pH changes

[StarryNights]

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Can someone please explain how to do this problem? It asks about pH and then the solution doesn't mention anything relevant to pH at all so I'm kind of confused. Thanks!

The pH of the solution that results after mixing 10 mL of 1.0 M HCl and 10 mL of 1.0 M NH3 is the same as the pH of:
a. A 0.5 M solution of NH4Cl
b. A 1.0 M solution of NH4Cl
c. A 2.0 M solution of NH4Cl
d. Pure Water

 

[RogueUnicorn]

what would happen in this reaction

HCl + NH3 + H2O --> ??

this is not really a pH question, it's just merely disguised as such.

 

[StarryNights]

Yea, you're exactly right. It pretty much dilutes the concentration and that is all that's mentioned. I chose D at first though because I thought that HCl mixed with NH3 would lead to acid-base neutralization. Can you point out what's wrong with my approach to this problem, because I can see this as something that would trip me up if it appears on the test. Thanks!

 

[RogueUnicorn]

Yea, you're exactly right. It pretty much dilutes the concentration and that is all that's mentioned. I chose D at first though because I thought that HCl mixed with NH3 would lead to acid-base neutralization. Can you point out what's wrong with my approach to this problem, because I can see this as something that would trip me up if it appears on the test. Thanks!

it would be neutralization if it were an arrhenius acid-base reaction. but of note here is that nh3 is a bronsted lowry base. really, what i'm trying to say is:

HCl + H2O --> H3O(+) + Cl(-)
NH3 + H2O --> NH4(+) + OH(-)

thus if you add it up
HCl + NH3 + 2 H2O --> NH4(+) + Cl(-) + OH(-) + H3O(+)

and to finaly simplify...
HCl + NH3 --> NH4(+) + Cl(-)

see what happened here? now, given this, what's the correct answer?


edit: pro-tip: something i do with the mcat is, with a seemingly very simple question (or even complex ones to get a hint), take a second to look at the answer choices. why on earth would they be talking about NH4Cl solutions in the answer stem? how do you even get NH4Cl in this situation, when it's so obviously neutralization? oftentimes this will lead to a quick "wait!" moment.

 

[StarryNights]

You are awesome! Always new opportunities for me to learn.

 

[Jeff85]

I understand the products, but is 0.5M as it is now diluted or 1M because of stoichiometry?
There seem to be 2 ideas conflicting.

 

[boaz]

I understand the products, but is 0.5M as it is now diluted or 1M because of stoichiometry?
There seem to be 2 ideas conflicting.

Can you elaborate on that?

 

[StarryNights]

I understand the products, but is 0.5M as it is now diluted or 1M because of stoichiometry?
There seem to be 2 ideas conflicting.

Hey, it is now 0.5 M as it is now diluted with twice the volume 🙂

 

[HopefulOncoDoc]

I guess I got a little confuse because I was thinking of it at first as a pH question. But since you are adding a strong acid, the NH3 will all be converted into NH4 and since you're doubling the volume, you would the concentration to be 0.5 M and that's why the answer is a. A 0.5 M solution of NH4Cl. Is my reasoning correct?