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Author Topic: why is 0.1M HCl solution more acidic than 1.0M acetic acid solution?  (Read 5925 times)

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microDeth

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The Question:
One hundred mL of a solution of 1M acetic acid contains 10 times as many moles of acetic acid as there are moles of HCl in 100 mL of 0.1M HCL solution.  In spite of this fact, the HCl solution has a lower pH than does the acetic acid solution.  Explain how something like that can happen.

My answer so far:
Since HCl is monoprotic, meaning it dissociates completely, the H30 concentration is much higher than the H30 concentration in the acetic acid solution. 

Any help is really appreciated.  Should my answer be more precise, numerically?
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Arkcon

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Re: why is 0.1M HCl solution more acidic than 1.0M acetic acid solution?
« Reply #1 on: April 29, 2008, 06:02:16 AM »

There is an important difference between HCl and acetic acid.  Do their structures and compositions give you a hint that you can use?

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Since HCl is monoprotic, meaning it dissociates completely, the H30 concentration is much higher than the H30 concentration in the acetic acid solution.

Your statement is incorrect.  Can you fix it?
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microDeth

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Re: why is 0.1M HCl solution more acidic than 1.0M acetic acid solution?
« Reply #2 on: April 29, 2008, 03:38:11 PM »

Ok, so far i've figured out that after the H+ dissociates from the CH3COO-, the CH3COO- becomes a conjugate base and the H3O+ that is created becomes a conjugate acid of the former H2O.  Given that, the acetic acid won't all dissociate and it will exist in equilibrium.  This is what causes the H3O content to be less and therefore less acidic?

Have I covered what was wrong with my first stab at an answer?
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Arkcon

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Re: why is 0.1M HCl solution more acidic than 1.0M acetic acid solution?
« Reply #3 on: April 29, 2008, 04:05:31 PM »

Given that, the acetic acid won't all dissociate and it will exist in equilibrium.  This is what causes the H3O content to be less and therefore less acidic?

Have I covered what was wrong with my first stab at an answer?

Yes, this part is correct, and well explained.  But ...

Quote
Ok, so far i've figured out that after the H+ dissociates from the CH3COO-, the CH3COO- becomes a conjugate base and the H3O+ that is created becomes a conjugate acid of the former H2O. 

This is technically correct, but all acids dissociate into their conjugate base and a proton, this explanation of yours doesn't really have a bearing on the question at hand.  Likewise, from before ...

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Since HCl is monoprotic,


... is perfectly correct ...

Quote
meaning

No!

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it dissociates completely

Likewise correct, but your word "meaning" here ruins everything, because it is not the reason, or the actual definition of the terms.

Sometimes, I see a beginning chemistry student jumble random text from their book together like this, so I want to tell you to be more careful.  Chemistry doesn't follow English grammar rules -- just because two statements are physically close to one another in a text, doesn't mean they can be combined any old way.
« Last Edit: April 30, 2008, 01:19:43 AM by Arkcon »
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microDeth

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Re: why is 0.1M HCl solution more acidic than 1.0M acetic acid solution?
« Reply #4 on: April 29, 2008, 04:11:27 PM »

ohhh, ok.  I see what is wrong with that.  Thanks!
I'm hoping this post lab goes well, thankyou for your help.

I belong in the english department where everything is based on B.S. :D
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