Acid-Base Question: Equivalence Point

[churroes]

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The equation for the equivalence point is given as:

NaVa = NbVb

Where N is the normality and V is the volume.

My question is: why doesn't the strengths/identities of the respective acids or bases matter when using this equation? Wouldn't weak acids/bases only PARTIALLY dissociate?

Thank you.

 

[sillyjoe]

The equation for the equivalence point is given as:

NaVa = NbVb

Where N is the normality and V is the volume.

My question is: why doesn't the strengths/identities of the respective acids or bases matter when using this equation? Wouldn't weak acids/bases only PARTIALLY dissociate?

Thank you.

I have never seen this equation for equivalence point. Where did you get this? What is it solving for?

 

[churroes]

MaVa = MbVb

^ That would be the more common way of writing it out. The moles of the acid and base would equal each other at the equiv point.


The only explanation I can think of is to assume that even if the acid/base is weak, it is still completely dissociating. Is this a correct assumption???

 

[sillyjoe]

MaVa = MbVb

^ That would be the more common way of writing it out. The moles of the acid and base would equal each other at the equiv point.


The only explanation I can think of is to assume that even if the acid/base is weak, it is still completely dissociating. Is this a correct assumption???

When you are performing a titration you use a strong titrant so every equivalent of titrant removes a proton (in the case of a base) or adds a proton (in the case of an acid). This will happen regardless of the strength of the analyte.

 

[churroes]

That makes a lot of sense. So in essence the strong titrant is "forcing" the weaker analyte to dissociate in order to neutralize it?

 

[sillyjoe]

That makes a lot of sense. So in essence the strong titrant is "forcing" the weaker analyte to dissociate in order to neutralize it?

Think of it like this:

If the titrant is a base the negative charge will rip a proton H+ off the weak acid.

If it is an acid it forces it's proton onto the weak base.

 

[churroes]

Then what about normalities of weak bases/acids? (separate question, outside of titrations)

Ex: Is 1M acetic acid equal to 1N acetic acid, considering that the percentage of H+ and acetate ion formed is quite small (very small Ka)?

 

[sillyjoe]

Then what about normalities of weak bases/acids? (separate question, outside of titrations)

Ex: Is 1M acetic acid equal to 1N acetic acid, considering that the percentage of H+ and acetate ion formed is quite small (very small Ka)?

Normality is just a way of normalizing equivalents necessary due to polyprotic acids/bases

If we talk about HCl, it has one proton to give. If I have 10 ml .1 molar HCl (.1 mole HCl/liter) I would need .1 molar of 10 mL NaOH to neutralize it.

HCL is 1M but is also considered 1 N because you only need 1 equivalent of base to neutralize it.

Now if you do the same titration but with .1 M H2SO4 which has 2 protons to give (diprotic) you would need 20mL of .1 NaOH or two equivalents of NaOH per 1 equivalent H2SO4

In this case H2SO4 is .1M but .2N because you need 2 equivalents of base to neutralize.

The same can be said about .1 M H3PO4 which would be .3 N because it is triprotic and needs 3 equivalents of base to neutralize it.

 

[churroes]

I was more so wondering about the difference of normality between strong and weak substances:

Would the normality of 1L of 1M HCl be equal to the normality of 1L of 1M CH3COOH?

I ask this because I am concerned about how the incomplete dissociation of CH3COOH (as opposed to HCl's complete dissociation into H+ and Cl-) will affect normality values.

Thank you!

 

[sillyjoe]

I was more so wondering about the difference of normality between strong and weak substances:

Would the normality of 1L of 1M HCl be equal to the normality of 1L of 1M CH3COOH?

I ask this because I am concerned about how the incomplete dissociation of CH3COOH (as opposed to HCl's complete dissociation into H+ and Cl-) will affect normality values.

Thank you!

Molarity and normality have nothing to do with strength. It has to do with moles of protons it carries. Or said differently, the ratio of protons to molecules in the acid.

 

[churroes]

Sorry, I don't think I'm getting it.

Won't you have a larger concentration of H+ from the dissociation of HCl when compared to the H+ concentration from CH3COOH? This is just from the respective Ka's of the two acids.

In this case, 1M of HCl would yield 1M H+, so wouldn't 1M CH3COOH yield less than 1M H+?

Doesn't that mean CH3COOH produces less equivalents of H+ ion?

 

[sillyjoe]

Doesn't that mean CH3COOH produces less equivalents of H+ ion?

Don't worry about it nothing to apologize for.

I think you are looking at it from the wrong perspective. It is not about how many protons will dissociate in neutral aqueous solution. That is pKa/pH stuff. Remember, this terminology was generally developed for titrations. Again, the titrant is strong and will cause the proton to dissociate whether weak or strong.

If you are using 1 equivalent of H2CO3 (weak acid) you will need 2 equivalents of NaOH (strong base).

Therefore, H2CO3 = 2N

If you are using 1 equivalent H2SO4 (strong acid) you will need 2 equivalents of NaOH (strong base)

Therefore, H2SO4 = 2N

You are looking at the perspective of how many protons it can theoretically add to solution. Not whether it will dissociate in neutral water. It has nothing to do with strength.

 

[churroes]

Don't worry about it nothing to apologize for.

I think you are looking at it from the wrong perspective. It is not about how many protons will dissociate in neutral aqueous solution. That is pKa/pH stuff. Remember, this terminology was generally developed for titrations. Again, the titrant is strong and will cause the proton to dissociate whether weak or strong.

If you are using 1 equivalent of H2CO3 (weak acid) you will need 2 equivalents of NaOH (strong base).

Therefore, H2CO3 = 2N

If you are using 1 equivalent H2SO4 (strong acid) you will need 2 equivalents of NaOH (strong base)

Therefore, H2SO4 = 2N

You are looking at the perspective of how many protons it can theoretically add to solution. Not whether it will dissociate in neutral water. It has nothing to do with strength.

I think this clinched it for me. So you are just simply looking for the total possible amount of H+/OH- it has, that is able to react with the titrant?

Thank you so much!!