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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: steph_r on August 19, 2008, 03:40:24 AM

Title: pH of HCl and H2SO4
Post by: steph_r 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 ..
Title: Re: pH of HCl and H2SO4
Post by: Astrokel 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.
Title: Re: pH of HCl and H2SO4
Post by: steph_r 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?
Title: Re: pH of HCl and H2SO4
Post by: sameeralord on August 19, 2008, 04:21:01 AM
Quote from: steph_r 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?

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.
Title: Re: pH of HCl and H2SO4
Post by: Astrokel 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.
Title: Re: pH of HCl and H2SO4
Post by: yukiheart 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%
Title: Re: pH of HCl and H2SO4
Post by: MikeS 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.
Title: Re: pH of HCl and H2SO4
Post by: AWK on August 22, 2008, 06:45:27 AM
Quote from: MikeS 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.
H2SO4 ionizes its second proton in about 70 %.