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Topic: pH....  (Read 4954 times)

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kct

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pH....
« on: May 23, 2005, 11:48:03 AM »
I've been posting a couple of times this week because I have a test coming up! Uck. :'( . I have a question about pH's now...

This one question asks to solve for the pH of the buffer. I did it and got the correct answer. The next part said that  HCL is added to this buffer (the one I just solved) what will be the pH now? I'm getting confused here. Where should I start? With the original problem or the finished equation after equilibrium?

Maybe writing out the entire problem may help:

A)Determine the pH of a buffer made by mixing .20 mol of Na2HPO4 and .10 mol of NaOH and raising the volume of the osln to 1.0L.
B)What will be the pH if 10 ml of .20 M HCL is added to 100.ml of the uffer in Part A?

Thank you!

Offline Borek

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Re:pH....
« Reply #1 on: May 23, 2005, 11:58:24 AM »
Treat it as if the HCl reacted with phosphoric acid protonating it (it is a conjugated base of weak acid after all). Use simple stocihiometry to find out new amounts of all forms and do buffer calculations once again.
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kct

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Re:pH....
« Reply #2 on: May 23, 2005, 12:04:22 PM »
got it! Thanks! Just one more!!

pH of .20 NH4NO3 . I just have trouble writing out the chemical equation.

In water I know that NH4 goes to NH3 + H2O ... so what now? I just need help with the equation. I think I know how to do the calculations... THanks again

Offline Borek

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Re:pH....
« Reply #3 on: May 23, 2005, 12:53:11 PM »
In this case NH4(+) is a weak acid:

NH4(+) = NH3 + H+

If you have pKb only, remember that pKa + pKb = pKw
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kct

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Re:pH....
« Reply #4 on: May 23, 2005, 01:18:31 PM »
I thought that NO3, being the conjugate base of a strong acid, would do something to the pH. But it doesn't?

Offline Borek

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Re:pH....
« Reply #5 on: May 23, 2005, 01:52:57 PM »
In salt solution both cation and anion - both being conjugates of something - hydrolise. However, the weaker acid/base the stronger hydrolysis. Nitric acid is much stronger acid than ammonia is a base (nitric acid pKa = -1, ammonia pKb = 4.75) so its hydrolysis is neglectable.
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