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Topic: pH of Solutions  (Read 6161 times)

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kissoftalons

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pH of Solutions
« on: August 29, 2005, 03:53:56 PM »
Sorry for the numerous questions...just trying to get through the AP package. I appreciate all your *delete me*!

The two questions are as follows. Calculate the pH of the following solutions.
a) 0.025mol/L of HCL (aq)
b) 0.025 mol/L of H2SO4 (aq)

From my calculations I've figured out that for a) the pH is about 1.6
I was wondering if the pH for b) was 1.6 also, or do you have to take into account the pH of the acid (H2SO4) or the percentage of dissociation. And if there is, is there any way to figure out the percentage of dissociation? (or just have to commit it to memory?)

Thank you!
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Blueshawk

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Re:pH of Solutions
« Reply #1 on: August 29, 2005, 04:27:08 PM »
Quote from: kissoftalons on August 29, 2005, 03:53:56 PM
Sorry for the numerous questions...just trying to get through the AP package. I appreciate all your *delete me*!

The two questions are as follows. Calculate the pH of the following solutions.
a) 0.025mol/L of HCL (aq)
b) 0.025 mol/L of H2SO4 (aq)

From my calculations I've figured out that for a) the pH is about 1.6
I was wondering if the pH for b) was 1.6 also, or do you have to take into account the pH of the acid (H2SO4) or the percentage of dissociation. And if there is, is there any way to figure out the percentage of dissociation? (or just have to commit it to memory?)

Thank you!


I dont have the necessery numbers with me to do the problem, but assuming for a), that 1.6 is right....
I would have to guess that H2SO4 is not as strong a pH or an acid as HCl, though it is a strong acid.

Also H2SO4 is diprotic so it technically has 2 steps....

H2SO4 -> H+   +   HSO4-

HSO4- ->  H+   +  SO4-2  , but this step does not effect the pH to a considerable amount so can most often be skipped.
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Blueshawk

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Re:pH of Solutions
« Reply #2 on: August 29, 2005, 04:31:45 PM »
if you want a complete write out for the pH calculation for  H2SO4  let me know and i'll see what I can do.
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Offline Borek

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Re:pH of Solutions
« Reply #3 on: August 29, 2005, 04:39:32 PM »
Quote from: kissoftalons on August 29, 2005, 03:53:56 PM
b) 0.025 mol/L of H2SO4 (aq)

http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-simplified

Problem is, one never know what to do, as the correct answer often dpeneds on the teacher. Some want you to use just first dissociation step, others want you to assume both protons are fully dissociated, others want you to give accurate answer using right Ka values :(
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Offline Borek

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Re:pH of Solutions
« Reply #4 on: August 29, 2005, 04:58:56 PM »
Quote from: kissoftalons on August 29, 2005, 03:53:56 PM
b) 0.025 mol/L of H2SO4 (aq)

From my calculations I've figured out that for a) the pH is about 1.6
I was wondering if the pH for b) was 1.6 also, or do you have to take into account the pH of the acid (H2SO4) or the percentage of dissociation. And if there is, is there any way to figure out the percentage of dissociation? (or just have to commit it to memory?)

Oops, I haven't read your post precisely before.

Percentage of dissociation is of no use, as it changes with concentration. The anly thing that makes sense is a dissociation constant. Read introduction in my lectures.
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two39plutonium

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Re:pH of Solutions
« Reply #5 on: September 02, 2005, 03:15:37 AM »
Ahhh!  normality rears its ugly head in this problem!
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