2
$\begingroup$

When a weak acid is added to water , will the dissociation of water reduce due to common ion effect caused by increased H+ concentration due to acid , my text book said common ion effect is caused by a strong electrolyte Moreover this doubt is a part of a larger doubt - if multiple weak acids are added in water will they reduce each others and water's dissociation as compared to their dissociation in their individual solutions ( due to common ion effect caused by H+ ions ) They should ? Having this doubt because the textbook I'm using just adds up their normal dissociations irrespective of common ion.

$\endgroup$

closed as unclear what you're asking by andselisk, MaxW, Mithoron, Mathew Mahindaratne, Todd Minehardt Jan 22 at 21:59

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ Yes, if you have multiple equilibria with common species, they will influence each other. If you put an acid in neutral water, the $\ce{H+}$ will increase of course, but at the same time, the $\ce{OH-}$ concentration will decrease. For the case with multiple weak acids, adding the first acid will change the pH, so the other acids will dissociate less just like when you add strong acid to a weak acid. $\endgroup$ – Karsten Theis Jan 20 at 21:19
2
$\begingroup$

Adding acid to the water will not stop the auto-ionization of the water, so: the auto-ionization will continue to give equal concentrations of hydroxide and hydronium. But the addition of acid to the water increases the concentration of the hydronium ions in the solution, pushing water balance with its ions to the left to resist the increase in the concentration of hydronium and reach a new equilibrium. The concentrations of hydronium and hydroxide produced by auto-ionization of water are reduced, where the concentrations of $\ce{H_3 O^+}$ or $\ce{OH^-}$ from water ionization is less than$\pu{ 10^{-7}}$.

$$\ce{{[H_3O^+]}_\text{(after acid addition)}=\ce{[H_3O^+]}_\text{(produced by acid dissociation)} +\ce{[OH^-]}_\text{(produced by water auto-ionization )}}$$

$\endgroup$
  • 1
    $\begingroup$ so how do I know what is the new dissociation. $\endgroup$ – ADITYA PRAKASH Jan 21 at 19:03
  • 1
    $\begingroup$ You have two equilibria: water auto-ionization and acid dissociation, and you can use the following equation to calculate $\ce{[H_3O^+]}_\text{(after acid addition)}$:$$\ce{{[H^+]}=\frac{K_\mathrm{w}}{[H^+]}+\frac{K_\mathrm{a}\text{[Acid]}_\mathrm{I}}{\ce{[H^+]}+K_\mathrm{a}}}$$ $\endgroup$ – Adnan AL-Amleh Jan 21 at 21:26

Not the answer you're looking for? Browse other questions tagged or ask your own question.