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

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Discussion Overview

The discussion revolves around a chemistry pre-lab exercise involving acid/base titration calculations. Participants explore the correct application of formulas and concepts related to pH, equilibrium, and the use of percentages in determining concentrations during titration.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes their approach to calculating pH during a titration of a weak acid with a strong base, using ICE tables and percentages to represent concentrations.
  • Another participant challenges the use of percentages, stating that the percentage should not represent [A-]/[HA] but rather [A-]/[analytical concentration of the acid].
  • A later reply suggests that the method used in class may have led to confusion regarding the interpretation of percentages in the context of titration.
  • Participants discuss the implications of using incorrect percentages, with one noting that while a ratio of 25/75 is acceptable, it does not equate to 25% in this context.
  • One participant expresses frustration over their misunderstanding and acknowledges the need for further practice.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the correct application of percentages in the calculations, indicating that multiple competing views remain regarding the methodology and interpretation of the concepts involved.

Contextual Notes

There are limitations in the understanding of how to apply percentages in the context of acid/base titration, particularly regarding the distinction between different types of concentrations and their representations.

Who May Find This Useful

Students engaged in chemistry coursework, particularly those focused on acid/base titration and related calculations, may find this discussion relevant.

Lancelot59
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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.