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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: RaelAerosolKid on December 17, 2020, 03:02:40 PM

Title: Help with titration involving weak base and strong acid or vice-versa
Post by: RaelAerosolKid on December 17, 2020, 03:02:40 PM

So I'm just beginning to have analytical chemistry classes and I already got pretty confused about something.

When you calculate the resulting pH of, say, NaOH added to acetic acid, the concept of subtracting the amount that is being titrated with the amount that reacts and dividing it by the initial volume + the added volume of the solution with the titrant seems completely fine to me.

But, you would have to take into account that acetic acid being a weak acid is in a chemical equilibrium like H+ + Ac- <---> HAc , right?

So now I don't know if NaOH reacts to everything (I'm thinking of the chemical equation NaOH + HAc --> H2O + NaAc), regardless if it's dissociated or not, or if you have to only consider one part of it. My teacher just skipped this part and gave us the titration graph.

I am really confused,

Thanks in advance

Title: Re: Help with titration involving weak base and strong acid or vice-versa
Post by: RaelAerosolKid on December 17, 2020, 03:42:13 PM

I might have just found my answer. This site explains it well: https://groups.chem.ubc.ca/courseware/pH/section14/index.html#:~:text=Calculate%20the%20pH%20at%20any,this%20is%20not%20the%20case.

Title: Re: Help with titration involving weak base and strong acid or vice-versa
Post by: Borek on December 17, 2020, 03:56:43 PM

This is a bit chaotic, but I assume you mean calculating pH during titration by using Henderson-Hasselbalch equation.

Yes, the assumption that concentrations calculated from the neutralization stoichiometry are exact it not always valid, as they always shift a bit due to dissociation/hydrolysis. In most typical cases the error is negligible though.

Title: Re: Help with titration involving weak base and strong acid or vice-versa
Post by: RaelAerosolKid on December 17, 2020, 04:46:11 PM

Quote from: Borek on December 17, 2020, 03:56:43 PM

This is a bit chaotic, but I assume you mean calculating pH during titration by using Henderson-Hasselbalch equation.

Yes, the assumption that concentrations calculated from the neutralization stoichiometry are exact it not always valid, as they always shift a bit due to dissociation/hydrolysis. In most typical cases the error is negligible though.

Yeah what I wrote was kinda confusing but thank you for naming the equation I didn't even know had a name

Title: Re: Help with titration involving weak base and strong acid or vice-versa
Post by: Borek on December 17, 2020, 05:34:05 PM

Technically it is just a rearranged definition of dissociation constant, so the name is not that important. Still, used often enough to be worth knowing.