Introduction:
Acetic acid (CH3COOH) is stronger than propionic acid (C2H5COOH) due to several factors related to its chemical structure and properties. The strength of an acid is determined by its ability to donate a proton (H+) in an aqueous solution.

1. Molecular Structure:
The molecular structure of acetic acid and propionic acid plays a significant role in their acidic strength. Acetic acid has a smaller alkyl group (methyl) attached to the carboxylic acid functional group, while propionic acid has a larger alkyl group (ethyl). The size of the alkyl group influences the stability of the conjugate base formed after donation of a proton. In acetic acid, the smaller methyl group allows for greater electron density on the carboxylate ion, making it more stable. This stability facilitates the release of a proton, resulting in a stronger acid.

2. Inductive Effect:
Another factor contributing to the stronger acidity of acetic acid is the inductive effect. The electronegative oxygen atom in the carboxylic acid group withdraws electron density from the carbon atom attached to the alkyl group. In acetic acid, this effect is more pronounced due to the closer proximity of the electronegative oxygen to the methyl group. As a result, the carbon atom in acetic acid is more electron-deficient, making it easier to donate a proton.

3. Resonance Stabilization:
Acetic acid exhibits resonance stabilization, which further enhances its acidic strength. The presence of a double bond between the carbon and oxygen atoms in the carboxylic acid group allows for delocalization of electron density. This resonance stabilization spreads the negative charge over the oxygen atom and the adjacent carbon atom, making the conjugate base more stable. In propionic acid, the absence of a double bond limits this resonance stabilization, resulting in a weaker acid.

4. Solubility in Water:
The solubility of an acid in water also affects its strength. Acetic acid is more soluble in water compared to propionic acid. The presence of the smaller methyl group in acetic acid allows for stronger hydrogen bonding with water molecules. This increased solubility facilitates the dissociation of acetic acid into its ions, leading to a stronger acid.

Conclusion:
In summary, acetic acid is stronger than propionic acid due to its molecular structure, including a smaller alkyl group, inductive effect, resonance stabilization, and higher solubility in water. These factors collectively contribute to the greater ability of acetic acid to donate a proton, making it a stronger acid than propionic acid.