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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: sterlinski on October 08, 2015, 11:32:45 AM

Title: Mixture of weak base and weak acid equation.
Post by: sterlinski on October 08, 2015, 11:32:45 AM

Hi,
could anyone derive a formula on the bottom, please?

[A^-]=[BH⁺], because [OH^-] and [H⁺] << [A^-] and [BH⁺]
(https://scontent-fra3-1.xx.fbcdn.net/hphotos-xtp1/v/t34.0-12/12092745_1012912415439800_1817929591_n.jpg?oh=806ed9942b8ac2df2f1af67dfe733f18&oe=56198E3C)
Don't mind this polish words :P
Thanks a lot.

Title: Re: Mixture of weak base and weak acid equation.
Post by: Borek on October 08, 2015, 12:08:28 PM

Have you tried? Solve K_a definition for [A^-] and plug it into the [A^-]=[BH⁺]...

Title: Re: Mixture of weak base and weak acid equation.
Post by: sterlinski on October 08, 2015, 12:25:40 PM

Quote from: Borek on October 08, 2015, 12:08:28 PM

Have you tried? Solve K_a definition for [A^-] and plug it into the [A^-]=[BH⁺]...

Yes I've tried many times but I can't gain this final formula. So… K_a*[HA]/[H⁺]=[BH⁺], and then i can plug [BH⁺]=c_B-[B ] or solved K_b definition for [BH⁺]. And I really don't know what to do then.

Title: Re: Mixture of weak base and weak acid equation.
Post by: Borek on October 08, 2015, 05:39:52 PM

I got the same they did.

Many ways to skin that cat. Plug [HA]=C_A-[A^-] and [B]=C_B-[HB⁺] into K_HA and K_HB, solve for [A^-] and [HB⁺], plug into [A^-]=[HB⁺], rearrange.

Title: Re: Mixture of weak base and weak acid equation.
Post by: sterlinski on October 09, 2015, 04:24:31 PM

Yeah, I see now, thanks a lot. :)