2SO₂ + O₂ → 2SO₃

Synthesis of sulfur trioxide — balanced chemical equation, step by step.

This is the key oxidation step of the contact process used to manufacture sulfuric acid.

How to balance SO2 + O2 = SO3

A synthesis (combination) reaction joins two or more reactants into a single product. Balancing means choosing coefficients so that every element has the same number of atoms on both sides of the arrow — the Law of Conservation of Mass. Here is how it's done, step by step.

Step 1 — Write the unbalanced equation

Start with the correct formulas for every reactant and product:

SO2 + O2 = SO3

Step 2 — Count the atoms of each element

With no coefficients yet (everything counted once), the atoms do not match. The ✗ marks show which elements are unbalanced:

Step 3 — Add the smallest whole-number coefficients

Adjust the coefficients in front of each formula until every element balances. The smallest whole-number coefficients are 2 SO₂, 1 O₂, 2 SO₃, giving:

2SO2 + O2 = 2SO3

For a synthesis reaction, balance the element that appears in only one reactant and one product first, then adjust the remaining free elements.

Step 4 — Verify the balance

Recount every element. Each one now matches on both sides:

All elements are balanced and the coefficients are the smallest whole numbers, so 2SO2 + O2 = 2SO3 is the correct balanced equation.

Molar masses and mole ratio

The balanced coefficients are also the mole ratio of the reaction. Using standard atomic masses, the molar mass of each substance is:

For this reaction the mole ratio is 2 : 1 : 2. Combine these molar masses with the ratio in the free stoichiometry calculator to convert between moles, grams, and the number of particles for any reactant or product.

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