chemistry

What is oleum? Difference between oleum and H2SO4

Oleum, also known as fuming sulfuric acid, is a highly reactive compound consisting of dissolved sulfur trioxide gas (SO₃) in 100% sulfuric acid.

What do you mean by fuming?

When we say “fuming” (like fuming sulfuric acid = oleum), we mean:

  • The liquid gives off visible vapors (“fumes”) when exposed to air.
  • In the case of oleum, those vapors are actually SO₃ gas, which escapes easily because SO₃ is volatile.
  • When this SO₃ gas meets moisture in the air, it reacts immediately to form tiny droplets of H₂SO₄, which look like white, smoky fumes.

That’s why:

  • Concentrated H₂SO₄ does not “fume” much in air.
  • Oleum does, because of the escaping SO₃.

So fuming = releasing visible vapors (SO₃ + H₂SO₄ mist) into the air

Fumes of H2SO4 dangerous?

Yes ✅ the fumes of H₂SO₄ (sulfuric acid) are very dangerous.

Here’s why:

  • What the fumes are:
    Pure H₂SO₄ itself doesn’t evaporate much, but when it’s very concentrated (or as oleum) it releases SO₃ vapors.
    These vapors react with moisture in the air → form fine droplets of H₂SO₄ mist.
  • Effects on health:
    • Extremely corrosive to skin, eyes, and mucous membranes.
    • Causes severe burns on contact.
    • If inhaled, can damage the nose, throat, lungs (leading to breathing difficulty, pulmonary edema).
    • Even short exposure to strong fumes is harmful.

That’s why handling concentrated H₂SO₄ / oleum requires fume hoods, protective gloves, goggles, and masks.

H₂S₂O₇ is the chemical formula of oleum?

Oleum is also called fuming sulfuric acid.

It is not a single pure compound but rather a solution of SO₃ dissolved in H₂SO₄.

H₂S₂O₇ = pyrosulfuric acid, which represents one particular composition of oleum (when there is exactly one mole of SO₃ combined with one mole of H₂SO₄).

Oleum in general can have different amounts of SO₃, not just the 1:1 case.

If it is mixture, so why it is represented by molecular formula H2S2O7?

Oleum is really a mixture (H₂SO₄ + SO₃), but in chemistry we often like to represent it in a molecular way. That’s why one of its possible compositions is written as H₂S₂O₇ (pyrosulfuric acid).

Convenience in Representation

  • Writing H₂S₂O₇ makes it easier to handle in equations and discussions, instead of always writing “H₂SO₄·SO₃”.
  • For example, when balancing reactions in textbooks, H₂S₂O₇ is simpler to use.

But in Reality

  • Oleum is not just H₂S₂O₇ — it may have more (or less) SO₃ dissolved in H₂SO₄.
  • So H₂S₂O₇ is only one specific case of oleum.

Why Sulphonation of OF BENZENE IS Carried out with Oleum not pure Conc. H2SO4?

leum already contains a large amount of free SO₃ dissolved in H₂SO₄.

That SO₃ directly acts as the electrophile in the sulphonation of benzene.

The H₂SO₄ in oleum helps by protonating SO₃ to form the stronger electrophile

MCQs on Solutions – Class 12

MCQs on Solutions – Class 12

1.

If 2 moles of NaCl are dissolved in 1 kg of water, the molality of the solution is:
a) 1 m
b) 2 m
c) 0.5 m
d) 4 m

2.

Which of the following is not a colligative property?
a) Relative lowering of vapour pressure
b) Osmotic pressure
c) Depression of freezing point
d) Molal mass

3.

A 0.1 M solution of glucose and a 0.1 M solution of NaCl are prepared. Which will have higher osmotic pressure?
a) Glucose
b) NaCl
c) Both same
d) Cannot be predicted

4.

The vapour pressure of pure water at 25 °C is 23.8 mmHg. What will be the vapour pressure of a solution containing 1 mol of non-volatile solute in 55.5 mol of water?
a) 23.38 mmHg
b) 23.0 mmHg
c) 22.0 mmHg
d) 21.5 mmHg

5.

Henry’s law constant for CO₂ in water is high. This means:
a) CO₂ is very soluble in water
b) CO₂ is sparingly soluble in water
c) Solubility is independent of pressure
d) Solubility increases rapidly with temperature

6.

If 1 mole of NaCl is dissolved in 1 L water, the van’t Hoff factor (i) is:
a) 1
b) 2
c) 3
d) 0

7.

A solution boils at 100.52 °C. The ebullioscopic constant of water (Kb) = 0.52 K kg mol⁻¹. The molality of solute is:
a) 1 m
b) 0.5 m
c) 2 m
d) 0.1 m

8.

Which statement is correct?
a) Colligative properties depend on nature of solute.
b) Colligative properties depend on number of solute particles.
c) Colligative properties depend on solvent only.
d) Colligative properties are independent of concentration.

9.

The mole fraction of solute in a solution is 0.2. What is the mole fraction of solvent?
a) 0.8
b) 0.2
c) 1.2
d) 0.02

10.

A 5% (w/w) solution of glucose in water means:
a) 5 g glucose in 95 g water
b) 5 g glucose in 100 g solution
c) 5 g glucose in 100 mL solution
d) 5 g glucose in 100 g water

11.

The units of Henry’s law constant KH are:
a) mol L⁻¹ atm⁻¹
b) atm mol⁻¹ L
c) atm
d) mol L⁻¹

12.

An azeotrope is:
a) A mixture of liquids with the same composition in liquid and vapour phase
b) A mixture that boils at variable temperature
c) A mixture of solid and liquid
d) A mixture showing abnormal colligative properties

13.

Which of the following pairs shows positive deviation from Raoult’s law?
a) H₂O + C₂H₅OH
b) CHCl₃ + C₆H₆
c) H₂O + HCl
d) Acetone + CS₂

14.

Relative lowering of vapour pressure is equal to:
a) Mole fraction of solute
b) Mole fraction of solvent
c) Molality of solution
d) Molarity of solution

15.

The freezing point of 1 molal NaCl solution is nearly double the depression shown by 1 molal urea solution. Why?
a) NaCl is volatile
b) NaCl ionises giving more particles
c) NaCl reduces vapour pressure more
d) NaCl is heavier

16.

At higher altitudes, people use pressure cookers because:
a) Boiling point of water increases with pressure
b) Boiling point of water decreases with pressure
c) Vapour pressure of water decreases with temperature
d) None of the above

17.

The value of van’t Hoff factor (i) for K₂SO₄ is:
a) 2
b) 3
c) 1
d) 4

18.

Which one forms a maximum boiling azeotrope with water?
a) HNO₃
b) HCl
c) C₂H₅OH
d) CH₃COOH

19.

The molarity of pure water is approximately:
a) 1 M
b) 18 M
c) 55.5 M
d) 100 M

20.

If osmotic pressure of 0.01 M solution of a substance is 2.46 atm at 27 °C, the substance is:
(R = 0.0821 L atm K⁻¹ mol⁻¹)
a) Nonelectrolyte
b) Strong electrolyte
c) Non-volatile liquid
d) Colloid

21.

Cryoscopic constant (Kf) depends on:
a) Nature of solute
b) Nature of solvent
c) Both solute and solvent
d) Temperature only

22.

If solute associates in a solution, van’t Hoff factor (i) is:
a) > 1
b) < 1
c) = 1
d) Infinite

23.

In osmosis, solvent moves:
a) From concentrated solution to dilute
b) From dilute solution to concentrated
c) From high pressure to low pressure
d) In both directions equally

24.

Raoult’s law is applicable to:
a) Non-ideal solutions only
b) Ideal solutions only
c) Electrolyte solutions
d) Colloidal solutions

25.

A solution shows negative deviation from Raoult’s law when:
a) A–B interactions < A–A or B–B
b) A–B interactions > A–A or B–B
c) Solution behaves ideally
d) Solute associates in solution

Answer Key

  1. b
  2. d
  3. b
  4. a
  5. b
  6. b
  7. a
  8. b
  9. a
  10. b
  11. a
  12. a
  13. d
  14. a
  15. b
  16. a
  17. b
  18. a
  19. c
  20. b
  21. b
  22. b
  23. b
  24. b
  25. b