Titration Gen chem TBR Confusion

[Dochopeful13]

Hi All,

In the TBR Gen Book it says that in a weak acid strong base titration, the weaker an acid is the stronger the conjugate base it forms, which in turn means the higher the PH. This makes sense to me. However, at the end of the chapter the book mentions that the stronger the acid the more (stronger) base you will need which means a higher equivalent point. Is this true? To me both of these statements are saying the opposite. Can someone please clear this up for me?

Thank you!

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[desperadoflakes]

If you dissolve HCl into solution, it will readily dissociate into H+ and Cl-. The increase in [H+] means a decrease in the pH. It is the amount of this dissociation that defines the strength of an acid. We call HCl a strong acid and acetic acid a weak acid because if we dissolve 1M of both of them into separate solutions of pure water, HCl will dissociate into much more H+ (and thus lower the pH) of its solution far more significantly than acetic acid would. We say that strong acids dissociate completely, while weak acids dissociate a certain degree as determined by their Ka.

The stronger the acid, the stronger the increase in [H+] once the acid is dissolved. In order to neutralize all of that H+, we need a lot of OH-. Just like with acids and H+, stronger bases dissociate more readily into OH-. So, in order to neutralize a strong acid that will cause a large increase in [H+], we need to add a strong base which will cause a large increase in [OH-].

 

[NextStepTutor_3]

Hi @Dochopeful13 ! Good questions - acid-base chem can be tricky.

Your first statement is correct here. The weaker the acid, the stronger the conjugate base. Consider a case where HF (a weak acid) is being titrated by NaOH (a strong base). Once we reach the equivalence point, all of the H+ has been neutralized by OH-, so we'll be left with F-, Na+, and H2O. Since F- is the conjugate of a weak acid, it is a fairly decent base (we won't say it's a strong base, because that only applies to select bases - but it is certainly basic). As a result, the solution present at our equivalence point will be basic, with pH > 7.

Regarding your second statement, are you sure that's exactly what the book says? Titrating a strong acid does not require more base than titrating the same amount of weak acid, nor does it require a larger amount of base. For example:

Titrating 1L of 1M HCl (a strong acid) requires 1 mol of OH-. Titrating 1L of 1M HCN (a weak acid) also requires 1 mol of OH-.

HCl + NaOH --> NaCl + H2O
The above reaction involves titrating HCl with a strong base. The stoichiometric ratio is 1:1, so if we have 1 mol of HCl, we'll need 1 mol of NaOH to reach the equivalence point.

HCN + NaOH --> NaCN + H2O
This reaction also has a 1:1 ratio. So if we started with 1 mol of HCN, we'll need 1 mol of NaOH to reach the equivalence point.

See how we actually didn't need any more base for the first scenario, even though the acid was much stronger? The only real difference is that the first reaction produces a neutral solution (pH = 7), while the second produces a basic solution, since CN- is the conjugate of a weak acid and thus basic. This is what determines our pH at the equivalence point of a titration.

Maybe the chapter said that the more acid we have, the more base we need? That's certainly true, but it also wouldn't impact the equivalence point. Titrating 1 mol of H+ ions with NaOH will have the same equivalence point as titrating 10 mol of H+ ions with NaOH.

Hope you find this helpful! Good luck

 

[Dochopeful13]

Hi @Dochopeful13 ! Good questions - acid-base chem can be tricky.

Your first statement is correct here. The weaker the acid, the stronger the conjugate base. Consider a case where HF (a weak acid) is being titrated by NaOH (a strong base). Once we reach the equivalence point, all of the H+ has been neutralized by OH-, so we'll be left with F-, Na+, and H2O. Since F- is the conjugate of a weak acid, it is a fairly decent base (we won't say it's a strong base, because that only applies to select bases - but it is certainly basic). As a result, the solution present at our equivalence point will be basic, with pH > 7.

Regarding your second statement, are you sure that's exactly what the book says? Titrating a strong acid does not require more base than titrating the same amount of weak acid, nor does it require a larger amount of base. For example:

Titrating 1L of 1M HCl (a strong acid) requires 1 mol of OH-. Titrating 1L of 1M HCN (a weak acid) also requires 1 mol of OH-.

HCl + NaOH --> NaCl + H2O
The above reaction involves titrating HCl with a strong base. The stoichiometric ratio is 1:1, so if we have 1 mol of HCl, we'll need 1 mol of NaOH to reach the equivalence point.

HCN + NaOH --> NaCN + H2O
This reaction also has a 1:1 ratio. So if we started with 1 mol of HCN, we'll need 1 mol of NaOH to reach the equivalence point.

See how we actually didn't need any more base for the first scenario, even though the acid was much stronger? The only real difference is that the first reaction produces a neutral solution (pH = 7), while the second produces a basic solution, since CN- is the conjugate of a weak acid and thus basic. This is what determines our pH at the equivalence point of a titration.

Maybe the chapter said that the more acid we have, the more base we need? That's certainly true, but it also wouldn't impact the equivalence point. Titrating 1 mol of H+ ions with NaOH will have the same equivalence point as titrating 10 mol of H+ ions with NaOH.

Hope you find this helpful! Good luck

Thank you very much for your help. Please see attached for what the book says.I am reading this statement as the higher the initial acid the higher base equlivant you need. Can you please tell me the flaw in my logic? Titrations are not my strongest point.

Thank you!

 

[NextStepTutor_3]

You're very welcome! Yes, titrations can be super confusing.

The book is just saying that the higher the concentration of initial acid, the higher the concentration of its conjugate base at the equivalence point, so the more basic the equivalence point will be. This is what it's referring to with that "...because higher [HOAc]initial..." part.

This is certainly true! But all this means is that (for example) if you start with a 10 M solution of HF in 1 L and you titrate it with NaOH, your solution at the equivalence point will now have 10 M of F-, since our ratio is 1:1 according to the equation below:

HF + NaOH ---> H2O + Na+ + F-

A solution of 10 M F- will naturally be more basic than, say, a solution of 1 M F- (which we'd get if we were titrating only 1 M of HF in 1 L. The takeaway is the larger your initial concentration of weak acid, the higher (or more basic) your equivalence point will be, because you'll have a large concentration of conjugate base in solution.

In turn, the weaker your initial weak acid, the higher (or more basic) your equivalence point will be, because the conjugate base of that weak acid is going to be pretty strong. These two statements are not contradictory! They're just two things that can lead to a high equivalence point pH for the titration of a weak acid. If both were true - if we titrated a really large concentration of a really weak acid - the equivalance point would just be really basic.

What is not true is the statement that "the stronger the acid, the more base we need." Don't confuse strength with concentration! We need more base if we have a higher initial concentration of acid, but this isn't related to the strength of the acid. When trying to predict how much base we need to perform a titration, just think about the stoichiometry of the reaction (even write it out if you need to) and you should be fine.

Let me know if any of this is unclear