Chemistry Question: shouldn't the blue litmus paper remain blue when COO- reacts with HCl?

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lotus0618
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Chemistry Question: shouldn't the blue litmus paper remain blue when COO- reacts with HCl?

Post by lotus0618 » Mon Jul 06, 2020 3:19 am

When COO- (carboxylate ion) is added with HCl, it is obvious that the blue litmus paper will turn red because the carboxylate gets protonated. However, I'm also confused by this reaction:
COO- + H20 => COOH + OH-
Based on this reaction, since COO- acts as a base here and produces OH-, shouldn't the litmus paper stay blue because the solution becomes basic?
NS_Tutor_Nancy
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Re: Chemistry Question: shouldn't the blue litmus paper remain blue when COO- reacts with HCl?

Post by NS_Tutor_Nancy » Thu Jul 09, 2020 1:15 pm

Hi,

COO- is the conjugate base of a carboxylic acid. Even though your equation is written with OH- on the right, that's not what you would expect in solution. A more typical way to write this equation is COOH + H2O > COO- + H3O+. And this equation goes back and forth in the solution at equilibrium. Carboxylic acids are the most acidic organic compounds and can turn blue litmus paper red.

I hope this helps!
lotus0618
Posts: 182
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Re: Chemistry Question: shouldn't the blue litmus paper remain blue when COO- reacts with HCl?

Post by lotus0618 » Thu Jul 09, 2020 10:53 pm

But the question states that carboxylate ion, not carboxylic acid that reacts with HCl.
NS_Tutor_Yuqi
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Re: Chemistry Question: shouldn't the blue litmus paper remain blue when COO- reacts with HCl?

Post by NS_Tutor_Yuqi » Sun Jul 12, 2020 1:51 pm

Hi! Just to clarify, are you asking about the status of the litmus paper when COO- reacts with HCl or with H20? When HCl is added, the litmus paper should turn red as per your reasoning in the original post. However, if H2O is being added to COO-, you're essentially just diluting a basic solution, so the paper should remain blue.
lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

Re: Chemistry Question: shouldn't the blue litmus paper remain blue when COO- reacts with HCl?

Post by lotus0618 » Sun Jul 12, 2020 4:57 pm

ohhhhh.
Okay so I got it. Can you please look at my reasoning one more time to make sure it's correct?
HCl is the same as H+, which is the same as H30. Therefore, COO- + H30 => COOH + H20. And that's why the color of the litmus paper is pink
Now, if COO- reacts with water, then like the other reaction I wrote, the reaction would yield OH-, and that's why the litmus paper remains blue.

H20 is obviously not the same as H30. I confused myself by using H20 instead of H30.
NS_Tutor_Nancy
Posts: 48
Joined: Wed May 27, 2020 7:25 pm

Re: Chemistry Question: shouldn't the blue litmus paper remain blue when COO- reacts with HCl?

Post by NS_Tutor_Nancy » Thu Jul 16, 2020 10:00 am

Yep! You've got it now.
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