Question about oleum

[Larus99]

Hello! I want to ask how much oleum (20% SO3) and water should be mixed to make concentrated H2SO4? Thanks!

[chenbeier]

If you have 1 kg Oleum (20% SO₃ dissolved in 100% H₂SO₄) then you have 200 g of sulfur trioxide and 800 g sulfuric acid.  With the molare weight of 80 g/mol for SO₃  you get 2,5 mol.

You need also 2,5 mol water according H₂O + SO₃ => H₂SO_{4 .} This is equal to 45 g. So take 45 g = 45 ml water first!!  and add carefully the Oleum.

Edited August 3, 2019 by chenbeier

[Larus99]

Thank you!

[John Cuthber]

Given the extremely violent reaction of  oleum with water it's probably better to add the oleum to ice.

It may also be a better bet to let it absorb water from the air (until it gains the right mass), but the fumes would be a problem.

 

Fundamentally, if you can't do basic chemistry like the calculation above, you shouldn't be working with oleum.