C2H3O2- concentration given pH of 30mL NaC2H3O2 solution

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Discussion Overview

The discussion revolves around calculating the concentration of acetate ions (C2H3O2-) and sodium ions (Na+) in a 30 mL solution of sodium acetate (NaC2H3O2) based on a measured pH of 8.4458. Participants explore the relationships between pH, pOH, and ion concentrations, as well as the implications of using the correct equilibrium constant (Kb) for acetate.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant calculates the OH- concentration from the pH and attempts to use it in the Kb equation to find the concentration of C2H3O2- and Na+, expressing confusion over the resulting value being lower than expected.
  • Another participant questions the accuracy of the reported pH and suggests that the last digit may not be reliable, indicating that pH can vary with different volumes.
  • Concerns are raised regarding the use of an incorrect Kb value for acetate, with one participant correcting their earlier calculation of Kb and expressing uncertainty about the implications for ion concentration calculations.
  • There is a brief agreement on the stoichiometry of the reaction, noting that the concentrations of C2H3O2- and Na+ should be equivalent due to a 1:1 ratio.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the correct Kb value and its impact on the calculations. While there is some agreement on the stoichiometric relationship between C2H3O2- and Na+, the overall discussion remains unresolved with competing views on the calculations and assumptions made.

Contextual Notes

Limitations include potential inaccuracies in pH measurement, the use of an incorrect Kb value, and the implications of volume changes on pH. The discussion does not resolve these issues.

Phyzwizz
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I'm working on a lab for my chem class and we have to calculate all of the different ion concentrations within some titrations that we did. Here I have to calculate the concentration before anything is added. Na+ + C2H3O2- + H2O → HC2H3O2 + OH- + Na+
the pH is 8.4458 for a 30.0 mL NaC2H3O2 solution. I tried finding the concentration of the Na+ and C2H3O2- ions by: 14.00 - 8.4458 = pOH = 5.55452 then 10-5.55452 = 2.79 X 10-6 which is the OH- concentration. In the Kb? equation this would be x so: 5.3 X 10-6 = (2.79 X 10-6)2/(x-(2.79 X 10-6). Calculating all this through I get 4.25 X 10-6. With the Liters being 30.0 mL I take this concentration and divide it by .0300 L and I get 1.42 X10-4. This just seems incorrect because the Molarity was supposed to be somewhere near 1.0M, what did I do wrong?
 
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Phyzwizz said:
the pH is 8.4458

You really know pH with that high accuracy? Wow. Most pH meters I know will show 8.45, and I would not believe the last digit.

for a 30.0 mL NaC2H3O2 solution

You mean - if you will take different volume, pH will have different value?

14.00 - 8.4458 = pOH = 5.55452

Probably typo. 5.5542, if anything. Not that it changes the result by much.

2.79 X 10-6 which is the OH- concentration

OK

In the Kb? equation this would be x so: 5.3 X 10-6

5.3x10-6 is not Kb of the acetate ion, so anything you did past this point can't be correct. Still, you managed to write something that I can't left uncommented:

With the Liters being 30.0 mL I take this concentration and divide it by .0300 L and I get 1.42 X10-4.

So you took concentration and you divided it by volume to get... what? Concentration again?
 
yeah I don't know what I did to get that Kb value I know its (1.0 X 10-14)/(1.8 X 10-5) = 5.6 X 10-10.

Looking at it now I'm unsure why I would have gone any farther than the 4.25 X 10-6 this answer would definitely be different with the correct Kb value but wouldn't this give me the ion concentrations of both C2H3O2- and Na+ since they are 1:1?
 
Yes.
 

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