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# Why would the pH of HCl be virtually the same as the pH of H2SO4? watch

1. The longer question...

'Why would the pH of 1 moldm-3 of HCl be virtually the same as the pH of 1 moldm-3 of H2SO4?'

... despite the fact that H2SO4 can donate 2H+ 's?

Well thats a question I've been asked to ponder on and I can't find anything in my books to explain this! Help into why the statement it is true would be appreciated! Thanks x
2. Work out the PH's of both solutions.
3. i'm still confused as i still wouldn't have answered the question
4. Is that the only info you're given
5. yes! um i think its to make me think about theory rather than calcualte a calculation if that helps in any way ... i'm stuck and confused myself!
6. (Original post by adellie)
The longer question...

'Why would the pH of 1 moldm-3 of HCl be virtually the same as the pH of 1 moldm-3 of H2SO4?'

... despite the fact that H2SO4 can donate 2H+ 's?

Well thats a question I've been asked to ponder on and I can't find anything in my books to explain this! Help into why the statement it is true would be appreciated! Thanks x
This is probably wrong (until and unless somebody vouches for it but) I think HCl is a strong acid, dissociating fully into H+ ions, whereas H2SO4 is a weak acid, dissociating only partially into H+ ions; therefore the pH is almost the same.

Thanks.
7. Assuming that HCl and H2S04 both completely disociate, the respective H+ concentrations will be 1moldm-3 and 2(1moldm-3). So the pH of HCl will be -log10(1) = 0 and the pH of H2S04 will be -log10(2) = 0.01. It's saying because the pH values are so close they may as well be the same.
8. (Original post by DJkG.1)
This is probably wrong (until and unless somebody vouches for it but) I think HCl is a strong acid, dissociating fully into H+ ions, whereas H2SO4 is a weak acid, dissociating only partially into H+ ions; therefore the pH is almost the same.

Thanks.
No mate, sorry to tell anyone they're wrong. but you are here

But both HCl and H2SO4 are strong acids.... thats why OP and I are kinda confused, lol

Jez
9. (Original post by joelmd)
Assuming that HCl and H2S04 both completely disociate, the respective H+ concentrations will be 1moldm-3 and 2(1moldm-3). So the pH of HCl will be -log10(1) = 0 and the pH of H2S04 will be -log10(2) = 0.01. It's saying because the pH values are so close they may as well be the same.
Thats what i was thinking...... but if you say it too.. then thats another vouch.. so im happy with that

So yeh

pH of HCl = 0
pH og H2SO4 = -0.3
So they're roughly the same
10. thanks, although i made the assumation that they both fully disociate.
11. The second dissociation of sulphuric acid (i.e. the dissociation of a proton from hydrogensulphate ion) is far from complete. Only around 10% of those protons dissociate.
12. the pH will still be -log10(1.1) = 0.079 which is still close to the pH of HCl
13. ... so is the answer that becuase the second H+ ino from H2SO4 will not easily dissociate so its like they both only donate a single H+

and the pH values calculated from that would be similar???
14. (Original post by adellie)
... so is the answer that becuase the second H+ ino from H2SO4 will not easily dissociate so its like they both only donate a single H+

and the pH values calculated from that would be similar???
Yeah, that's it really. The same effect holds for, say, phosphoric acid. The first dissociation is (more-or-less) complete, the second less so, and the third dissociation is pretty feeble. If you want data, the pKa data is available on Wikipedia, on the phosphoric acid entry, which also has a handy explanation of this
15. (Original post by cpchem)
Yeah, that's it really. The same effect holds for, say, phosphoric acid. The first dissociation is (more-or-less) complete, the second less so, and the third dissociation is pretty feeble. If you want data, the pKa data is available on Wikipedia, on the phosphoric acid entry, which also has a handy explanation of this
Thank you and to everyone else that replied!

However, I was wondering, why is the second dissociation for sulhpuric less complete (...if that's a phrase)?
16. (Original post by adellie)
Thank you and to everyone else that replied!

However, I was wondering, why is the second dissociation for sulhpuric less complete (...if that's a phrase)?
In the second dissociation: HSO4- ---> SO42- + H+

We are removing a positive charge from an already negatively charged species, so it has electrostatic attraction working against it.
17. disassociation in HCl is much stronger (i.e more ionising) than that of H2SO4, even though it is diprotic.

This is why we say HCl is a stronger acid than H2S04, it is fully ionising unlike H2S04

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