[vhpk]

I want to know that in industry, how is the max concentration of H₂SO₄ can we produce

[AWK]

Up to 10% oleum

[DevaDevil]

only 10%?, I remember reading somewhere about 65% oleum (sth like 112% equivalent sulf. Acid?)

Anyway, disregarding oleum, in principle 100% Sulfuric Acid can be made, but usually the highest grade sold is 98% (more stable form to store)

[P-man]

Is 100% sulphuric acid even legal (in the U.S., let's say)?

[constant thinker]

Is 100% sulphuric acid even legal (in the U.S., let's say)?

I don't see why it would be illegal. You can't directly make explosives or drugs with it as far as I'm away of.

As has been already stated, 100% is theoretical, but Sulfuric acid is hydrophilic. It will quickly become contaminated by atmospheric oxygen. The hydrophilic property of sulfuric acid is demonstrated nicely when you add concentrated (~98%) sulfuric acid to sugar. The sugar will actually be dehydrated by the sulfuric acid (in a highly exothermic reaction).

[P-man]

So, you can't have the pure H₂SO₄ molecule (therefore 100% sulphuric acid) because it gets oxidized by the oxygen in the air, reducing it to 98% at the max. So does the acid become H₂SO₆? Or do you get an ionic solution of H₂O and SO₄? Sorry to get off topic a bit but I'm confused about this oxidizing thing.

[Borek]

Oxygen doesn't do anything, concentrated sulphuric acid sucks water from the air.

It is enough to keep it in closed container to avoid water contamination.

OTOH in practice there is no such thing as not contaminated substances. In most cases even high purity means ppm contaminations.

IIRC highest purity obtained was something about 1 atom of contamination per 10¹⁴ atoms of main component, but I can be completely off. Besides, preparing such a thing is a rocket science.