Examples of Weak Acids: Formula, Properties & Uses

Weak Acids are acids that do not dissociate their constituent ions completely in water or an aqueous solution. Acids are chemical substances that donate a hydrogen ion (H⁺) to another substance in an aqueous solution. Weak Acids and Strong Acids are two major types of acids based on ion dissociation in water.

Examples of Weak Acids are: 

• Formic Acid (HCOOH)
• Nitrous Acid (HNO₂)
• Sulfurous Acid (H₂SO₃)
• Phosphoric Acid (H₃PO₄)
• Acetic Acid (CH₃COOH)
• Benzoic Acid (C₆H₅COOH)
• Oxalic Acid (C₂H₂O₄)
• Hydrofluoric Acid (HF)

Equilibrium is established between the concentration of the weak acid and its constituent ions when dissolved in water. 

Read More: Importance of pH in Everyday Life

Key Terms: Acids, Weak Acids, Ions, Proton, Hydrogen, Salts, Anions, Litmus Paper, Strong Acids, Bases, pH Value

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What are Acids?

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Acids are ionic compounds that produce H⁺ ions when dissolved in water.

• Acids donate a hydrogen ion (H⁺) to another substance in an aqueous solution. 
• They are sour in taste and feel sticky to the touch.
• Acids have a pH value of less than 7 and turn blue litmus paper to red. 
• They react with bases to form salts and water. 

What are Acids?

On the basis of Ion Dissociation in water, Acids are categorized into two major types: 

1. Weak Acids: Acids that do not dissociate completely into their constituent particle ions are called weak acids.
2. Strong Acids: Acids that completely dissociate into their ions in a solution are called Strong Acids.

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What are Weak Acids? 

Weak Acids are those acids that partially dissociate into their ions in an aqueous solution or water.

• They have a higher pH than strong acids at the same concentration.
• The conjugate base of a weak acid is a weak base, while its conjugate acid is a weak acid.
• They have small values for K_a (Quantitative measure of the Acidic Strength) when compared to the strong acids.
• Weak Acids are sour in taste just like most of the other acids and are sticky in nature.
• The range of pH value of weak acids is from 2 to 7.

Read More: NCERT Solutions for Class 10 Science Acids, Bases and Salts

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Examples of Weak Acids

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Weak Acids are all those acids that are not classified as Strong Acids. There are only seven strong acids in Chemistry. Rest of them are weak acids as they do not dissociate their constituent ions completely in water. 

Examples of Weak Acids are as follows: 

• Formic Acid (HCOOH)
• Nitrous Acid (HNO₂)
• Hydrofluoric Acid (HF)
• Acetic Acid (CH₃COOH)
• Benzoic Acid (C₆H₅COOH)
• Sulfurous Acid (H₂SO₃)
• Phosphoric Acid (H₃PO₄)
• Oxalic Acid (C₂H₂O₄)

Strong and Weak Acids 

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Formic Acid

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Formic Acid is also known as Methanoic Acid, and it is the simplest form of Carboxylic acid. The chemical formula of formic acid is HCOOH or CH₂O₂. 

• Formic acid is considered to be one of the most important weak acids.
• It appears to be a fuming liquid that is colorless under standard temperature and pressure.
• It also has a strong and penetrating pungent smell providing an acrid odor.
• One of the most common uses of formic acid is that it is used in the processing of leather and textiles.
• Formic acid is found naturally inside the bodies of some ants.
• It can also be prepared in the form of its esters. 

Properties of Formic Acid

• The molar mass of this acid is 46.03 grams, and the density is 1.22 grams per milliliter.
• The boiling point of formic acid is 100.3 degrees celsius whereas the freezing point is 8.4 degrees Celsius.
• Formic acid is also miscible with other organic solvents like Acetone, Glycerol, Ethanol, and Methanol.
• It can easily be miscible in water as its pKa value is 3.745.
• It can also be partially soluble in some aromatic compounds like benzene and toluene.
• This acid is a little bit miscible in hydrocarbons forming hydrogen-bonded dimers and not as individual molecules.
• Formic acid does not obey the ideal gas law. 

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Acetic Acid

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Acetic Acid, also known as Ethanoic Acid, is a weak acid with a chemical formula of CH₃COOH.

• Acetic acid is a weak acid as it partially dissociates into its constituents when it is dissolved in water.
• It is a form of carboxylic acid, the second simplest form in which methane is attached to the COOH group.
• Acetic acid is the main component of Vinegar after water as it is 4-7 percent of the solution in water by volume.
• Vinegar is acetic acid diluted in water, formed after fermentation and further oxidation with ethanol.
• The concentrated form of acetic acid is corrosive to the human skin and should be handled with care.
• It is an important chemical reagent and is also an industrial chemical.
• It also helps in the production of synthetic fibers, and also polyvinyl acetate for wood glue.

Properties of Acetic Acid

• The molar mass of Acetic Acid is 60.052 grams per mole.
• It exists in the form of a colorless liquid and has a vinegar-like smell under standard temperature and pressure.
• In the form of liquid, the density of acetic acid is 1.049 grams per cubic centimeter.
• In the solid form, the density is higher which is 1.27 grams per cubic centimeter.
• The solid form of acetic acid is known to have hydrogen bonds in them.
• The melting point of acetic acid is between 16 to 17 degrees Celsius, and the boiling point is 118 degrees Celsius.
• It forms miscible mixtures with water. 

Read More: Acids Bases and Salt Important Questions

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Benzoic Acid

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Benzoic acid is the simplest aromatic carboxylic acid and has the chemical formula of C₆H₅COOH.

• It is also referred to as Benzene Carboxylic Acid and Carboxy Benzene.
• It can easily be found in nature in plants and in gum benzoin and is considered a weak acid.
• Its salts are used in food industries as preservatives on a large scale.
• Benzoic acid is not very soluble in water.
• It is present in the form of crystalline solid colorless or white color under standard temperature and pressure.
• Its odor is faintly pleasant because of the presence of an aromatic system in the benzoic acid. 

Properties of Benzoic Acid

• The molar mass of benzoic acid is 122.123 grams.
• Its density is 1.26 grams per cubic centimeter under standard temperature and pressure conditions.
• The melting point of this acid is 122 degrees Celsius, and the boiling point is 250 degrees Celsius.
• The solubility of benzoic acid in water increases with the increase in temperature.
• At 25 degrees celsius of temperature, the solubility of benzoic acid in water is 3.44 grams.
• If the temperature is increased to 100 degrees Celsius, its solubility increases to 56.31 grams per liter.
• It is soluble in some organic solvents like benzene, acetone, carbon tetrachloride, and hexane.

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Hydrofluoric Acid

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Hydrofluoric Acid is made by creating a solution of hydrogen fluoride in water. It has the chemical formula HF.

• It is considered a weak acid as it cannot be dissociated into its constituents Hydrogen and Fluorine.
• Hydrofluoric acid is very dangerous to human skin and it can cause severe damage to human health.
• One must be careful while handling the acid as it may burn the skin deeply.

Properties of Hydrofluoric Acid

• The molar mass of hydrofluoric acid is 20.0063 grams per mole and the density is 1.5 grams per milliliter.
• It appears colorless under standard temperature and pressure conditions.
• The melting point of this acid is -83.55 degrees Celsius whereas the boiling point is 19.5 degrees Celsius.
• The pK_a of hydrofluoric acid corresponds to 3.17.
• Hydrofluoric acid is mainly used for the etching of silicon and glass wafers.
• It is also used for making refrigerants, pharmaceuticals, herbicides, high-octane gasoline, aluminum, plastics, etc.

Read More: Acids Bases and Salt Revision Notes

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Oxalic Acid

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Oxalic Acid is the simplest form of a dicarboxylic acid with the chemical formula C₂H₂O_4.

• It is considered a weak acid as it does not dissociate in its constituent forms when dissolved in water.
• When compared to acetic acid, the acidity of Oxalic acid is much higher.
• It acts as a reducing agent and has hydrogen bonding in its structure.

Properties of Oxalic Acid

• The molar mass of oxalic acid is 90.03 grams per mole.
• The dihydrous of this acid has a molar mass corresponding to 126.06 grams per mole.
• It has a melting point of 190 degrees Celsius, and a boiling point of 149 to 160 degrees Celsius.
• It is perfectly soluble in water.

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Things to Remember

• Weak Acids are defined as acids that don't completely dissociate in solution.
• They are sour in taste, and sticky in nature just like other acids.
• Weak acids are much more common than strong acids and are found in daily life items.
• Examples of Weak Acids include Formic Acid, Acetic Acid, Benzoic Acid, Hydrofluoric Acid, Oxalic Acid, etc.
• They are essential in making buffer solutions. 
• Weak acids can be corrosive and dangerous in high concentrations.
• Strong Acids are those acids that dissociate their H⁺ ions completely in water or an aqueous solution.