Sulfur Trioxide(SO3) Properties(25 Facts You Should Know)

Sulfur trioxide is non-flammable and is formed by the oxidation reaction of sulfur dioxide. Let us study sulfur trioxide in detail.

Sulfur trioxide is one of the constituents of acid rain. The gaseous form of sulfur trioxide is considered a most pollutant chemical compound. It also acts as a fire hazard when in contact with organic materials such as fiberboard, cotton and wood.

This article discusses the facts and properties of sulfur trioxide, such as chemical formula, molar density, viscosity, density, structure, and reaction with a base.

SO3 IUPAC name

The IUPAC name of SO3 is Sulfur trioxide.

SO3 chemical formula

The chemical formula of sulfur trioxide is SO3. It consists of two components, Sulfur and Oxygen, in which three oxygen atoms surround the central sulfur atom.

SO3 CAS number

The CAS number of SO3 is 7446-11-9.

SO3 ChemSpider ID

The ChemSpider ID of SO3 is 23080.

SO3 chemical classification

The chemical classification of sulfur trioxide is as follows,

  • Sulfur trioxide is an inorganic anhydride.
  • SO3 exists in crystalline trimer, gaseous monomer and solid polymer.
  • SO3 is a highly reactive substance, and its vapor form is highly corrosive.
  • SO3 is a strong oxidizing agent.

SO3 molar mass

The molar mass of sulfur trioxide is 80.06 g/mol, in which Sulfur and oxygen atom have a molar mass of 32.065 g/mol and 16.00 g/mol, respectively.

SO3 color

Sulfur trioxide is a colorless liquid.

SO3 viscosity

The viscosity of SO3 is 1.3 cP at 38 ⁰C.

SO3 molar density

The molar density of SO3 is 1.92 g/cm3 at 20 ⁰C.

SO3 melting point

The melting point of SO3 is 16.9 ⁰C.

SO3 boiling point

The boiling point of SO3 is 45 ⁰C.

SO3 state at room temperature

SO3 is a liquid that will fumes in the air at room temperature. It turns a white crystalline solid below room temperature.

SO3 covalent bond

In sulfur trioxide, there are 3 covalent bonds between the Sulfur and oxygen atom, which are double-bonded to each other. Here, the electrons are equally shared between the two nonmetals; Sulfur and oxygen atoms form covalent bonds.

SO3 covalent radius

The covalent radius of Sulfur and Oxygen is 104 pm and 74 pm, respectively.

SO3 electron configurations

The electron configuration represents how many electrons are distributed among the orbital shells. Let us discuss the electronic configuration of atoms of SO3.

  • The electronic configuration of Sulfur is [Ne] 3s2 3p4
  • The electronic configuration of Oxygen is [He] 2s2 2p4

SO3 oxidation state

The sulfur atom has an oxidation state of +6, and Oxygen carries a -2 charge in the molecules of sulfur trioxide. Therefore sulfur trioxide is a strong oxidizing agent.

SO3 acidity

SO3 is acidic. It reacts violently with water, producing strong acid, such as sulfuric acid, that turns litmus solution red.

Is SO3 odorless?

SO3 acid mist is odorless and in its vapor state. It has a pungent smell.

Is SO3 paramagnetic?

The magnetic field feebly attracts paramagnetic substances, and they acquire feeble magnetism in the same direction as the magnetic field. Let us see the magnetic property of SO3.

SO3 is diamagnetic. The outermost shell of the Sulfur and oxygen atom has 6 and 12 electrons, respectively. The total number of electrons is 18; therefore, the molecule has paired electrons.

SO3 hydrates

SO3 is the anhydride of sulfuric acid. It rapidly hydrated in the presence of moisture in the atmospheric air to form sulfuric acid.

  • SO3 + H2O → H2SO4

SO3 crystal structure

SO3 has trigonal planar geometry. It undergoes sp2 hybridization, and the O-S-O bond angle is 120⁰.

Trigonal planar geometry of Sulfur trioxide
Trigonal planar geometry of Sulfur trioxide

SO3 polarity and conductivity

  • SO3 is nonpolar. It is due to its trigonal planar geometry. Three bonds across the Sulfur and oxygen atom are at 120⁰ to each other, which cancels their overall polarity and makes the molecule nonpolar.
  • SO3 shows conductivity when it is added to the solution of SbF5 and HSO3F.

SO3 reaction with acid

SO3 reacts with hydrochloric acid to form chlorosulfonic acid (HSO3Cl), a strong acid.

  • HCl (aq) + SO3 (l) → HSO3Cl (l)

SO3 reaction with base

SO3 reacts with the base, such as sodium hydroxide, to form sodium hydrogen sulfate.

  • SO3+NaOH→NaHSO4

SO3 reaction with oxide

SO3 reacts with oxide, such as calcium oxide, to form calcium sulfate. It is the acid-base reaction used to remove the smoke from the burning coal containing Sulfur.

  • CaO (s) + SO3 (g) →CaSO4 (s)

SO3 reaction with metal

SO3 reacts with a metal compound such as a trialkylphosphine to form an adduct.

  • R3P +SO3 → R3P+-SO3


Sulfur trioxide is a colorless liquid that is very reactive. It acts as a strong oxidizing agent. It undergoes sp2 hybridization with the trigonal planar geometry.