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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Kritkatten on February 18, 2010, 11:13:21 AM

Title: pH by the equivalence point
Post by: Kritkatten on February 18, 2010, 11:13:21 AM
Hello all! Nice to see a forum for chemistry! :)

I'm new here and a beginner in the awesome subject of chemistry. Glad to meet you all! Please excuse my eventual bad grammar/spelling, English is not my first language...

So, here's something I've tried to solve all day:

1.24 gram benzoic acid solution was diluted to 50.0 cm3. After shaking it, the solution was titrated with 0.180 M NaOH. What was the pH in the solution by the equivalence point?

So, I know that by the eq. point, the amount of mol acid should be equal to the amount of mol base, but that's about as far as I've got. :(
Title: Re: pH by the equivalence point
Post by: Borek on February 18, 2010, 01:42:25 PM
Benzoate is a weak base.
Title: Re: pH by the equivalence point
Post by: cpncoop on February 18, 2010, 01:55:14 PM
You may be overthinking this a bit.... pH is a measure of H+ concentration.  If the amount of H+ you have present (from benzoic acid) is neutralized by an equal amount of NaOH you have a neutral solution.... What is the pH of a neutral solution?
Title: Re: pH by the equivalence point
Post by: Kritkatten on February 18, 2010, 02:00:23 PM
Quote from: cpncoop on February 18, 2010, 01:55:14 PM
You may be overthinking this a bit.... pH is a measure of H+ concentration.  If the amount of H+ you have present (from benzoic acid) is neutralized by an equal amount of NaOH you have a neutral solution.... What is the pH of a neutral solution?

Well, the pH of a neutral solution is 7, right? But that is not the correct answer. The final solution will be basic.
Title: Re: pH by the equivalence point
Post by: Borek on February 18, 2010, 06:16:43 PM
I hate to repeat myself, but

Quote from: Kritkatten on February 18, 2010, 02:00:23 PM
The final solution will be basic

- because benzoate is a weak base.
Title: Re: pH by the equivalence point
Post by: Kritkatten on February 19, 2010, 03:04:17 AM
Quote from: Borek on February 18, 2010, 06:16:43 PM
I hate to repeat myself, but

Quote from: Kritkatten on February 18, 2010, 02:00:23 PM
The final solution will be basic

- because benzoate is a weak base.

Yes, thank you. That's how I know it'll be a basic solution. :) But I still don't know how to "start". I've written down the reaction, also calculated the concentration of the benzoate (0.02 M).
I'm new to these kind of equilibrium schemes. Don't know where to begin.  ???
Title: Re: pH by the equivalence point
Post by: Borek on February 19, 2010, 04:16:22 AM
Use Kb and concentration to calculate pOH.