Volume KOH needed to neutralize nitrous acid...

[confusedliberal]

---

Members do not see this ad.

What volume of 0.5M KOH would be necessary to neutralize 15mL of 1.0M nitrous acid?

Answer is 30 mL.

My question: Why do we assume nitrous acid dissociates completely? I am pretty sure it is a weak acid. Its Ka is 4 x 10^-4.....I am confused. Perhaps a typo exists and the problem is supposed to have nitric acid instead? This practice question is from Kaplan btw

 

[Dr Gerrard]

My understanding is that unless it dissociates completely, it is not completely neutralized.

 

[confusedliberal]

My understanding is that unless it dissociates completely, it is not completely neutralized.

sorry, can you clarify what you mean, in the context of the problem?

 

[Dr Gerrard]

I am not positive on this, since acids and bases are a weak point for me.

Anyways, I am saying that if all of the nitrous acid does not dissociate, then there woulds till be some nitrous acid in solution and thus all of the acid would not be neutralized since some acid would remain in solution.

Not sure if that makes any sense at all.

 

[johndoe3344]

Nitrous acid has a pKa of 3.4 (nitric acid has a pKa of -1.4 by comparison).

Nitrous acid is a weak acid, and does not dissociate completely. I would think that it was a typo, and the question meant nitric acid instead.

 

[drchem]

I'll try to explain it as best I can, and if someone disagrees they can say something.

Equation: HNO2 <--> H[+] + NO2[-]

I think the pKa of nitrous acid is irrelevant. Le Chatelier's principle states that if the acid starts being neutralized, the equilibrium will shift to produce more hydrogen ion. So, even though the acid is weak, it will eventually become completely dissociated, because the hydrogen is constantly reacting with the hydroxide in KOH.

 

[combatwombat]

I think the pKa of nitrous acid is irrelevant. Le Chatelier's principle states that if the acid starts being neutralized, the equilibrium will shift to produce more hydrogen ion. So, even though the acid is weak, it will eventually become completely dissociated, because the hydrogen is constantly reacting with the hydroxide in KOH.

This is what I'd reason as well. With chemistry questions, it always helps to think about how the question relates to equilibrum.

I could be wrong on this, but I doubt the MCAT would expect you to factor the pKa of nitrous acid in to the question, especially if it is not given somewhere.

 

[confusedliberal]

This is what I'd reason as well. With chemistry questions, it always helps to think about how the question relates to equilibrum.

I could be wrong on this, but I doubt the MCAT would expect you to factor the pKa of nitrous acid in to the question, especially if it is not given somewhere.

Well, the Ka of nitrous acid was given in the passage, and the passage also indicated nitric acid dissociated completely.