Two questions about the pH of buffers after solutions are added questions

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Discussion Overview

The discussion revolves around the pH calculations of buffer solutions, specifically involving the addition of HCl to a sodium formate solution and the behavior of weak acids in solution. Participants explore the implications of strong acids on buffer systems and the calculations related to pH in these contexts.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the validity of using HCl in a buffer system, noting that buffers typically consist of a weak acid and its conjugate base.
  • Another participant inquires about the reaction of formate ions (HCOO-) with strong acids, suggesting that it may produce formic acid (HCOOH) and questioning the need to calculate moles of reactants to determine the remaining concentrations.
  • There is a discussion about the calculations involving the concentration of formic acid and sodium formate after the reaction, with one participant asserting their results and another expressing confusion about the derived concentrations.
  • Participants discuss the use of an ICE table versus assuming complete reaction for calculating pH, with differing opinions on the necessity of the ICE table in this context.

Areas of Agreement / Disagreement

Participants express differing views on the appropriate method for calculating pH in buffer solutions involving strong acids, with some agreeing on the need for mole calculations while others question the necessity of an ICE table. There is no consensus on the correctness of the calculations presented.

Contextual Notes

Some participants express uncertainty regarding the calculations and the assumptions made about the reactions involved, particularly concerning the concentrations of the species after the addition of HCl.

Who May Find This Useful

Students studying buffer solutions in general chemistry, particularly those grappling with the interactions of strong acids with weak acid-base pairs.

TeenieBopper
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I have two questions about pH of buffers. It's a general chemistry class, but the questions aren't similar to anything we've had yet.

Homework Statement


A buffer is prepared by adding 0.96 L of 0.96 M HCl to 762 mL of 1.4 M NaHCOO. What is the pH of this buffer? [Ka(HCOOH) = 1.7 × 10-4]


Homework Equations


pH=pKa+log([A-]/[HA]


The Attempt at a Solution



It's the fact that it's HCl that's really throwing me off. I thought a buffer could only be made with a weak acid and it's conjugate base (and vice versa). I can figure out the pH of the HCl and the NaHCOO solutions without a problem, but it's the combining that's throwing me.



Homework Statement


A certain monoprotic weak acid with Ka = 0.37 can be used in various industrial processes. (a) What is the [H+] for a 0.234 M aqueous solution of this acid and (b) what is its pH?

Homework Equations


Ka=[H+][A-]/[HA]
pH=-log[H+]

The Attempt at a Solution


I did an ICE table and set up the equation .37=x^2/(.234-x) to get [H+]. Once I had that, -log[H+] gave me the pH. I got [H+]=.1625 and pH=.79, which doesn't make sense at all considering it's supposed to be a weak acid, but I'm not sure what I'm doing wrong.
 
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How does HCOO- react with a strong acid?

Acid with Ka=0.37 is quite strong.
 
Borek said:
How does HCOO- react with a strong acid?

Does it create Formic Acid (HCOOH)? Do I need to determine the amount of moles of HCl and NaHCOO and then that will give me how much NaHCOO is left along with how much Formic acid is made? Which I can then plug into the pH=pKA + log([A-]/[HA]) with A- being the cation of HCOOH?

Acid with Ka=0.37 is quite strong.

So my math is probably correct then?
 
TeenieBopper said:
Does it create Formic Acid (HCOOH)? Do I need to determine the amount of moles of HCl and NaHCOO and then that will give me how much NaHCOO is left
along with how much Formic acid is made?

Correct so far.

Which I can then plug into the pH=pKA + log([A-]/[HA])

Yes, that's what you should do.

with A- being the cation of HCOOH?

but this part doesn't make any sense.

So my math is probably correct then?

It IS correct.
 
Borek said:
but this part doesn't make any sense.
Okay, so i did the math, and after the formation of the Formic Acid, , I have .5352 M formic acid and .0843 sodium formate. After doing an ICE table, I find I have .003702+.009726=.013428 M COOH-. So the pH should be:

pH= -log(1.7E-4)+log(.013428/.0843)

Correct?
 
TeenieBopper said:
I have .5352 M formic acid and .0843 sodium formate

I don't see where you got these numbers from (unless your first post contains wrong data).

After doing an ICE table

You don't need ICE table, simply assume reaction went to completion.
 

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