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Sulfurous acid is a distinct inorganic compound with the formula H₂SO₃. Unlike its more familiar counterpart, sulfuric acid (H₂SO₄), sulfurous acid has never been isolated as a stable liquid; it has only been detected in the gas phase under laboratory conditions.
A chemical formula lists the elements that make up a molecule and the number of atoms of each. For H₂SO₃, the structure consists of a central sulfur atom bonded to three oxygen atoms: two single bonds and one double bond. The two hydrogen atoms attach to the singly bonded oxygens, giving the overall stoichiometry of two hydrogens, one sulfur, and three oxygens.
The molecular mass (or formula mass) of a compound is the sum of the relative atomic masses of its constituent atoms. Using the standard atomic masses from the periodic table—hydrogen (1), sulfur (32), and oxygen (16)—the calculation is straightforward:
Formula mass = (2 × 1) + (1 × 32) + (3 × 16) = 82
Thus, the molecular mass of sulfurous acid is 82 g/mol. While the periodic table lists more precise atomic masses that account for isotopic variations, rounding to the nearest integer is typically sufficient for most practical calculations.
