acid/base chem question...help!

[uclakid]

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Ok, my biochem TA asked what the pH of 10^-8 molar HCl solution would be. Some people answered 8. That's incorrect because a pH greater than 7 is basic...and HCl is a strong acid. So, he said that you have to take into account the ionization of water (which has a 10^-7 H+ concentration). So, I don't understand why you must take water into account when you have a strong acid..the strong acid completely dissociates and your H concnetration should be equal to the initial concentration of the HCl.

Ok, second question: Do you take ionization of water into account for WEAK acids??? why/why not?

third: why can't you use henderson-hasselbalch equation for strong acids?

thanks everyone!

 

[Street Philosopher]

1. B/c the acid contribution by water is greater than by the acid, regardless of whether it is a strong acid or not.
2. Yes if the weak acid concentration is low enough. I don't remember the rule of thumb but it wouldn't hurt to include the water's contribution each time.
3. I don't remember.

 

[Nutmeg]

Since the pH is the negative log of the H+ conc, I would guess you'd have to add the H+ from the dissociation of HCl and that from the dissociation of water. So it should be the negative log of 1.1*10^-7, or about 6.96. Basically, the conc is too low to have a large impact on the pH.

For number two, I would guess no, because water is by itself a very weak acid. Thus, water and the weak acid would both be in equilibrium with the H3O+ concentration. Besides, the H-H eqn doesn't acount for it, so I think that means my guess is likely right.

Finally, you can't use the H-H eqn for strong acid because if you assume they ionize completely, then the [HA] term in the denominator is zero, which would be undefined.

My guesses are for novelty purposes only, and I cannot be held accountable for it's content. 😉 But I think that's right.

 

[babinski bob]

Nutmeg is correct about #3. By definition, a strong acid is one which completely dissociates when put into solution. The Henderson-Hasselbach assumes a distribution between an acid and it's conjugate base (or vice versa) which will only occur with weak acids. The def of a weak acid is one which partially dissociates when put into solution which means there will exist an equilibrium between protonated/unprotonated form...

 

[uclakid]

bump

 

[coldchemist]

You do have to take the ionization into account for weak acids and bases. Why? Because, weak acids are going to ionize, but to a much lesser extent (95% less) than strong acids. Depending on the concentration of the acid, the amount of dissociating H+ may be comparable to the amount attributed to the autoionization of water.

 

[uclakid]

bump

 

[CD]

Originally posted by uclakid
Ok, my biochem TA asked what the pH of 10^-8 molar HCl solution would be. Some people answered 8. That's incorrect because a pH greater than 7 is basic...and HCl is a strong acid. So, he said that you have to take into account the ionization of water (which has a 10^-7 H+ concentration). So, I don't understand why you must take water into account when you have a strong acid..the strong acid completely dissociates and your H concnetration should be equal to the initial concentration of the HCl.

Ok, second question: Do you take ionization of water into account for WEAK acids??? why/why not?

third: why can't you use henderson-hasselbalch equation for strong acids?

thanks everyone!

Okay....first, the influence of H2O only needs to be considered when the influence of the acid is within 100*the ionization of water....or inother words anytime the acids contribution is 10^-5 or LESS. This is independent of the strong/weak acid argument.

Second....see above. If the influence of the acid is 10^-5 or less then the autoionization of water must be considered.

Third....the henderson-hasselbach equation ONLY applies to BUFFER systems. In fact, when the concentration of acid to base is between 10 acid/100base to 100base/10acid the henderson-hasselbach equation may be used. BUT when the ratio falls outside the 10%-90% mark you no longer have a buffer system and you CANNOT use the henderson-hasselback equation but must instead use the ICE scheme and pKa's. Because of this, you cannot use the H-H equation on strong acids!!! Strong acids completely dissociate so there is no acid present (only H+ and base) and you are not within the 10%-90% range and you don't have a buffer!!!!!!!!

hope that helps!

 

[Cerberus]

idontremember🙁

 

[edinOH]

I wish I remembered this stuff.

 

[91Bravo]

So is your TA hot? 😍

 

[CD]

Originally posted by 91Bravo
So is your TA hot? 😍

I "AM" the T.A.!! 😉