Which of the following is a diprotic acid

1. Acetic acid
2. Phosphoric acid
3. Sulphuric acid
4. Nitric acid

Answer (Detailed Solution Below)

Option 3 : Sulphuric acid

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Concept:

Diprotic Acid

A diprotic acid is an acid that dissociates in two stages and yields two H+ ions per acid molecule.

1. H2X → H+ + HX-
2. HX- → H+ + X2-

Explanation:

• Sulphuric acid (H2SO4)

Sulfuric acid is a diprotic acid, it contains two acidic hydrogen atoms. 

H2SO4 → H+ + HSO4-

HSO4- → H+ + SO42-

•  Acetic acid (CH3COOH)

Acetic acid is monoprotic acid due to its structure. The three hydrogen atoms attached to the carbon are in weak polar bonds. They do not ionize. Only the H bonded to the highly electronegative oxygen can be ionized.

CH3COOH → CH3COO- + H+

• Phosphoric acid (H3PO4)

Phosphoric acid is tricrotic acid, it contains three hydrogen ions.

H3PO4 + H2O → H+ + H2PO4-

H2PO4- + H2O → H+ + HPO42-

HPO42- +H2O → H+ + PO43-

• Nitric acid (HNO3)

Nitric acid is monoprotic acid, it contains one acidic hydrogen atom.

HNO3 → H+ + NO3-

Among the given acids Sulfuric acid is a diprotic acid.

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The correct answer is option E.

Nitric acid (HNO3) is a monoprotic acid which means that each molecule of HNO3 will donate a proton on dissociation. Thus, this choice is incorrect.

Chloric acid is a monoprotic acid as it releases a single proton on dissociation. Thus, this option is incorrect.

This option is incorrect as phosphoric acid is a triprotic acid and can donate three protons.

The ionizable protons are highlighted in red.

This option is incorrect as hydrofluoric acid is a monoprotic acid. Each molecule of HF will dissociate to produce a single proton and a fluoride ion.

This is the right option. Sulfuric acid (H2SO4) is a diprotic acid. A diprotic acid can be defined as an acid in which each molecule dissociates to give two protons.

Examples of diprotic acid in the following topics:

• Diprotic and Polyprotic Acids

  • Diprotic and polyprotic acids contain multiple acidic protons that dissociate in distinct, sequential steps.
  • Two common examples are carbonic acid (H2CO3, which has two acidic protons and is therefore a diprotic acid) and phosphoric acid (H3PO4, which has three acidic protons and is therefore a triprotic acid).
  • Diprotic and polyprotic acids show unique profiles in titration experiments, where a pH versus titrant volume curve clearly shows two equivalence points for the acid; this is because the two ionizing hydrogens do not dissociate from the acid at the same time.
  • A diprotic acid (here symbolized by H2A) can undergo one or two dissociations depending on the pH.
  • For example, a generic diprotic acid will generate three species in solution: H2A, HA-, and A2-, and the fractional concentration of HA-, which is given by:

• Polyprotic Acid Titrations

  • Polyprotic acids, also known as polybasic acids, are able to donate more than one proton per acid molecule.
  • Common examples of monoprotic acids in mineral acids include hydrochloric acid (HCl) and nitric acid (HNO3).
  • Certain types of polyprotic acids have more specific names, such as diprotic acid (two potential protons to donate) and triprotic acid (three potential protons to donate).
  • For example, oxalic acid, also called ethanedioic acid, is diprotic, having two protons to donate.
  • The diprotic acid has two associated values of Ka, one for each proton.

• Heterogeneous and Multiple Equilibria

  • Consider the case of a diprotic acid, such as sulfuric acid.
  • Diprotic acids can be written as H2A.
  • Sulfuric acid, the molecule pictured here, is an example of a diprotic acid.

• Calculating Equilibrium Concentrations of Polyprotic Acids

  • Polyprotic acids can lose more than one proton.
  • Common polyprotic acids include sulfuric acid (H2SO4), and phosphoric acid (H3PO4).
  • For a diprotic acid for instance, we can calculate the fractional dissociation (alpha) of the species HA- using the following complex equation:
  • When a weak diprotic acid such as carbonic acid, H2CO3, dissociates, most of the protons present come from the first dissociation step:
  • The chemical structure of phosphoric acid indicates it has three acidic protons.

• Strong Acids

  • The strength of an acid refers to the ease with which the acid loses a proton.
  • where HA is a protonated acid, H+ is the free acidic proton, and A- is the conjugate base.
  • Strong acids yield weak conjugate bases.
  • For sulfuric acid, which is diprotic, the "strong acid" designation refers only to the dissociation of the first proton:
  • p-Toluenesulfonic acid is an example of an organic soluble strong acid, with a pKa of -2.8.

• Acid Dissociation Constant (Ka)

  • The acid dissociation constant (Ka) is the measure of the strength of an acid in solution.
  • The acid dissociation constant (Ka) is a quantitative measure of the strength of an acid in solution.
  • Acid dissociation constants are most often associated with weak acids, or acids that do not completely dissociate in solution.
  • Acids with a pKa value of less than about -2 are said to be strong acids.
  • Acetic acid is a weak acid with an acid dissociation constant $K_a=1.8\times 10^{-5}$ .

• Weak Acids

  • The majority of acids are weak.
  • Examples of weak acids include acetic acid (CH3COOH), which is found in vinegar, and oxalic acid (H2C2O4), which is found in some vegetables.
  • Acids with a Ka less than 1.8×10−16 are weaker acids than water.
  • The Ka of acetic acid is $1.8\times 10^{-5}$.
  • Although it is only a weak acid, a concentrated enough solution of acetic acid can still be quite acidic.

• The Brønsted-Lowry Definition of Acids and Bases

  • Originally, acids and bases were defined by Svante Arrhenius.
  • A wide range of compounds can be classified in the Brønsted-Lowry framework: mineral acids and derivatives such as sulfonates, carboxylic acids, amines, carbon acids, and many more.
  • The conjugate acid is the species that is formed when the Brønsted base accepts a proton from the Brønsted acid.
  • Here, acetic acid acts as a Brønsted-Lowry acid, donating a proton to water, which acts as the Brønsted-Lowry base.
  • Chemistry 12.1 What are Acids and Bases?

• Oxoacids

  • Halogen oxoacids include hypochlorous acid (HOCl); chlorous acid(HOClO); chloric acid(HOClO2); oerchloric acid(HOClO3); oerbromic acid (HOBrO3)
  • Consider the simple oxyacids HOI (hypoiodous acid), HOBr (hypobromous acid), and HOCl (hypochlorous acid).
  • The strongest acid is perchloric acid on the left, and the weakest is hypochlorous acid on the far right.
  • Carboxylic acids are the most common type of organic acid.
  • Mellitic acid is an example of a hexacarboxylic acid.

• Carboxylic Acids

  • Carboxylic acids are organic acids that contain a carbon atom that participates in both a hydroxyl and a carbonyl functional group.
  • As proton donors, carboxylic acids are characterized as Brønsted-Lowry acids.
  • Salts and esters of carboxylic acids are called carboxylates.
  • Generally, in IUPAC nomenclature, carboxylic acids have an "-oic acid" suffix, although "-ic acid" is the suffix most commonly used.
  • Carboxylic acids are characterized as weak acids, meaning that they do not fully dissociate to produce H+ cations in a neutral aqueous solution.

Is H3PO4 a diprotic acid?

Is H2CO3 a diprotic acid?

Is ch3cooh a diprotic acid?

Is H3BO3 a diprotic acid?