Solve pH of NaOH + Acetic Acid Mixture

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Discussion Overview

The discussion revolves around calculating the pH of a mixture of sodium hydroxide (NaOH) and acetic acid. Participants explore the chemical reaction between these two substances, the concept of buffer solutions, and the implications of their concentrations and ratios.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Exploratory
  • Technical explanation

Main Points Raised

  • One participant expresses uncertainty about how to approach the problem of calculating pH after mixing NaOH and acetic acid.
  • Another participant notes that acetic acid and sodium hydroxide react in a 1:1 mole ratio and suggests determining which reactant will be in excess after mixing.
  • A participant mentions that understanding the molar ratio of the reactants may eliminate the need for calculations to find the pH.
  • There is a suggestion that the resulting solution may act as a buffer, with unreacted acetic acid remaining in the solution, and questions whether the salt produced will significantly affect the pH.
  • One participant reflects on their learning experience with buffers and weak acids/bases, emphasizing the importance of understanding equilibrium concepts.
  • A later reply indicates that there is a shortcut to solving the problem, as noted by another participant, and suggests starting with identifying all chemical species present and their concentrations.

Areas of Agreement / Disagreement

Participants exhibit a mix of uncertainty and exploration regarding the calculation of pH, with no consensus reached on the final approach or solution. Some participants propose that the mixture may form a buffer, while others focus on the need for calculations.

Contextual Notes

Participants mention the need for analytical calculations and the potential effects of the salt on pH, indicating that assumptions about the system may influence the outcome. The discussion includes references to specific chemical principles and concepts that may require further clarification.

rmarsino
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Homework Statement



What is the pH after 12.50 mL of 0.10 M NaOH has been added to 25.00 mL of 0.10 M acetic acid?


Homework Equations





The Attempt at a Solution


i have no idea how to do this.
 
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The concentration of each starting solution is the same. Acetic acid and sodium hydroxide react in a 1:1 mole ratio. One of those will be in excess when the quantities of solutions are mixed. Which one, and what concentration will it be?
 
Once you know what buffer is, and once you realize what is molar ratio of both reactants, you don't need any calculations to find out the answer.


&
 
Borek said:
Once you know what buffer is, and once you realize what is molar ratio of both reactants, you don't need any calculations to find out the answer.

Borek
--
pH calculator & buffer calculator

The exact truth would be a little different from that. Note that the solution will have unreacted acetic acid. 37.50 ml. of entire solution containing 0.0125*0.10 moles of unneutralized acetic acid. The salt present should likely not have much effect on pH. I'm assuming that there is enough acid present to overcome any effect of the salt, but I can't be sure unless I actually perform the analytical calculation-------- wait! Maybe this really IS a buffer solution, being half the moles are the salt and half the moles are the acetic acid. There is a simple way to find pH of this buffer using pK value, right?
 
symbolipoint said:
Maybe this really IS a buffer solution, being half the moles are the salt and half the moles are the acetic acid. There is a simple way to find pH of this buffer using pK value, right?

Bingo :smile: Hopefully rmarsino will get to this point as well.
 
In post #5, I began writing using only half my mind. The concept caught up with me as I wrote and figured. rmarsino, when you study equilibrium of weak acids and weak bases, you will learn about buffers. You will also learn how to find pH of solutions containing weak bases, weak acids, and weak acids or bases and salts of these.

I actually learned this stuff best when I was NOT enrolled in a course to learn it; but while I was reviewing on my own. For monoprotic or monofunctional compounds, this stuff is full of details, but not necessarily difficult. Learning it well takes time, patience, and much exercise and repetition.
 
These things are always done the same way from three-and-a-half principles, stated in

https://www.physicsforums.com/showpost.php?p=1754973&postcount=3

You say you have no idea how to do. In general start by writing all the chemical species present. Then write what you know about their concentrations...

It is true that in this particular case there is a (instructive) shortcut as pointed out by symbolipoint.
 

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