Examples of Weak Acids

Weak Acids are the acids that do not completely dissociate into their constituent ions when dissolved in solutions. When dissolved in water, an equilibrium is established between the concentration of the weak acid and its constituent ions. Some common examples of weak acids are listed below.

  • Formic acid (chemical formula: HCOOH)
  • Acetic acid (chemical formula: CH3COOH)
  • Benzoic acid (chemical formula: C6H5COOH)
  • Oxalic acid (chemical formula: C2H2O4)
  • Hydrofluoric acid (chemical formula: HF)
  • Nitrous acid (chemical formula: HNO2)
  • Sulfurous acid (chemical formula: H2SO3)
  • Phosphoric acid (chemical formula: H3PO4)

A brief description of the properties of some of these weak acids is provided below.

Formic Acid

Formic acid, also known as methanoic acid, is one of the simplest carboxylic acids. The chemical formula of this compound is HCOOH or CH2O2. Formic acid is an important example of a weak acid and is known to occur naturally inside the bodies of some ants. Under standard conditions for temperature and pressure, formic acid exists as a colourless liquid (which sometimes evolves fumes). In its liquid state, formic acid is known to possess strong and penetrating pungent odour.

The density of this compound corresponds to 1.22 grams per millilitre. The melting point of formic acid is 8.4 degrees celsius whereas the boiling point of this compound corresponds to 100.8 degrees celsius. This weak acid is known to form a miscible mixture with water. The pKa of this weak acid is equal to 3.745. The conjugate base formed from the deprotonation of formic acid is commonly referred to as formate.

It can also be noted that formic acid forms miscible mixtures with several other organic solvents such as acetone, glycerol, ethanol, and methanol. Furthermore, this compound is known to be partially soluble in certain aromatic compounds such as benzene and toluene. The molar mass of formic acid is equal to 46.025 grams per mole.

Acetic Acid

Acetic acid, also known as ethanoic acid, is a weak acid with the chemical formula CH3COOH. It is known to be the active component of vinegar, which is a 4% – 7% solution of acetic acid in water. Acetic acid is a weak acid because it only partially dissociates into its constituent ions when dissolved in water. However, it is important to note that concentrated acetic acid is known to be corrosive to human skin.

The molar mass of acetic acid corresponds to 60.052 grams per mole. Under standard conditions for temperature and pressure, this weak acid is known to exist as a colourless liquid that emanates a strong, vinegar-like odour. In its liquid state, the density of acetic acid is equal to 1.049 grams per cubic centimetre. However, solid acetic acid is known to have a relatively greater density, equal to 1.27 grams per cubic centimetre.

The melting point of acetic acid usually ranges from 16 to 17 degrees celsius. Similarly, the boiling point of acetic acid is also known to range from 118 to 110 degrees celsius. This weak acid is known to form miscible mixtures with water. The conjugate base of acetic acid is commonly referred to as acetate. The pKa value of acetic acid corresponds to 4.756. It can also be noted that solid acetic acid is known to feature hydrogen bonding as well.

Benzoic Acid

Benzoic acid is an aromatic carboxylic acid with the chemical formula C6H5COOH. This compound is a weak acid which is known to naturally occur in many plants and also in gum benzoin. Salts of this weak acid are widely used in the food industry as preservatives. It can be noted that benzoic acid is also referred to as benzene carboxylic acid and carboxy benzene. The molar mass of this compound corresponds to 122.123 grams per mole.

Under standard conditions for temperature and pressure, benzoic acid is known to exist as a crystalline solid which does not have any characteristic colour. Due to the presence of an aromatic system in the compound, benzoic acid is known to possess a faintly pleasant odour. Under standard conditions for temperature and pressure, the density of benzoic acid is roughly equal to 1.26 grams per cubic centimetre.

The melting point of benzoic acid is equal to 122 degrees celsius whereas the boiling point fo benzoic acid is equal to 250 degrees celsius. Benzoic acid is not very soluble in water. At a temperature of 25 degrees celsius, the solubility of benzoic acid in water corresponds to 3.44 grams per litre. However, it can be noted that the solubility of benzoic acid in water increases as the temperature is increased. At a temperature of 100 degrees celsius, the solubility of benzoic acid in water increases to 56.31 grams per litre. It can also be noted that this weak acid is also soluble in certain other organic solvents like benzene, acetone, carbon tetrachloride, and hexane.

Hydrofluoric Acid

Hydrofluoric acid is a weak acid with the chemical formula HF. This compound is considered to be a weak acid because it does not completely dissociate into hydrogen and fluoride ions when dissolved in water. However, it is important to note that hydrofluoric acid is dangerous to human beings and that the contact of hydrofluoric acid with human skin can result in deep burns. This compound is widely used in the etching of silicon and glass wafers.

Under standard conditions for temperature and pressure, a solution of hydrogen fluoride in water (which yields hydrofluoric acid) is known to be colourless. A 48% solution of HF in water is known to have a density of approximately 1.15 grams per millilitre. The pKa of hydrofluoric acid is equal to 3.17.

Oxalic Acid

Oxalic acid is a weak acid with the chemical formula C2H2O4. This organic compound is known to exist as a crystalline solid which is white in colour under standard conditions for temperature and pressure (often abbreviated to STP). Oxalic acid is regarded to be one of the simplest dicarboxylic acids. The condensed formula of this compound can be written as HOOC-COOH. However, it can be noted that the acidity of this compound is much higher than that of acetic acid.

The molar mass of anhydrous oxalic acid corresponds to 90.03 grams per mole. The dihydrate of this organic compound is known to have a molar mass of 126.06 grams per mole. The density of this compound under STP is known to be equal to 1.9 grams per cubic centimetre (in its anhydrous state). Anhydrous oxalic acid is known to have a melting point of roughly 190 degrees celsius. This organic acid is known to be fairly soluble in water. The solubility of oxalic acid in water (at a temperature of 20 degrees celsius) ranges from 90 to 100 grams per litre.

To learn more about the examples of weak acids and their properties along with other related concepts such as the examples of strong acids, register with BYJU’S and download the mobile application on your smartphone.

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