sketch the titration curve to show how the pH changes during the addition of NaOH to propanoic acid?

Question

I am seeking clarification on the titration curve for the reaction between propanoic acid and sodium hydroxide. Specifically, I would like to understand how the pH changes during the addition of 40 cm³ of 0.10 mol/dm³ NaOH to 30 cm³ of 0.10 mol/dm³ propanoic acid. The provided answer states that the curve starts between pH 2 and 4, flattens at pH 12-14, and exhibits a steep rise of 3 to 4 units in the pH range of 6 to 11, with a particularly steep rise occurring at 30 cm³ of NaOH added. 
I am confused about how these conclusions were reached, especially considering the different volumes of the two reactants. Is there a specific calculation involved in determining the pH at these points? Additionally, why is there a steep rise at the 30 cm³ mark? For titration curves involving strong bases and weak acids, I thought the vertical pH range typically spans from 7 to 10. Any insights or explanations would be greatly appreciated, as I am struggling to grasp these concepts. Thank you for your help!

Anonymous · Accepted Answer

http://www.kentchemistry.com/links/AcidsBases/titr...see the way way bottom of the page

Anonymous · Answer

Initial pH is between 2 and 4 because that is the pH of weak acid propanoic acid.

NaOH and the acid reacts as such:

NaOH + CH3CH2COOH --> CH3CH2COONa + H2O

As titration proceeds, we get a buffer system consisting of the unreacted weak acid and the salt that is formed. (Gentle rise in pH)

Then there is a sharp rise in pH at 30cm3 because that is the equivalence point, as given in the question, meaning all the acid and base have reacted in stoichiometric amounts.

And finally, at equivalence point, we have a basic salt which undergoes anion hydrolysis:

CH3CH2COO- + H2O <--> CH3CH2COOH + OH-

Equivalence pH should be above 7. I don't think the range is restricted to 7 and 10...

(Hope this helps. The explanation I have given is according to the A-level standard, not sure whether it's applicable to you)

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Marlin Muller DVM · Answer

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Oswaldo Gutmann

sketch the titration curve to show how the pH changes during the addition of NaOH to propanoic acid?

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