C2H3O2- concentration given pH of 30mL NaC2H3O2 solution

[Phyzwizz]

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 K_b? 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?

 

[Borek]

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 K_b? 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?

 

[Phyzwizz]

yeah I don't know what I did to get that K_b 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 K_b value but wouldn't this give me the ion concentrations of both C₂H₃O₂^- and Na⁺ since they are 1:1?

 

[Borek]

Yes.

 

[DeeNos]

Your calculations seem to be correct, but there may be some errors in the data or assumptions. It's important to double check the given pH value and make sure it is accurate. Also, make sure the volume of the solution is actually 30 mL and not 30.0 mL, as this could affect your final concentration calculation. Additionally, the concentration of a solution can vary depending on its preparation and storage, so it's possible that the actual concentration of the NaC2H3O2 solution is not exactly 1.0M.

If you're still unsure, it may be helpful to consult with your instructor or a classmate to compare calculations and make sure you're on the right track. Remember to always double check your work and use the correct units in your calculations to ensure accuracy. Good luck with your lab!