PH concentrations, strong acids and water

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Discussion Overview

The discussion revolves around the calculation of pH for a strong acid solution, specifically 0.05 mol dm-3 of HCl, and whether the contributions of [H+] and [OH-] from water should be considered in this context.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant questions whether the pH of a strong acid solution should account for the [H+] and [OH-] concentrations from water, given that the acid is dissolved in water.
  • Another participant clarifies that the term mol/dm3 indicates the solution is in water, which is neutral with equal concentrations of H+ and OH-.
  • A subsequent post reiterates the concern about including the [H+] from water in the overall pH calculation.
  • It is suggested that at high concentrations of acid, such as above 5 x 10-7 M, the contribution from water can be ignored for pH calculations.
  • A participant expresses gratitude for the clarification and mentions their prior exposure to the topic in class, indicating some uncertainty about the details.

Areas of Agreement / Disagreement

Participants express differing views on whether the contributions from water should be included in pH calculations for strong acids, indicating that the discussion remains unresolved.

Contextual Notes

The discussion includes assumptions about the concentration thresholds for ignoring water's contribution to pH and the context of educational exposure to the topic.

nobahar
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Hello!
I was wondering, with questions such as "What is the pH of 0.05 mol dm-3 of HCl?", you specify the pH by simply using the concentration, as it completely dissociates. However, I assume that at such a concentration like this it is not 'pure', and would be present in water. Shouldn't I then have to consider the [H+] and [OH-] concentrations of water?
Thanks for any help.
 
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They are in water, that's what mol/dm3 means.
So in every liter of solution, there is 0.05 mol of HCl. The rest is pure water, which is neutral (in the sense that the concentration of H+ and OH- is equal to great accuracy).
 
so shouldn't the [H+] introduced be included with the [H+] of the water for an overall pH value?
Sorry, it has not been a good day...
 
nobahar said:
so shouldn't the [H+] introduced be included with the [H+] of the water for an overall pH value?

If concentration of acid is high enough they can be safely ignored. High enough in this context means more than 5*10-7M.

See calculation of pH of strong acid for more details.
 
Last edited by a moderator:
Ah, many thanks Compuchip and Borek. I looked into calculating pH, and I have covered it in class, I couldn;t find it mentioned on a site, and I don't think it came up in class, although it may have done...
Once again, many thanks!
 

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