# Strong or weak acid?

1. Jan 23, 2009

### Physics197

1. The problem statement, all variables and given/known data

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

2. Relevant equations

3. 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

Last edited: Jan 23, 2009
2. Jan 23, 2009

### Staff: Mentor

Take two titration curves and compare them.

You may try (there is a free trial).

3. Jan 23, 2009

### 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

4. Jan 23, 2009

### 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.

5. Jan 23, 2009

### Physics197

Yes, it happens at pH = 8.7

It just occured 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

6. Jan 23, 2009

### Staff: Mentor

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

7. Jan 23, 2009

### Physics197

Well whats another way I could show that this acid is weak?

8. Jan 23, 2009

### symbolipoint

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.