[Anthasci]

I have a non-numeric problem relating the titration of CH₃COONa with HCl. The idea is to sketch the titration curve, which I'm guessing would start somewhere above 7 and then asimptotically approach the value of
[itex] -log[HCl] [/itex]. The more problematic thing is, I have to define an equation for [H₃O⁺] in 4 points of interest:
-at the beginning of titration (I'm guessing before we add any HCl)
-before the equivalence point
-at the equivalence point
-after the equivalence point.

[H₃O⁺] after the equivalence point would, in my opinion, equal the [HCl] and is defined as such:
[itex] \frac {c_{HCl}V_{HCl}}{V_{tot}} [/itex]

at the equivalence point, I figured:
CH₃COONa + HCl   CH₃COOH + NaCl
and as for [H₃O⁺];

[H₃O⁺] = [itex] \frac {K_a[CH_3COOH]}{[CH_3COO^-]} [/itex]

However I have no idea how to form the expression at the beginning and before the EQ. The pH has to, in my opinion, gradually decrease, so the concentrations of oxonium ions have to be lower, thus something additional must be in the denominator. I'm really lost here.

[Borek]

Do you know how to calculate titration curve for titration of a weak base?

[Anthasci]

I know how to perform a titration curve for titration of a weak acid with a strong base, but I can't imagine the reverse being much different.

[Darren]

At the beginning of titration, before acid is added, you have the conjugate base. So you can find its pH by finding the K_b of the conjugate base. Multiply this by its concentration, and square root the whole value. This gives you the concentration for the hydroxide ions in solution since it is a base and you use the K_b value. Then pOH= -log(concentration of hydroxide ions). So from there you can find pH.

[Borek]

I know how to perform a titration curve for titration of a weak acid with a strong base, but I can't imagine the reverse being much different.

It is not much different. Do everything exactly the same way, just calculate pOH using Kb instead of calculating pH using Ka.

Multiply this by its concentration, and square root the whole value. This gives you the concentration for the hydroxide ions

Unless it doesn't. Do you know limitations of this method?

[Darren]

@borek
Limitations? What do you mean by limitations of that calculation method?

[Borek]

@borek
Limitations? What do you mean by limitations of that calculation method?

http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base

You don't have to follow the derivation, but scroll down to tables and try to understand what they say about the accuracy of calculations. You are trying to use equation 8.13.

[Darren]

@borek
Limitations? What do you mean by limitations of that calculation method?

http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base

You don't have to follow the derivation, but scroll down to tables and try to understand what they say about the accuracy of calculations. You are trying to use equation 8.13.

Ooh so thats what you were referring to. Yup ive learnt about that approximation thing so its not perfectly accurate in calculations. But its roughly an estimate since the salt used is the conjugate base of its acid and has a low dissociation.