Calculating pH of Strong and Weak Acids with 10^-7 M Concentration - Explained

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Discussion Overview

The discussion revolves around calculating the pH of strong and weak acids at a concentration of 10^-7 M. It explores the necessity of considering the contribution of hydrogen ions from water in this context.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant requests an explanation of the method for calculating pH for strong and weak acids at low concentrations.
  • Another participant explains that the contribution of H+ ions from water becomes significant at low acid concentrations, specifically when comparing it to the dissociation from the acid itself.
  • A specific example is provided regarding the pH of 10^-8 M HCl, highlighting the common ion effect and the need to account for both the H+ from the acid and from water.
  • One participant inquires about a simpler method to avoid solving a quadratic equation in the calculations.
  • A later reply asserts that solving the quadratic equation is necessary for accurate results.

Areas of Agreement / Disagreement

Participants express differing views on the complexity of the calculation method, with some seeking simpler approaches while others maintain that a quadratic solution is unavoidable.

Contextual Notes

The discussion highlights the limitations of calculations at low concentrations, particularly the assumptions regarding the contributions of H+ from both the acid and water, and the implications of the common ion effect.

benzun_1999
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Can anyone tell me how to calculate pH of an strong and weak acid with conc 10^-^7 M . Please just explain the method. i read a little about it. Why is it necessary to take into consideration the change in conc of H^+ from water?

-Benzun
 
Last edited:
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benzun_1999 said:
Why is it necessary to take into concideration the change in conc of H^+ from water?

That is because the amount of H+ which comes from water is now not negligible when compared to the H+ which dissociates from the acid.

For example, what is the pH of 10^-8 M of HCl ?

What happens is that, the dissociation of H20 is suppressed by HCl by the common ion-efect.

So if you consider the dissociation of water
H20----> H+ + OH-
let x be the conc of H+
so, the total conc of H+ is x (from water) + 10^-8 (from the strong acid HCl). And the conc of OH- is x(from water). You also know Kw. So, can you take it from here?
 
Last edited:
ok understood but is there some easy and clean way to proceed in this sum without getting quadratic equation?
 
No. You will have to solve the quadratic.
 

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