Acetic Acid Formula: Structure and Properties

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

Acetic acid is a two-carbon monocarboxylic acid with the molecular formula CH3COOH, also written as C2H4O2, and a molar mass of 60.05 g/mol. It consists of a methyl group bonded to a planar carboxyl group, and it ionizes in water with a pKa of 4.76 at 25 degrees C.

That single formula sits behind a striking range of everyday and laboratory chemistry. The same molecule that gives vinegar its sour taste at roughly 5 percent w/v is sold as a nearly pure liquid, glacial acetic acid, that freezes just below room temperature. It dissolves in water in all proportions, it is the workhorse weak acid of countless molecular biology protocols, and its protonation state controls how it behaves toward proteins, polysaccharides, and small molecules. Understanding the formula of acetic acid, its Lewis structure, and its physical constants is the foundation for using it correctly at the bench.

The Chemical Formula of Acetic Acid

The molecular formula of acetic acid is C2H4O2. The condensed structural formula is CH3COOH, which makes the functional groups explicit: one methyl carbon bearing three hydrogens, and one carboxyl carbon bonded to an oxygen by a double bond and to a hydroxyl oxygen by a single bond.

The empirical formula is CH2O, the simplest whole-number ratio of the atoms. The empirical formula is useful for combustion analysis and percent-composition problems, but it hides the carboxyl group, so it is never the formula you use to reason about acidity. The condensed formula CH3COOH is the one that tells you where the acidic hydrogen lives.

Reading the Formula Atom by Atom

Count the atoms to confirm the molar mass. Two carbons contribute 24.02 g/mol, four hydrogens contribute 4.03 g/mol, and two oxygens contribute 32.00 g/mol. The total is 60.05 g/mol. A 1 M solution therefore contains 60.05 g of acetic acid per liter of final volume.

The formula also predicts the acid's reactivity. The carboxyl group is a composite of two functional groups, a carbonyl (C=O) and a hydroxyl (O-H), joined on the same carbon. That arrangement is what makes the O-H proton unusually acidic compared with an alcohol, and it is why acetic acid is classified as a weak acid rather than a strong one.

Structure and Bonding of the Carboxyl Group

The carboxyl group, written -COOH, is the reactive heart of the molecule. Its carbon is sp2 hybridized, meaning it mixes one s orbital and two p orbitals to form three sigma bonds arranged in a trigonal planar geometry with roughly 120 degree bond angles. The remaining unhybridized p orbital on the carbon overlaps side-on with a p orbital on the carbonyl oxygen to form a pi bond.

The result is a flat, rigid unit. The carboxyl carbon, both oxygens, and the hydroxyl hydrogen all lie close to a single plane, and the methyl carbon is also roughly in that plane because it bonds to the sp2 carbon by a sigma bond. This planarity matters because it allows the carboxyl group to engage in resonance and in hydrogen bonding with defined geometry.

Resonance and Why the Two Oxygens Are Not Identical in Behavior

The carboxyl group is stabilized by resonance. The lone pair on the hydroxyl oxygen can delocalize toward the carbonyl carbon while the carbonyl pi electrons shift onto the carbonyl oxygen. The two resonance structures place a formal negative charge on one oxygen or the other, and the true structure is a hybrid in which the negative charge is shared.

This delocalization has two consequences. First, it stabilizes the deprotonated form, the acetate ion, which is why acetic acid is far more acidic than ethanol. Second, it means that when acetic acid donates a proton, the resulting acetate ion has two equivalent carbon-oxygen bonds, each with partial double-bond character, rather than one pure single bond and one pure double bond.

Where the Acidic Proton Sits

The acidic hydrogen is the one bonded to the carboxyl oxygen, the hydrogen in the -COOH group. It is not one of the three hydrogens on the methyl carbon. This is the single most common structural misconception about acetic acid.

Methyl hydrogens sit on an sp3 carbon bonded to other carbons and hydrogens. Removing one of them would require a very strong base and would generate a carbanion, a species that is far less stable than the resonance-stabilized acetate anion. The O-H bond, by contrast, is polar, and the resulting negative charge on the conjugate base is delocalized across two electronegative oxygen atoms. That energetic difference is enormous, and it is why the pKa of the carboxyl proton is 4.76 while the pKa of a typical alkane C-H bond is above 40.

Physical Properties of Acetic Acid

Pure acetic acid is a colorless liquid with a sharp, pungent odor and a sour taste. It is miscible with water in all proportions, meaning any volume of acid mixes with any volume of water to give a single phase with no separation. It is also miscible with many polar organic solvents, including ethanol and acetone.

The density of pure liquid acetic acid is 1.049 g/mL at 20 degrees C, so it is slightly denser than water. The boiling point is 118 degrees C at atmospheric pressure, noticeably higher than water, which reflects hydrogen bonding between carboxyl groups in the liquid. The pKa is 4.76 at 25 degrees C in water.

Acidity and the pKa

A pKa of 4.76 means that in dilute aqueous solution at pH 4.76, half the molecules are protonated and half are deprotonated. At physiological pH near 7.4, the equilibrium lies far toward the acetate anion. At the acidic pH values used in many microbial growth media, a larger fraction remains protonated.

This pKa is not a fixed constant under every condition. When formic and acetic acids are confined within silica nanopores roughly 4 nm in diameter, the apparent pKa increases, meaning the protonated form is stabilized relative to bulk water [1]. The effect is attributed to a reduced dielectric response in the confined solution and to a locally higher proton concentration that shifts the equilibrium toward the undissociated acid [1]. Computational work on acetic acid deprotonation has also shown that the calculated dissociation constant depends on how the reaction coordinate is defined, and that biasing two collective variables rather than one gives far more efficient and box-size-independent sampling [2]. For ordinary bench work in bulk water, however, 4.76 is the value to use.

Why the pKa Matters in Practice

The dissociation constant of a weak acid determines how much undissociated acid is present at a given pH, and that in turn shapes biological effects. A study of Salmonella and Escherichia coli growth in laboratory media found a simple relationship between the pKa of the acid used to control pH and the minimum pH at which growth begins, allowing prediction of the minimum growth pH for a given strain and acid type from as few as two measured values [3]. The same principle explains why acetic acid is more inhibitory to many organisms than a strong acid at the same nominal pH: the undissociated molecule can cross membranes, while the fully dissociated strong acid cannot.

Glacial Acetic Acid Versus Vinegar

The two forms of acetic acid that people encounter most often differ by concentration, not by identity. Glacial acetic acid is essentially pure acetic acid, while vinegar is a dilute aqueous solution of the same molecule.

Glacial acetic acid is so named because it solidifies into ice-like crystals at cool room temperatures. Its freezing point is about 16.6 degrees C, which is close to typical laboratory and pantry temperatures, so a bottle stored in a cool room can freeze. The term "glacial" refers to this solidification behavior, not to any water content.

Vinegar is roughly 5 percent w/v acetic acid in water, a concentration low enough to be safe for food use and for many household applications. The difference in concentration is roughly twentyfold or more, and it drives a large difference in hazard. Glacial acetic acid is corrosive and must be handled in a fume hood with appropriate gloves and eye protection. Vinegar is not.

Concentration Conversions

Because the density of pure acetic acid is 1.049 g/mL and its molar mass is 60.05 g/mol, pure acetic acid is about 17.4 M. That figure is worth remembering because it is the stock concentration from which dilute working solutions are prepared. A 1 M solution is a roughly 17-fold dilution of the glacial stock.

Vinegar at 5 percent w/v contains about 50 g of acetic acid per liter, which corresponds to roughly 0.83 M. That is a useful anchor when comparing a kitchen product with a laboratory reagent.

How Acetic Acid Is Observed and Measured

Several standard techniques let you confirm the identity, concentration, or protonation state of acetic acid in a sample.

Titration

Acid-base titration with a standardized strong base such as sodium hydroxide is the classic method for quantifying acetic acid. The titration curve of a weak acid shows a characteristic shape with a shallow buffer region centered near the pKa and a steep rise near the equivalence point. The half-equivalence point, where half the acid has been neutralized, occurs at a pH equal to the pKa, which is a direct way to read 4.76 off a titration curve.

Spectroscopy

Infrared and Raman spectroscopy report directly on the vibrational modes of the carboxyl group. The carbonyl stretch appears in the infrared near 1710 reciprocal centimeters for the protonated acid, and the pattern shifts when the acid is deprotonated to acetate. These spectroscopic signatures were used to quantify the pKa shift of acetic acid under nanoconfinement in silica pores [1].

Computational Determination

Molecular dynamics with reactive force fields can model the deprotonation event directly and estimate the dissociation constant. For acetic acid, the choice of collective variables strongly affects the result, and adding a second variable that tracks the distance between the hydronium cation and the acetate anion produces substantially more efficient sampling and removes the dependence on simulation box size [2].

Structural Characterization in the Solid State

X-ray crystallography can reveal how acetic acid packs and how it interacts with other molecules. In one cavitand cocrystal, the asymmetric unit contained one cavitand molecule and two acetic acid molecules, one of which was encapsulated inside the aromatic cavity and disordered over two positions with a refined occupancy ratio of 0.344 to 0.656 [4]. The guest interacted with the host mainly through its methyl group, forming C-H to pi interactions with the benzene rings, while the overall crystal packing was dominated by O-H to O and C-H to O hydrogen bonding involving the acetic acid and the cavitand carboxyl group [4]. That study illustrates a general point: the methyl end and the carboxyl end of acetic acid engage in chemically distinct interactions.

Acetic Acid in Molecular Biology and Biochemistry

Acetic acid shows up throughout molecular biology, sometimes as a reagent and sometimes as a probe of protein chemistry.

As a Solvent and Processing Agent

Acetic acid is a common solvent for biopolymers. In a study of chitosan filament production, the choice of dissolving acid mattered: filaments dissolved in aqueous acetic acid and coagulated with ethanol showed higher crystallinity, while lactic acid induced greater structural disorder, and coagulation with ethanol gave more homogeneous surfaces than methanol [5]. Acetic acid also participates in mixed organic acid solvent systems used to dissolve lignin, where dissolution behavior follows solubility parameter theory rather than simple like-dissolves-like reasoning [6].

Acetic acid also serves as a reagent in materials chemistry. Washing carbon-supported palladium-copper nanoparticles with acetic acid chemically dealloyed them and produced porous structures with at least threefold enhancement of palladium mass activity toward formic acid oxidation compared with a commercial catalyst [7]. In cements, an acetic acid hardening liquid produced higher dry compressive strengths than a standard phosphate hardener, though wet strengths were nearly identical [8].

As a Probe of Protein Microenvironments

Acetic anhydride, the acid anhydride of acetic acid, reacts with nucleophilic side chains and is used to measure the reactivity of individual residues in a folded protein. In the sodium and potassium ion activated ATPase, competitive labeling with tritiated acetic anhydride allowed assignment of the acid dissociation constant and apparent nucleophilicity of lysine-501, which had a normal pKa of 10.4 [9]. The rate constant for reaction of the free base of that lysine with acetic anhydride at 10 degrees C was 400 per molar per second, only about 30 percent of the value expected for a fully accessible lysine, which indicated that the residue is partly shielded by the tertiary structure in a pocket that forms the active site [9]. The takeaway is that acetic acid derivatives can report on local environment, not just bulk solution chemistry.

As a Degradation Variable

The pKa of the acid solvent influences polymer stability. A comparative study of chitosan degradation in 3 percent w/v solutions of lactic, acetic, malic, and formic acids tracked intrinsic viscosity and chromatographic profiles over up to 168 hours at 20 degrees C, and the results tied degradation kinetics to the dissociation constant of the solvent [10]. For anyone preparing chitosan solutions for spinning, encapsulation, or drug delivery, the choice between acetic acid and another organic acid is a stability decision, not just a solubility decision.

Quick Review

• The molecular formula of acetic acid is C2H4O2, condensed as CH3COOH, with a molar mass of 60.05 g/mol.
• The carboxyl carbon is sp2 hybridized and trigonal planar, and the carboxyl group is stabilized by resonance.
• The acidic proton is on the carboxyl oxygen, not on the methyl carbon.
• The pKa is 4.76 at 25 degrees C, the density of the pure liquid is 1.049 g/mL, and the boiling point is 118 degrees C.
• Acetic acid is miscible with water in all proportions.
• Glacial acetic acid is essentially pure and freezes near 16.6 degrees C, while vinegar is about 5 percent w/v.
• The pKa of a weak acid predicts how much undissociated acid is present at a given pH, which drives many of its biological effects.

Common Mistakes and Limitations

Confusing the acidic hydrogen with a methyl hydrogen is the most frequent structural error. The three methyl hydrogens are not acidic in any practical sense. Only the O-H hydrogen dissociates under normal aqueous conditions.

Treating glacial acetic acid as interchangeable with vinegar on a molar basis is another common slip. The concentrations differ by more than an order of magnitude, and so do the hazards. Dilution calculations must start from the actual stock concentration, and pure acetic acid is about 17.4 M.

Assuming the pKa is a universal constant ignores real effects. Confinement in nanometer-scale pores raises the apparent pKa of acetic acid, and the magnitude of the shift depends on pore size and chemistry [1]. Calculated pKa values also depend on the simulation protocol, and single-variable sampling can give box-size-dependent results [2]. Use 4.76 for bulk aqueous work and treat it as a working value rather than an immutable number.

Extrapolating from one polymer or one organism to another is unsafe. Chitosan degradation kinetics differ between acetic, lactic, malic, and formic acid solutions [10], and the relationship between acid pKa and minimum growth pH was established for specific organisms and media [3]. The general principle transfers, the exact numbers do not.

Individual experimental systems vary, and any protocol that depends on acetic acid concentration, pH, or solvent identity should be validated in your own hands before it is scaled up.

Frequently Asked Questions

What is the chemical formula of acetic acid?

The molecular formula is C2H4O2, and the condensed structural formula is CH3COOH. The condensed form is preferred when reasoning about structure because it shows the methyl group and the carboxyl group separately.

What is the molar mass of acetic acid?

The molar mass is 60.05 g/mol. It is the sum of two carbons, four hydrogens, and two oxygens, and it means a 1 M solution contains 60.05 g per liter.

Which hydrogen in acetic acid is acidic?

The hydrogen bonded to the carboxyl oxygen is the acidic one. The three methyl hydrogens are not acidic under ordinary aqueous conditions because removing them would give an unstable carbanion rather than a resonance-stabilized carboxylate.

What is the pKa of acetic acid?

The pKa is 4.76 at 25 degrees C in water. At that pH, half the molecules are protonated, and at higher pH the acetate anion dominates.

What is the difference between glacial acetic acid and vinegar?

Glacial acetic acid is essentially pure acetic acid and freezes near 16.6 degrees C. Vinegar is about 5 percent w/v acetic acid in water, roughly a twentyfold or greater dilution.

Is acetic acid miscible with water?

Yes, acetic acid is miscible with water in all proportions. Any ratio of the two liquids forms a single phase with no separation.

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Sources

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3. Simple relationship between acid dissociation constant and minimal pH for microbial growth in laboratory medium.
4. Host-guest supra-molecular inter-actions between a resorcinarene-based cavitand bearing a -COOH moiety and acetic acid.
5. How Molar Mass, Acid Type, and Coagulation Bath Composition Influence Coagulation Kinetics, Mechanical Properties, and Swelling Behavior of Chitosan Filaments: A Full Factorial Approach.
6. Lignin dissolution model in formic acid-acetic acid-water systems based on lignin chemical structure.
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