Examples of Weak Acids

A weak acid partially dissociates into its ions in an aqueous solution or water. A strong acid, on the other hand, completely dissociates into its ions when immersed in water. A weak acid's conjugate base is a weak base, while a weak acid's conjugate acid is a weak acid. Weak acids have a higher pH than strong acids at the same concentration. Below are some common examples of weak acids. 

• 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)

• Sulphurous acid (chemical formula: H₂SO₃)

• Phosphoric acid (chemical formula: H₃PO₄)

Formic Acid

One of the most basic carboxylic acids is formic acid, also known as methanoic acid. This compound's chemical formula is HCOOH or CH₂O₂. Formic acid is a good example of a weak acid that is found naturally in the bodies of some ants. Formic acid exists as a colourless liquid under standard temperature and pressure conditions (which sometimes evolves fumes). Formic acid has a strong and penetrating pungent odour in its liquid state. This compound has a density of 1.22 grams per millilitre. Formic acid has a melting point of 8.4 degrees Celsius and a boiling point of 100.8 degrees Celsius. This weak acid is known to be miscible with water. 

This weak acid's pKa value is 3.745. Formate is the name given to the conjugate base formed by the deprotonation of formic acid. Formic acid also forms miscible mixtures with several other organic solvents, including acetone, glycerol, ethanol, and methanol. Furthermore, it is known that this compound is partially soluble in aromatic compounds such as benzene and toluene. Formic acid has a molar mass of 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% acetic acid solution in water. Acetic acid is a weak acid because it only partially dissociates into its constituent ions when dissolved in water. It is important to note, however, that concentrated acetic acid is known to be corrosive to human skin. Acetic acid has a molar mass of 60.052 grams per mole. This weak acid is known to exist as a colourless liquid with a strong, vinegar-like odour under standard temperature and pressure conditions. Acetic acid has a density of 1.049 grams per cubic centimetre when it is liquid. 

Solid acetic acid, on the other hand, has a higher density, measuring 1.27 grams per cubic centimetre. Acetic acid's melting point is typically between 16 and 17 degrees Celsius. Similarly, the boiling point of acetic acid is reported to be between 118 and 110 degrees Celsius. This weak acid is known to combine with water to form miscible mixtures. It should also be noted that solid acetic acid is known to have hydrogen bonding.

Benzoic Acid

Benzoic acid is an aromatic carboxylic acid with the chemical formula C₆H₅COOH. This compound is a weak acid that is found naturally in many plants, including gum benzoin. These weak acids’ salts are widely used as preservatives in the food industry. It should be noted that benzoic acid is also known as benzene carboxylic acid and carboxy benzene. This compound's molar mass is 122.123 grams per mole. Under standard temperature and pressure conditions, benzoic acid is known to exist as a crystalline solid with no discernible colour. Because of the presence of an aromatic system in the compound, benzoic acid has a faintly pleasant odour. 

The melting point of benzoic acid is 122 degrees Celsius, while the boiling point of benzoic acid is 250 degrees Celsius. In water, benzoic acid is not very soluble. The solubility of benzoic acid in water at 25 degrees Celsius corresponds to 3.44 grams per litre. It should be noted, however, that the solubility of benzoic acid in water increases with increasing temperature. The Benzoic acid is a weak acid which is also soluble in certain other organic solvents such as benzene, acetone, carbon tetrachloride, and hexane.

Oxalic Acid

Oxalic acid, with the chemical formula C₂H₂O₄, is a weak acid. Under standard temperature and pressure conditions, this organic compound is known to exist as a crystalline solid that is white in colour. One of the most basic dicarboxylic acids is oxalic acid. The abbreviated formula for this compound is HOOC-COOH. It should be noted, however, that the acidity of this compound is much higher than that of acetic acid.

Anhydrous oxalic acid has a molar mass of 90.03 grams per mole. This organic compound's dihydrate has a molar mass of 126.06 grams per mole. This compound's density under STP is known to be 1.9 grams per cubic centimetre. This organic acid is known to be water-soluble. The solubility of oxalic acid in water (at 20 degrees Celsius) ranges between 90 and 100 grams per litre.

Hydrofluoric Acid

Hydrofluoric acid, abbreviated as HF, is a weak acid. Because it does not completely dissociate into hydrogen and fluoride ions when dissolved in water, this compound is classified as a weak acid. It is important to note, however, that hydrofluoric acid is hazardous to humans and that contact with hydrofluoric acid can result in severe burns. This substance is commonly used in the etching of silicon and glass wafers.

A solution of hydrogen fluoride in water is known to be colourless under standard temperature and pressure conditions. A 48 per cent HF solution in water has a density of approximately 1.15 grams per millilitre. Hydrofluoric acid has a pKa value of 3.17.

Nitrous Acid 

Nitrous acid (HNO₂) is an unstable weak acidic compound that can only be prepared in cold, dilute solutions. Nitrous acid is broken down into nitric oxide (NO) and nitric acid (HNO₃). It can react as an oxidising or reducing agent, which means that its nitrogen atom can gain or lose electrons in reactions with other substances. It is used to remove the potentially explosive compound sodium azide's toxic nature. It is also used in the Sandmeyer reaction to produce diazonium salts from amines and to produce azo dyes. It has an unpleasant bitter or pungent odour. This acid's molar mass is 47.013 g/mol. The boiling point is approximately 158 °C. The density of this acid is 1.1 g/cm³. It has a specific density of 1.35 g/cm³.

Sulphurous Acid

Sulphur dioxide solution, dihydrogen trioxosulfate, or trioxosulfuric acid are all names for sulphurous acid. It serves as an intermediate species in the formation of acid rain from sulphur dioxide (SO₂). It is used as a disinfectant as well as a reducing agent. It's a clear, colourless liquid with a strong, pungent odour. This acid has a density of 1.03 g/cm³. This acid has a molecular weight of 82.07 g/mol. This acid has a boiling point of -60 °C.

Phosphoric Acid

Phosphoric acid is used in a variety of industries, most notably in the production of fertilisers. Phosphates are the salts of this acid that are primarily used in agriculture and even at home. It is also known as orthophosphoric acid, which distinguishes it from other phosphoric acids like polyphosphoric acid. In its pure form, this acid is non-toxic, and it is solid at room temperature. The molar mass of it is 97.99 g/mol. This acid has a boiling point of 407 °C. This acid's melting point is approximately 42.4 °C. This acid has a density of 2.030 g.cm-3.

More About Weak Acids

We can define acids as substances that dissolve in water to produce H⁺ ions; it is capable of donating a proton (hydrogen ion) to another substance. Acids are usually identified by their sour taste. An acid is a molecule that can donate H+ ions and can remain favourable even after losing H+ ion, Acids turn blue litmus into red. Now the question arises: what is a weak acid and strong acid?

Classification of Acids

Depending on the elements present, acids can be classified as follows.

Oxyacid: 

Acids that contain both hydrogen and oxygen are termed oxyacids. Eg: Nitric acid (HNO₃), sulphuric acid (H₂SO₄), and phosphoric acid (H₃PO₄).

Hydracid: 

Acids that contain hydrogen and other nonmetallic elements (s), except oxygen, are termed hydracids. For example, hydrochloric acid (HCl) and hydrocyanic acid (HCN) are hydracids.

Acids can also be Classified as:

• Organic acids and inorganic acids

• Concentrated acids and dilute acids 

• Strong acids and weak acids 

In the topic, we have discussed the weak acid definition, classification of acid. Now let’s discuss some weak acid examples:

1. Acetic acid (CH₃COOH)

2. Formic acid (HCOOH)

3. Oxalic acid (C₂H₂O₄)

4. Hydrofluoric acid (HF)

5. Nitrous acid (HNO₂)

6. Sulphurous acid  (H₂SO₃)

7. Phosphoric acid (H₃PO₄)

8. Benzoic acid (C₆H₅COOH)

1. Acetic Acid:

Acetic acid or ethanoic acid is a weak acid with the chemical formula CH₃COOH. It is a weak acid because it only dissociates partially into its constituent ions when dissolved in water. Acetic acid is a colourless liquid that emanates a strong, vinegar-like odour.

The molar mass of this acid is 60.052 grams/ mole.

The boiling point of acid ranges from 118 to 110 °C.

The density of acetic acid is 1.049 grams per cubic centimetre.

The melting point of acid ranges from 16 to 17 °C.

2. Formic Acid:

Formic acid, or methanoic acid, is one of the simplest carboxylic acids with the chemical formula is HCOOH or CH2O₂.It is used in processing textiles and leather. It is known to occur naturally inside the bodies of some ants. This acid is also prepared in the form of its esters by treatment of carbon monoxide with an alcohol such as methyl alcohol in the presence of a catalyst.  In the liquid state, formic acid is known to possess a strong and penetrating pungent odour.

The molar mass of formic acid is 46.03 grams per mole. 

The boiling point of this acid is about 100.8 °C.

The density of this acid is 1.22 grams per millilitre.

The melting point of this acid is 8.4 °C.

3. Oxalic Acid:

It is one of the simplest dicarboxylic acids with the chemical formula C₂H₂O₄. However, the acidity of this acid is much higher than that of acetic acid.

The molar mass of this acid is equal to 90.03 grams per mole and the dihydrate of this organic compound has a molar mass of 126.06 grams per mole.

The density of oxalic acid is 1.9 grams per cubic centimetre (in an anhydrous state).

The melting point of this acid is around 90°C.

The boiling point of this acid ranges from 149 - 160°C.

4. Hydrofluoric Acid :

It is a weak acid with a chemical formula HF. Hydrofluoric acid is dangerous to human beings and direct human skin contact with hydrofluoric acid can result in deep burns.

The molar mass of this acid is 20.0063 grams per mole. 

The melting point of this acid is around -83.55 °C.

The boiling point of this acid is around 19.5 °C.

The density of this acid is 1.15 g/mL.

5. Nitrous Acid:

Nitrous acid (HNO₂) is an unstable weak acidic compound that is prepared only in the form of cold, dilute solutions. Nitrous acid decomposes into nitric oxide (NO) and nitric acid (HNO₃). It can react either as an oxidizing or a reducing agent i.e, its nitrogen atom can either gain or lose electrons in reactions with other substances. It is used to remove the toxic nature of the potentially explosive compound sodium azide. It is also used in the preparation of diazonium salts from amines and in the preparation of azo dyes in the Sandmeyer reaction. Its smell is unpleasantly bitter or pungent.

The molar mass of this acid is 47.013 g/mol.

The boiling point of this acid is around 158 °C.

The density of this acid is 1.1 g/cm³.

Its specific density is 1.35 g/cm³

6. Sulphurous Acid :

Sulphurous acid is called Sulphur dioxide solution or dihydrogen trioxide sulphate or trioxosulfuric acid. It acts as an intermediate species in producing acid rain from sulphur dioxide (SO₂). It is used as a reducing agent

and also used as a disinfectant. It is a colourless liquid with a strong pungent odour.

The density of this acid is 1.03 g/cm³

The molecular weight of this acid is 82.07 g/mol.

The boiling point of this acid is  -60 °C.

7. Phosphoric Acid:

Phosphoric acid is used in many industries, especially in the manufacturing of fertilisers. The salts of this acid known as phosphates are mainly used in agriculture and even at home.  It is also termed orthophosphoric acid which helps us to distinguish it from other phosphoric acids such as polyphosphoric acid. This acid is non-toxic in its pure form, it is a solid at room temperature. 

It has a molar mass of 97.99 g/mol.

The boiling point of this acid is  407 °C.

The melting point of this acid is around 42.4 °C.

The density of this acid is 2.030 g.cm-3

8. Benzoic Acid:

It is an aromatic carboxylic acid with the chemical formula C6H5COOH. It naturally occurs in many plants and also in gum benzoin. It is also referred to as benzene carboxylic acid and carboxy benzene. It is not very soluble in water. At a temperature of 25 °C, the solubility of benzoic acid in water is 3.44 grams per litre. It is soluble in certain other organic solvents like benzene, acetone, carbon tetrachloride, and hexane.

The molar mass of this acid is 122.123 grams per mole. 

The melting point of this acid is around 122 °C.

The boiling point of benzoic acid is equal to 250°C.

This is a list of weak acids. Now let’s discuss the Ionization of weak acids and some uses of acids.

Ionisation of Weak Acids

The reaction symbol for a strong acid ionising in water is represented by an arrow facing from left to right. On the other hand, the reaction arrow for a weak acid ionising in water is a double arrow, indicating both the forward and reverse reactions occur at equilibrium. At equilibrium conditions, the weak acid, its conjugate base, and the hydrogen ion all are present in the aqueous solution. The general form of the ionisation reaction is: HA  ⇌  H+ + A⁻

The concentration ratio for both sides is constant given fixed analytical conditions and is referred to as the acid dissociation constant (Ka). Ka is defined by the following equation:

\[Ka=[H^{+}][A^{-}][HA]\]

The above square brackets indicate the concentration of respective components. Based on the above equation, Ka expresses how easily the acid releases a proton (or in other words, its strength as an acid). Besides, the equation shows how the dissociation state of weak acids varies according to the [H+] level in the solution. 

Carboxylic acids (containing -COOH as a functional group), like acetic and lactic acids, have a Ka constant of about 10^-3 to 10^-6. Expressing acidity in terms of the Ka constant alone is not very intuitive.

So, pKa was introduced as an index to express the acidity of weak acids, where pKa is defined as follows.

pKa = -log₁₀ka

The smaller the value of pKa, the stronger the acid is. Eg: The pKa value of lactic acid is 3.8, so it means that lactic acid is a stronger acid compared to acetic acid.

Uses of Some Common Acids:

1. Sulphuric Acid: 

• It is called the king of chemicals. Some of its major uses are:

• It is used in manufacturing paints, drugs, dyes, and to produce fertilisers.

• It is used in car batteries.

• Used in the manufacturing of hydrochloric acid and alum.

2. Hydrochloric Acid: 

• It is also used for cleaning sinks and sanitary ware.

• It is used in various industries that use heating applications.

• It is applied to remove deposits from the boilers.

3. Acetic Acid:

• It is used as a cleansing agent in products like cleaning windows, floors, utensils, etc.

• It is used for enhancing the flavour of food. Acetic acid is commonly known as vinegar.

• This acid helps to remove stains on woodwork such as furniture and carpets.

4. Phosphoric Acid:

• Used in fertiliser and detergent industries.

• It is a key ingredient in many soft drinks.

5. Citric Acid: 

• Used as a food preservative.

• Used as a flavouring agent.

6. Ascorbic Acid:

It is mostly used in the process of treatment of bone marrow and scurvy diseases.

7. Boric Acid:

• It is widely used in detergents.

• Used in the manufacturing of glass, leather, paper, adhesives, and explosives.

• The acid equilibrium constant Ka or pKa value can help to determine whether an acid is strong or weak. Strong acids have high Ka and small pKa values on the other hand weak acids have very small Ka values and large pKa values.

Conclusion:

Weak acids are that acid that can be ionised partially in their solutions, whereas strong acids are completely ionised when dissolved in water. A weak acid is more commonly used compared to strong acids. They are found in daily life products like vinegar (acetic acid) and lemon juice (citric acid). When they are dissolved in water, an equilibrium is established between the concentration of the weak acid and its constituent ions.