Examples of Weak Acids

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Weak Acid

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?

Weak acids are that acid that can be ionized partially in their solutions, whereas strong acids are completely ionized when dissolved in water. A weak acid is more commonly used compared to strong acids. They are found in daily life 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. 

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) is 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:

  • Acetic acid (CH3COOH)

  • Formic acid (HCOOH)

  • Oxalic acid (C2H2O4)

  • Hydrofluoric acid (HF)

  • Nitrous acid (HNO2)

  • Sulfurous acid  (H2SO3)

  • Phosphoric acid (H3PO4)

  • Benzoic acid (C6H5COOH)

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.

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.

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.

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.

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

Sulfurous Acid :

Sulfurous acid is called Sulfur dioxide solution or dihydrogen trioxosulfate or trioxosulfuric acid. It acts as an intermediate species in producing acid rain from sulfur 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.

Phosphoric Acid:

Phosphoric acid is used in many industries, especially in the manufacturing of fertilizers. The salts of this acid are 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

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 some weak acids list, Now let’s discuss the Ionization of weak acids and some uses of acids.

Ionization of Weak Acids

The reaction symbol for a strong acid ionizing in water is represented by an arrow facing from left to right. On the other hand, the reaction arrow for a weak acid ionizing 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 ionization 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 = \[\frac{[H^{+}][A^{-}]}{[HA]}\]

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:

Sulphuric Acid: 

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

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

  • It is used in car batteries.

  • Used in the manufacturing of hydrochloric acid and alum.

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.

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.

Phosphoric Acid:

  • Used in fertilizer and detergent industries.

  • It is a key ingredient in many soft drinks.

Citric Acid: 

  • Used as a food preservative.

  • Used as a flavouring agent.

Ascorbic Acid:

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

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.

FAQ (Frequently Asked Questions)

Question 1. How can the relationship between pH and pOH be defined?

Answer: The pH and pOH of a solution at 25oC can be written in the following way:

pH + pOH = 14

If any one of pH or the pOH of a solution is known, the other can be calculated easily.

Question 2. Which is the Weakest Acid?

Answer: Although there are so many weak acids, Hydrocyanic acid with Ka equals 4.9 X 10-10 is considered as the weakest acid.

Question 3. Differentiate between Concentrated acid and Dilute Acid?

Answer: A concentrated acids are those which contain a low amount of water. A dilute acid is an acidic solution that contains a lot of solvents. If we have 10 M acetic acid, it's concentrated, yet it is still a weak acid. No matter how much water we remove. On the other hand, A 0.0008 M HCl solution is dilute, yet it is still strong acid.

Question 4. Differentiate between Organic Acid and Inorganic Acid.

Answer: All sour things that we use in our daily food items contain acids. These acids are termed organic acids. Some of the acids that are commonly used in the laboratory such as hydrochloric acid (HCl), sulphuric acid (H2SO4), and nitric acid (HNO3) are inorganic acids or mineral acids.