How can the titration curve be used to determine the strength of an acid?

[Physics197]

Homework Statement

How can you determine if an acid is strong or weak given the titration curve? I need two reasons.

Homework Equations

The Attempt at a Solution

1 Reason that it is weak is that its being titrated by a strong base and the equivalence point is above pH of 7.

Need a 2nd

 

[Borek]

Take two titration curves and compare them.

You may try (there is a free trial).

 

[Physics197]

Not sure what the acid is, but through a tritaion calculation I found the concentration of the acid to be 0.2083M.

That may not be correct..

Heres the data:
NaOH: 0.125M
25mL
acid: 15mL
unknown concentration

But it is a monoprotic acid

 

[symbolipoint]

Depending on concentrations used, pH will change at a very high rate through pH 7 through the equivalence point. A weak acid will further show fairly small change in pH as the titrant is added over a "long" range of titrant addition until close to the equivalence point. Equivalence point should happen at about pH 8 or greater for a weak acid.

 

[Physics197]

Yes, it happens at pH = 8.7

It just occurred to me that weak acids do not 100% dissociate

So could I use the pH of 8.7 to determine the concentration of H+ and compare that number to the concentration of the acid, and if they are different, that means it wasnt 100% dissociated?

HX ---------> H+ + X-
0.2M -------> 2x10^-9M

[H+] = 10^-pH = 2x10^-9

 

[Borek]

At equivalence point there is (almost) no acid, there is (almost) only its salt.

 

[Physics197]

Well what's another way I could show that this acid is weak?

 

[symbolipoint]

Well what's another way I could show that this acid is weak?

You just did, according to some of your description in post #5.

Was the original acid available in relatively pure, dry form? Even a pH measurement before titrating might not be strong enough information compared to performing a titration. As long as you can refer to your titration curve, you can easily enough judge whether the acid is weak or strong. Note, that a strong acid will not present pH buffering as you begin and continue to approach the endpoint with titrant additions; and the equivalence point pH should be sudden and extremely close to 7.