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

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, 20196 yr by chenbeier

Thank you!

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.