[Kate]

Hi.

So I have a bottle of 96% of sulfuric acid in the lab but it doesn't say if it's 96% (w/v) or 96% (v/v).

So my question is, which one is it?

It probably reflects the way it is manufactured.  By bubbling SO₃ in a solution of H₂SO₄.

[kriggy]

Its mass percent. 1 litre is about 1800g of 96% solution which means about 1700g of pure H₂SO₄.

[Arkcon]

It probably reflects the way it is manufactured.  By bubbling SO₃ in a solution of H₂SO₄.

Yep, that's how.  If you bubble SO₃ into water, you get an exothermic reaction, and a mist of sulfuric acid.  And that's hard to work with.  So they bubble it into H₂SO₄, to produce oleum: http://en.wikipedia.org/wiki/Oleum#Sulfuric_acid_production  And that is diluted to make sulfuric acid.

[Kate]

Its mass percent. 1 litre is about 1800g of 96% solution which means about 1700g of pure H₂SO₄.

Oh, w/w. Thanks.

Yep, that's how.  If you bubble SO₃ into water, you get an exothermic reaction, and a mist of sulfuric acid.  And that's hard to work with.  So they bubble it into H₂SO₄, to produce oleum: http://en.wikipedia.org/wiki/Oleum#Sulfuric_acid_production  And that is diluted to make sulfuric acid.

Thanks.

So the w/w % reflects the amount of SO3 dissolved in the H2SO4 solution?

[Arkcon]

No, it apprx. 96-98% sulfuric acid, the balance is water.  Sulfuric acid is very hygroscopic, so it can't be made, stored and shipped at 100% easily.  You'll have to take the small amount of water and other impurities into account, when you try to make a useful working dilution out of concentrate sulfuric.

[Zyklonb]

Well, yes. But the main reason it is rarely produced at 100% conc. is because it's physical and chemical  properties change very significantly as it becomes anhydrous.
 For example, it's viscosity increases dramatically, and it no longer ionizes the same way:
H₂SO₄ + H₂O  H₃O⁺ + HSO₄^-
Vs. Anhydrous:  H₂SO₄ H⁺ + HSO₄^-.
[Correction:]
2 H₂SO₄ H₃SO₄⁺ + HSO₄^-.

[kriggy]

I think you ment
2 H₂SO₄ H₃SO₄⁺ + HSO₄^-

there is not much difference between the two you wrote, you just didnt specify the proton acceptor in the second eq.

[Zyklonb]

Yes, you're right.
Although it's still practically the same. I wasn't sure if the proton would react with H₂SO₄ like it does with water to produce a hydronium ion.

[Borek]

I wasn't sure if the proton would react with H₂SO₄ like it does with water to produce a hydronium ion.

Quite eagerly:

2H₂SO₄ H₃SO₄⁺ + HSO₄^-

K=790

[kriggy]

Yes, you're right.
Although it's still practically the same. I wasn't sure if the proton would react with H₂SO₄ like it does with water to produce a hydronium ion.

Well, it has to react with something, you cant have proton just floating around. Maybe at high temperatures in plasma or something but definitely not at usual lab temperature/pressure..

[Zyklonb]

OK, it's been a while since I've read anything on acids and bases. Some things I just forget cause I never really used the info. Thanks for the correction.

[snorkack]

Considering that sulphuric acid is commonly made from oleum, it should be easy to make 100 % sulphuric acid if desired. Say take some 20 % oleum, dilute it with suitable amount of 96 % sulphuric acid, and you have your 100 % sulphuric acid. Maybe you have some measurement errors in the concentration of your 96 % sulphuric acid, so you end up with 99,5 % sulphuric acid or 0,5 % oleum, but you should get close.

What are the practical reason why over 99 % sulphuric acid/under 1 % oleum is not commonly made, stored and shipped?

Also, what are the common cations in concentrated sulphuric acid?
HNO₃ might be protonated into H₂NO₃⁺, but the reactive cation is commonly reported as NO₂⁺ - so for some reason water is readily eliminated. Then is the sulphuric acid cation always H₃SO₄⁺, or can it ever be dehydrated to HSO₃⁺ cation?