Chemistry Pre-Lab Fail: Stuck with Acid/Base Work? Get Help Now!

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The discussion revolves around difficulties in performing acid/base titration calculations for a chemistry pre-lab. The user successfully calculated the initial pH using ICE tables but struggled with the application of the equilibrium expression for Ka. A key mistake was treating the percentage of dissociation as a direct ratio in the Ka expression, which led to an incorrect calculation. The correct approach requires using the analytical concentration of the acid instead of the percentage. The user acknowledges the need for further practice to improve understanding and avoid similar mistakes in the future.
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I Need Help With My Acid/Base Work

Apparently I've caught the stupid virus and absolutely fail at chemistry.

Todays prelab: 0.010L of .1000M HA with a pKa of 7.00 is titrated with 0.1000M NaOH over a series of volumes:

L NaOH
.0000
.0010
.0020
.0050
.0090
.0098
.0100
.01010
.01020
.01100
.01200

Step 1: Get the pH at zero. Hooray for ICE tables, I got the correct pH of 4.

Then I set sail for fail with a method my teacher showed us in class.

So we can see clearly that the equivalence point is at 10ml or 0.010. Oh my! It looks like we can cut everything up into nice neat percentages!

L NaOH
.0000 0%
.0010 10%
.0020 20%
.0050 50%
.0090 90%
.0098 98%
.0100 100%

For the rest of them Kb comes into the mix. Then I did this:

Ka = [H3O][A]/[HA]

Well it looks like I can just replace the [A]/[HA] with the percentage as a decimal!

Ka = [H3O](concentration coefficient)

Then after some more fun:

pH = -log(Ka/(concentration coefficient))

Which is exactly what my teacher did and class for a different setup, and wound up with me getting .25/2 on the pre-lab.

What went wrong here? Is this even right?
 
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Lancelot59 said:
Well it looks like I can just replace the [A]/[HA] with the percentage as a decimal!

No, percentage is not [A-]/[HA] but [A-]/[analytical concentration of the acid].

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methods
 
The way she used it seems to make percentage the percent until completion...

Here is the slide with the work on it...attachment pending.
 

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Right down: 25/75 is OK, but it is not 25%.

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...Ugh. That's a massive fail. Alright, something to remember for next time. So for my situation it would be things like 98/2 and whatnot?

Gotcha. Thanks for that...I need to practice a lot more.
 
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