1. ## H2so4

When calculating the pH of H2SO4 do we multiply the concentration by 2 i.e. -log[2H+]
This means it is stronger than HCL so is that always true?
What about H3PO4 and other polyprotics? or since they are weak we don't multiply the concentration by anything?

2. ## Re: H2so4

yes, when you're finding the concentration of hydrogen ions, you multiply the concentration of sulfuric acid by 2, then use -log[H+] to get the pH. however, remember that it's not -log[2H+], it's just -log[H+], but [H+] = 2[H2SO4]
and also, yes it will mean that sulfuric acid has a lower pH than HCl if the concentrations of HCl and H2SO4 are the same, so sulfuric acid would have a greater concentration of hydrogen ions, yet i'm not sure if you could say that it was stronger than HCl, since strength is based on ionisation levels, and both ionise completely (only first ionisation for sulfuric acid though, second ionisation isn't a complete ionisation)
the same goes with H3PO4, except it is triprotic, so you multiply by 3 instead

3. ## Re: H2so4

~Technically~ you do that with some questions BUT since the second proton only partially disassociates it's not really correct so you should proba write that in or something.

4. ## Re: H2so4

Originally Posted by Aesytic
yes, when you're finding the concentration of hydrogen ions, you multiply the concentration of sulfuric acid by 2, then use -log[H+] to get the pH. however, remember that it's not -log[2H+], it's just -log[H+], but [H+] = 2[H2SO4]
and also, yes it will mean that sulfuric acid has a higher pH than HCl if the concentrations of HCl and H2SO4 are the same, but remember that a higher pH means less acidic, which does not mean that sulfuric acid is stronger than HCl

the same goes with H3PO4, except it is triprotic, so you multiply by 3 instead
When you multiply by 2, it decreases the pH, making it more acidic not less. Think about it like this, you now have more h+ ions in the same space.

5. ## Re: H2so4

oh yeah, sorry about that, don't really know what i was thinking...

6. ## Re: H2so4

Originally Posted by Aesytic
yes, when you're finding the concentration of hydrogen ions, you multiply the concentration of sulfuric acid by 2, then use -log[H+] to get the pH. however, remember that it's not -log[2H+], it's just -log[H+], but [H+] = 2[H2SO4]
and also, yes it will mean that sulfuric acid has a higher pH than HCl if the concentrations of HCl and H2SO4 are the same, but remember that a higher pH means less acidic, which does not mean that sulfuric acid is stronger than HCl
the same goes with H3PO4, except it is triprotic, so you multiply by 3 instead
We just multiply [H2SO4] by 2 and then use -log[H+] to find the pH of the solution but that gives a lower pH meaning H2SO4 is more acidic. I just tried it on my calculator, type -log(0.001)=3 but if you type -log(2x0.001)=2.699 which means H2SO4 is stronger.

Also, in the 2008 (I think) JRAHS paper there is a multiple choise questions asking if you have equal concentrations of 0.001 of HCL, H2SO4, CH3COOH AND H3PO4 which one has the lowest pH and the answer in H2SO4.

Also, sometimes, H2SO4 isn't completely ionised in the second ionisation so we assume that it is, but I'm not sure about that/we don't need to know that?!?!?

7. ## Re: H2so4

In HSC Sulphuric Acid is assumed complete ionisation, unless otherwise stated in which case they have to give you a percentage of ionisation or unionised

8. ## Re: H2so4

Originally Posted by The Matrix
H2SO4 is stronger.
This is wrong, strength is the degree of ionisation which is pretty much what percentage of the molecules become ionised in solution at 1M.

To determine strength, you need to look at the dissociation constant, for HCl, it's -8.0 and for H2SO4, it's -3.0.
This makes HCl stronger.

9. ## Re: H2so4

Originally Posted by golgo13
In HSC Sulphuric Acid is assumed complete ionisation, unless otherwise stated in which case they have to give you a percentage of ionisation or unionised
Yes.
You assume it is completely ionised in both first and second ionisations which makes it stronger than HCl, which I think is wrong...
If they give you concentrations you can do [H+]/[Solution]x100=DOI...

10. ## Re: H2so4

Originally Posted by The Matrix
Yes.
You assume it is completely ionised in both first and second ionisations which makes it stronger than HCl, which I think is wrong...
If they give you concentrations you can do [H+]/[Solution]x100=DOI...
You're mixing strength and concentration. Strength refers to the degree of ionisation, concentration refers to how many H+'s there are. In your example, both completely ionise so even though H2SO4 has a higher concentration of H+, you can't say its stronger.

11. ## Re: H2so4

Originally Posted by deswa1
You're mixing strength and concentration. Strength refers to the degree of ionisation, concentration refers to how many H+'s there are. In your example, both completely ionise so even though H2SO4 has a higher concentration of H+, you can't say its stronger.
LOL
We use concentration of [H+], divide it by the concentration of the whole solution i.e. [H2SO4] then multiply by 100 to find the degree of ionisation...

12. ## Re: H2so4

Originally Posted by The Matrix
LOL
We use concentration of [H+], divide it by the concentration of the whole solution i.e. [H2SO4] then multiply by 100 to find the degree of ionisation...
Yeah, I don't exactly get what you're saying. They both ionise 100% in water so they are both equalling strong in water. If you use a different solvent however such as acetic acid, you can notice that hydrochloric is actually STRONGER than sulfuric even though it has a lower H+ concentration and a higher pH. You can notice this because it has a lower dissociation constant (not in syllabus).

13. ## Re: H2so4

If both H2SO4 and HCl have a 100% degree of ionisation then their pH should be equal to each other. However, since H2SO4 undergoes a second ionisation, it adds to the first ionisation, increasing the concentration of H+ [H+] making it more acidic, however, the second ionisation is theoratically not complete since HSO4- is a weak acid so technically we shouldn't multiply by 2 we should calculate DOI of the second ionisation then multiply by 1.22, 1.16, 1.478 or what ever it is for example when calculating pH of H2SO4 but for simplicity sake, in the HSC we assume that the second ionisation is complete so we multiply by 2, but H2SO4 is stronger than HCl even if we make this assumption. It makes sense because first ionisation of H2SO4 is 100% which makes it equal to HCl in terms of strength but since second ionisation of H2SO4 increases the concentration of H+ ions in solution, it makes H2SO4 stronger/more acidic ... ?

14. ## Re: H2so4

Originally Posted by The Matrix
If both H2SO4 and HCl have a 100% degree of ionisation then their pH should be equal to each other. However, since H2SO4 undergoes a second ionisation, it adds to the first ionisation, increasing the concentration of H+ [H+] making it more acidic, however, the second ionisation is theoratically not complease since HSO4- is a weak acid so technically we shouldn't multiply by 2 we should calculate DOI of the second ionisation then multiply by 1.22 for example when calculating pH of H2SO4 but for simplicity sake, in the HSC we assume that the second ionisation is complete so we multiply by 2, but H2SO4 is stronger than HCl. It makes sense because first ionisation of H2SO4 is 100% which makes it equal to HCl in terms of strength but since second ionisation of H2SO4 adds increases the concentration of H+ ions in solution, it makes H2SO4 stronger/more acidic ... ?
This is wrong. If they both ionise completely, H2SO4 will have a lower pH because it is diprotic.

Yes technically you should adjust for the incomplete second ionisation but at HSC we don't. You can't however draw the link between pH and strength since one acid is monoprotic and one is diprotic. That bit where you say the second ionisation of H2SO4 increases the concentration of H+ ions in solution makes H2O4 stronger/more acidic is half right.

Yes it makes it more acidic and gives it a lower pH because the H+ concentration is higher. However it is not stronger because, like I just said, the extent of the ionisation is the same for both in water.

Go to a textbook and read the definition of strength in regards to acids.

15. ## Re: H2so4

deswa1 is correct.

16. ## Re: H2so4

Originally Posted by deswa1
This is wrong. If they both ionise completely, H2SO4 will have a lower pH because it is diprotic.

Yes technically you should adjust for the incomplete second ionisation but at HSC we don't. You can't however draw the link between pH and strength since one acid is monoprotic and one is diprotic. That bit where you say the second ionisation of H2SO4 increases the concentration of H+ ions in solution makes H2O4 stronger/more acidic is half right.

Yes it makes it more acidic and gives it a lower pH because the H+ concentration is higher. However it is not stronger because, like I just said, the extent of the ionisation is the same for both in water.

Go to a textbook and read the definition of strength in regards to acids.
H2SO4 is more acidic and has a lower pH but HCl is stronger. The End.

17. ## Re: H2so4

Originally Posted by The Matrix
H2SO4 is more acidic and has a lower pH but HCl is stronger. The End.
That's better.

18. ## Re: H2so4

Thanks to everyone who made a contribution to the clearance of this misconception that I had, it is not a significant one, but it is always good to clear things up!
I thought that strong always means lower pH, but I went through my amazing set of notes to revise this concept again and realised that strong doesn't always mean more acidic, i.e. H2SO4 has a lower pH only because the second ionisation increases [H+].
Hehe, I sound like a nerd talking like this =)

19. ## Re: H2so4

Originally Posted by Aesytic
yes, when you're finding the concentration of hydrogen ions, you multiply the concentration of sulfuric acid by 2, then use -log[H+] to get the pH. however, remember that it's not -log[2H+], it's just -log[H+], but [H+] = 2[H2SO4]
and also, yes it will mean that sulfuric acid has a lower pH than HCl if the concentrations of HCl and H2SO4 are the same, so sulfuric acid would have a greater concentration of hydrogen ions, yet i'm not sure if you could say that it was stronger than HCl, since strength is based on ionisation levels, and both ionise completely (only first ionisation for sulfuric acid though, second ionisation isn't a complete ionisation)
the same goes with H3PO4, except it is triprotic, so you multiply by 3 instead
sorry i dont understand, how can u calculate the pH of a weak acid? even if u kno the concentration u dont know what degree it ionise

20. ## Re: H2so4

Originally Posted by get_back23
sorry i dont understand, how can u calculate the pH of a weak acid? even if u kno the concentration u dont know what degree it ionise
They have to give you the DOI.

21. ## Re: H2so4

oh right, cool

22. ## Re: H2so4

Originally Posted by get_back23
oh right, cool
I mean give you enough information to calculate it.

23. ## Re: H2so4

I doubt they'd ever ask to calculate things like that (pH of weak acids).

24. ## Re: H2so4

It is usually multiple choice questions asking you which acid is has the lowest pH. For example, the JRAHS question would confuse many people since both HCl (stronger) and H2SO4 (more acidic) are in the options. I have never seen a questions asking you to calculate pH other than the simple -log[pH] or calculating the pH of the resultant solution...

25. ## Re: H2so4

Sometimes they also give you a table and ask you to analyse which one is strong/weak etc.

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