thought to solve it was to multiply .020 L * 0.012 M to get the moles of Ca(OH)2 times 2 for OH normality, then assume 100% reaction with HBr (of course, there is no indication of HBr molarity so I got stuck here), and then the remaining HBr moles divided by the total volume (68 mL) to give the final concentration of HBr.

We never used normality in gen chem as far as I can remember so I don't have the concept down too well, so I assume I'm just missing how NV=NV works. Can someone enlighten me?

Thank you for your help

 

[lex1489]

I have another question that I'll throw in here because I don't want to monopolize the forum 😛

The question asks which is the weakest base. The answer is ClO4-, but I picked HClO4. Isn't HClO4 technically the extremely weak conjugate base of H2ClO4+?

 

[UCB05]

The question already starts out bad because it doesn't actually state that it's a complete neutralization reaction, so you just have to make that assumption.

That said, your reasoning is right on. Your work would be:
(.020 L) (0.012 M)(2)

Whereas the explanation already takes that into account as:
(0.024)(20)
*also, the explanation just keeps the units as mL, which is inconsequential as the units cancel.


HClO4 isn't a base. It's not even slightly basic and cannot accept any more protons.

 

[meant2be10]

Also, for the first problem, shouldn't the answer be 0.01M not 0.1M?

 

[lex1489]

The question already starts out bad because it doesn't actually state that it's a complete neutralization reaction, so you just have to make that assumption.

That said, your reasoning is right on. Your work would be:
(.020 L) (0.012 M)(2)

Whereas the explanation already takes that into account as:
(0.024)(20)
*also, the explanation just keeps the units as mL, which is inconsequential as the units cancel.


HClO4 isn't a base. It's not even slightly basic and cannot accept any more protons.

I see, so it's asking "If 20mL of 0.012M Ca(OH)2 is added to HBr with full neutralization, what was the concentration of HBr?"

@meant2be, I copied the answer wrong, you're right.

 

[SuperCool101]

HClO4 is a strong ACID!!! usually when you have a strong acid then its conjugate base will be very weak. Therefore the conjugate base is ClO4-.

weak acids and weak bases can have weak or strong conjugates.
Strong acids and strong bases have weak conjugates!!