The empirical formula for hydrogen cyanide is HCN.
Hydrogen cyanide is a simple molecule made up of one hydrogen atom, one carbon atom, and one nitrogen atom bonded together (H–C≡N). Because the ratio of atoms in the compound is already 1:1:1 in its molecular formula, the empirical formula (which expresses the smallest whole-number ratio of atoms of each element in a compound) is identical to its molecular formula: HCN.
To clarify the distinction: a molecular formula shows the actual number of atoms of each element in one molecule of a substance, while an empirical formula shows the simplest whole-number ratio of those atoms. For many compounds these two formulas differ — for example, glucose has a molecular formula of C6H12O6 but an empirical formula of CH2O, since the ratio 6:12:6 simplifies to 1:2:1. Hydrogen cyanide, however, is a case where the molecular and empirical formulas are the same because the atom ratio cannot be reduced any further.
Hydrogen cyanide has a molar mass of approximately 27.03 g/mol (1.008 for H + 12.01 for C + 14.01 for N). It is a colorless, extremely toxic, and volatile liquid or gas with a faint bitter-almond odor (though not everyone can detect this smell due to a genetic trait). It is used industrially in the production of various chemicals, including acrylonitrile and adiponitrile for plastics and synthetic fibers, and it also occurs naturally in trace amounts in some plants and combustion products.
Because of its high toxicity — it interferes with the body's ability to use oxygen at the cellular level — hydrogen cyanide is handled with strict safety precautions in industrial and laboratory settings.