Understanding pH: Solving Confusing Problems with Strong Acids and Bases

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Discussion Overview

The discussion revolves around understanding pH calculations for strong acids and bases, specifically focusing on the pH of concentrated hydrochloric acid (HCl) and sodium hydroxide (NaOH). Participants explore the implications of high concentrations on pH values and the validity of negative pH readings.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about calculating the pH of 12.1 M HCl, noting that their calculation yields -1.8.
  • Another participant explains that HCl is a strong acid that dissociates completely in aqueous solution, suggesting a pH calculation of -1.08 and questioning the concentration provided.
  • It is noted that negative pH values are permissible and that pH can exceed 14 in highly concentrated solutions.
  • A participant mentions that concentrations beyond the limits of Ostwald's dilution principle can lead to unusual pH results, affirming that negative pH and pOH values are expected in such cases.
  • The original poster acknowledges the possibility of negative pH but expresses surprise at the extent of negativity observed.

Areas of Agreement / Disagreement

Participants generally agree that negative pH values can occur in highly concentrated solutions, but there is no consensus on the specific calculations or implications of the pH values discussed.

Contextual Notes

Limitations include the potential misunderstanding of concentration effects on pH calculations and the applicability of Ostwald's dilution principle in extreme cases.

nautica
I know this is simple, but I am a bit confused

1) find the pH of 12.1 M HCl - the negative log gives me -1.8. What am I doing wrong

2) 50% NaOH (roughly 15 M) - I am not sure how to do this one either

thanks
nautica
 
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Well, HCL is a strong acid, so it dissociates completely in ions. Of course aqueous solution.

[tex]HCl + H_{2}O \rightarrow H_{3}O^{+} + Cl^{-}[/tex]

[tex]H_{3}O^+ \approx H^+[/tex]

I get pH -log(12.1) = -1.08, are you sure is 12.1 M?

Well, i barely remember my freshman chemistry, wait for more help.
 
Last edited:
What's wrong? You are allowed to have pH values that are negative and that are also above 14.
 
If the concentration is beyond the limits of Ostwald's dilution principle, then some weird results like this is likely to occur, as gravenenewworld said in a different way. Nautica, your findings of negative pH and pOH values are normal, as these are very concentrated and are not obeying the principle.
 
Thanks, I was aware that pH could be negative, but I did not realize they would get that negative. I was thinking around zero and negative 0.5 or so.

thanks
Nautica
 

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