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Topic: pH of HCl and H2SO4  (Read 40786 times)

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Offline steph_r

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pH of HCl and H2SO4
« on: August 19, 2008, 03:40:24 AM »
Hi everyone,
I was having a look at this question in my chem book and found it a little confusing..

Question: Explain why the pH of a 1.0M solution of Hydrochloric acid is greater than the pH of a 1.0M solution of sulfuric acid

My attempt: I would have thought that HCl acid is a stronger acid and therefore, would have a smaller pH (more acidic) than sulfuric acid  ???..

Help woudl be greatly appreciated ..

Offline Astrokel

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Re: pH of HCl and H2SO4
« Reply #1 on: August 19, 2008, 04:05:07 AM »
hey steph_r!

nono, why do you think HCl is a stronger acid? This question can actually be done intuitively, since the concentration of the acids are the same. Otherwise, you can calculate the concentration of H+ ions, thus the pH.
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

Offline steph_r

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Re: pH of HCl and H2SO4
« Reply #2 on: August 19, 2008, 04:15:40 AM »
Yes, i see where you're coming from, but how would you actually explain it? How do you explain that HCl has a greater pH than sulfuric acid?

Offline sameeralord

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Re: pH of HCl and H2SO4
« Reply #3 on: August 19, 2008, 04:21:01 AM »
Yes, i see where you're coming from, but how would you actually explain it? How do you explain that HCl has a greater pH than sulfuric acid?

A strong acid ionises more fully. So Ka value would be higher as more H30+ is produced. Look at the Ka tables for acids if you don't want to use formulas to work it out.

Offline Astrokel

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Re: pH of HCl and H2SO4
« Reply #4 on: August 19, 2008, 04:23:09 AM »
hey

can you show that 1.0M of HCl is less acidic than 1.0M of H2SO4? Therefore concluding, HCl has higher pH since it is less acidic.
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

Offline yukiheart

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Re: pH of HCl and H2SO4
« Reply #5 on: August 19, 2008, 12:26:03 PM »
hello all!!!

HCl ---> H++ Cl-
1,0         1,0M

but
H2SO4 ------> 2H++ SO42-
1,0                    2.0M

then pH[HCl] > pH[H2SO4]
note
HSO4- -------> H+ + SO42-  not all 100%

Offline MikeS

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Re: pH of HCl and H2SO4
« Reply #6 on: August 19, 2008, 10:50:27 PM »
Yuki just answered the question perfectly. All I can add is that the reason why H2SO4 can almost ionize both of its protons is because of the stability of the HSO4- and the SO4 2- ion due to resonance.

Offline AWK

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Re: pH of HCl and H2SO4
« Reply #7 on: August 22, 2008, 06:45:27 AM »
Yuki just answered the question perfectly. All I can add is that the reason why H2SO4 can almost ionize both of its protons is because of the stability of the HSO4- and the SO4 2- ion due to resonance.
H2SO4 ionizes its second proton in about 70 %.
AWK

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