CID 176

CH₃COOH

Acetic acid

Q: Is acetic acid a strong or weak acid?

Weak. OpenStax gives its acid ionisation constant, Kₐ, as 1.8 × 10⁻⁵, and HSDB gives the matching pKₐ, the same constant on a logarithmic scale, as 4.76 at 25 °C. In a solution of 0.100 moles per litre, OpenStax calculates, only 1.3 per cent of the molecules ionise. That share varies with concentration, typically falling as the acid gets more concentrated.

Ball-and-stick Stick Space-filling

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8 atoms, 7 bonds.

IUPAC name: acetic acid
Molar mass: 60.0520(68) g/mol
Atoms: 8
SMILES: CC(=O)O
InChIKey: QTBSBXVTEAMEQO-UHFFFAOYSA-N
PubChem CID: 176

Made of

C H O

Structural formula of acetic acid. Each unlabelled corner is a carbon atom. It shows which atoms are joined to which, not the shape. The model above shows the shape.

The sharp smell of vinegar and its sour taste both come from acetic acid, OpenStax notes. It belongs to the carboxylic acids, compounds carrying the group COOH: a carbon double-bonded to one oxygen and single-bonded to a second oxygen that holds a hydrogen. In acetic acid that group is joined to a methyl group, CH₃, giving the formula CH₃COOH and a molar mass of 60.05 g/mol.

It is a weak acid, which OpenStax defines as one that is not fully ionised in water. At any given moment only about 1 per cent of its molecules have handed the hydrogen of the COOH group to water, leaving the acetate ion behind. In NIOSH’s description the pure compound is colourless, a liquid or a crystalline solid, and smells sour, like vinegar. It melts at 16.6 °C, and in a cool room the undiluted acid turns solid; it boils at about 118 °C.

Structure and bonding

Eight atoms and seven bonds make up acetic acid, and its two carbons are easy to tell apart. One carries three hydrogens, at H–C–H angles of about 110.6°; the other carries both oxygens. In the Ball-and-stick view the single C–O bond is visibly longer than the double C=O: 1.355 Å against 1.222 Å in PubChem’s computed conformer.

Turned edge-on, the carboxyl end is flat. OpenStax calls the geometry around a carbonyl carbon trigonal planar, and in the model that carbon’s three angles sum to 360°. The two beside the double bond, 124.3° and 125.0°, are wider than the C–C–O angle of 110.7°. The Stick view leaves this flat fork of bonds clear, and Space-filling shows the two oxygens bulking out one end.

The drawing matches the neutral molecule’s real bonding, one double and one single C–O bond, as OpenStax describes the acid group. In water about one molecule in a hundred loses that hydrogen and becomes the acetate ion, CH₃CO₂⁻, which the model does not show. OpenStax calls the C=O bond polar, oxygen pulling the shared electrons towards itself; the viewer draws bonds, not that pull.

Where it turns up

Vinegar is the everyday form, and its strength varies from one vinegar to another. OpenStax gives 3 to 6 per cent acetic acid and the Hazardous Substances Data Bank (HSDB) about 4 to 12 per cent. Haz-Map says household vinegar is usually 5 per cent. OpenStax describes cider vinegar as apple juice left to ferment without oxygen. In its account, yeast in the juice turns the sugar first into ethanol and then into acetic acid.

Canners add the acid to pickles, fish, meat, sweets and glazes. JECFA, the food-additive committee run jointly by the Food and Agriculture Organization and the World Health Organization, classes it as an acid, an acidity regulator and a preservative. Beyond food it is the stop bath of film photography and a treatment for infections of the ear canal. One commercial weed and grass killer lists vinegar, at 25.0 per cent, as its active ingredient.

Why it matters

HSDB put world demand for acetic acid at about 10.25 million tonnes in 2011. For 2023, US producers reported between 7 and 8.5 billion pounds to the Environmental Protection Agency (EPA). About half of world output comes from methanol carbonylation, the reaction of methanol with carbon monoxide, and about a third from oxidising acetaldehyde. HSDB notes that radiocarbon analysis separates natural vinegar, above 112 per cent of modern activity, from petrochemical acid, at 0 per cent.

Most of that acid becomes other chemicals. HSDB’s breakdown sends 60 per cent to vinyl acetate monomer, 10 per cent to acetic anhydride, which includes the making of cellulose acetate, and 10 per cent to esters. Cellulose acetate, OpenStax says, is a component of many synthetic fibres such as rayon. With ethanol, acetic acid forms the ester ethyl acetate, CH₃CO₂CH₂CH₃. Acid above 99 per cent can be stored and shipped in aluminium, HSDB reports: the metal corrodes slowly, and a layer of basic aluminium acetate then stops further corrosion.

Properties

Properties of acetic acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 176.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₂H₄O₂
Molar mass	60.0520(68) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	2
Hydrogen atoms	4
Oxygen atoms	2
Bonds	7
Single bonds	6
Double bonds	1
PubChem molecular weight	60.05 g/mol	g/mol	PubChem

Safety

The dilute acid in vinegar is a food, affirmed by the US Food and Drug Administration as generally recognised as safe; the concentrated acid is corrosive. Under the Globally Harmonized System (GHS) it carries the signal word Danger, as a flammable liquid and vapour. Over 99.9% of company reports to the European Chemicals Agency say it causes severe skin burns and eye damage. The PubChem Laboratory Chemical Safety Summary and the NIOSH card cover handling and workplace exposure.

Questions

What shape is an acetic acid molecule?

Acetic acid has two carbon centres built differently, and no single shape word covers both. OpenStax gives the two carbons different hybridisation, the mixing of an atom's orbitals into new ones for bonding: the CH₃ carbon is sp³, the arrangement OpenStax links to four regions of electrons in a tetrahedral arrangement, and the carboxyl carbon is sp², trigonal planar about that carbon.

Is acetic acid a strong or weak acid?

Weak. OpenStax gives its acid ionisation constant, K_a, as 1.8 × 10⁻⁵, and HSDB gives the matching pK_a, the same constant on a logarithmic scale, as 4.76 at 25 °C. In a solution of 0.100 moles per litre, OpenStax calculates, only 1.3 per cent of the molecules ionise. That share varies with concentration, typically falling as the acid gets more concentrated.

Does acetic acid mix with water?

Yes, in any proportion: HSDB lists it as miscible with water, and with ethanol, ether, acetone and benzene as well. NIOSH notes it is often used in water solution, and vinegar is one: household vinegar is usually a 5 per cent solution. Dissolving it in water moderates its chemical reactivity, according to the CAMEO entry in PubChem's Safety and Hazards data.

Why is pure acetic acid called glacial?

Glacial is the name Haz-Map gives to acids, acetic and phosphoric among them, that freeze slightly below room temperature when highly pure. Pure acetic acid melts at 16.6 °C, and NIOSH notes that it is a solid below 62 °F. HSDB defines the glacial grade as at least 99.8 per cent acid, as distinct from the water solutions also sold as acetic acid.

Made of the same elements

CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
HCOOH Formic acid 5 atoms, 4 bonds.

All 51 compounds made of carbon, hydrogen and oxygen

C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.

Appears in these articles

An acid gives a proton and a base takes it: Arrhenius, Brønsted–Lowry and Lewis Hydrogen chloride gives its proton to water effectively completely; acetic acid gives only a small fraction.