Which one among the following compounds has same equivalent weight and molecular weight ?

1. Basics of Atomic and Molecular Mass (basic)

Concept: Basics of Atomic and Molecular Mass

2. The Mole Concept and Avogadro's Number (basic)

In the world of chemistry, atoms and molecules are so incredibly small that counting them individually is impossible. To solve this, scientists use a collective unit called the Mole. Much like we use the word 'dozen' to represent 12 items, or 'gross' to represent 144, a mole is a specific number used to count particles like atoms, molecules, or ions. For instance, when a substance like KOH dissociates in water Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.24, it breaks into individual ions; the mole allows us to quantify exactly how many of those ions are present in a measurable sample.

The exact number of particles in one mole is known as Avogadro’s Number (or Avogadro's Constant), which is approximately 6.022 × 10²³. This number is the vital bridge between the microscopic world of atoms and the macroscopic world of grams. The genius of the mole concept is that 1 mole of any substance will have a mass in grams equal to its atomic or molecular weight. For example, since the atomic mass of Carbon is 12, one mole of Carbon atoms weighs exactly 12 grams.

Concept	Daily Life Equivalent	Chemical Equivalent
Unit Name	Dozen	Mole
Quantity	12 items	6.022 × 10²³ particles
Usage	Counting eggs or bananas	Counting atoms, ions, or molecules

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.24

3. Valency and Formation of Ions (basic)

To understand how chemicals react, we must first look at the "social life" of atoms. Most atoms are inherently unstable and seek a state of equilibrium known as the noble gas configuration. This is a state where the outermost shell of an atom is completely filled with electrons (usually eight, known as an octet). To reach this state of "calm," atoms interact with one another by either losing, gaining, or sharing electrons Science Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59.

Valency is defined as the combining capacity of an atom. It is determined by the number of valence electrons—the electrons present in the atom's outermost shell. For example, if an atom has 1 valence electron, it is much easier to lose that 1 electron than to find 7 more to fill the shell. Conversely, an atom with 7 valence electrons will aggressively try to gain 1 more. This "give and take" is what creates ions.

When atoms gain or lose electrons, they lose their electrical neutrality and become charged particles called ions. There are two primary types:

Feature	Cation	Anion
Charge	Positive (+)	Negative (-)
Formation	Losing electrons (Protons > Electrons)	Gaining electrons (Electrons > Protons)
Typical Element	Metals (e.g., Na⁺, Ca²⁺)	Non-metals (e.g., Cl⁻, O²⁻)

Physical Geography by PMF IAS, Thunderstorm, p.348

When a metal transfers its electrons to a non-metal, they form ionic or electrovalent compounds, such as MgCl₂ or NaCl Science Class X (NCERT 2025 ed.), Metals and Non-metals, p.48. In these compounds, the valency factor (n-factor) is represented by the total positive or negative charge. For instance, in Calcium Chloride (CaCl₂), the calcium atom loses 2 electrons to become Ca²⁺, giving it a valency factor of 2. In Sodium Chloride (NaCl), the sodium atom loses only 1 electron to become Na⁺, resulting in a valency factor of 1.

Sources: Science Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59; Physical Geography by PMF IAS, Thunderstorm, p.348; Science Class X (NCERT 2025 ed.), Metals and Non-metals, p.48

4. Electrolytes and Ionization in Solutions (intermediate)

When we dissolve a substance like common salt (NaCl) in water, it doesn't just disappear; it undergoes a process called ionization or dissociation. In this state, the solid crystal lattice breaks down, and the substance splits into charged particles called ions. Positively charged ions are known as cations (like Na⁺), and negatively charged ions are anions (like Cl⁻). Any substance that produces an electrically conducting solution when dissolved in a polar solvent like water is called an electrolyte. This conductivity is the reason why electrolytes are essential components in a Voltaic cell, where a chemical reaction between electrodes and the electrolyte produces electricity Science, Class VIII (NCERT), Electricity: Magnetic and Heating Effects, p.55.

To understand the quantitative behavior of these chemicals in reactions, we use a concept called Equivalent Weight. While molecular weight tells us the mass of one mole of a molecule, equivalent weight tells us the mass that effectively contributes to a reaction. It is calculated by dividing the Molar Mass (M) by the n-factor (also called the valency factor). The n-factor depends on the nature of the substance:

Substance Type	n-factor Basis	Example
Acids	Basicity (Number of replaceable H⁺ ions)	H₂SO₄ (n=2)
Bases	Acidity (Number of replaceable OH⁻ ions)	NaOH (n=1)
Salts	Total positive or negative charge	CaCl₂ (Ca²⁺ has charge +2, so n=2)

For a salt like Sodium Chloride (NaCl), the total positive charge from the Sodium ion (Na⁺) is 1. Therefore, its n-factor is 1, meaning its equivalent weight is equal to its molecular weight. In contrast, for Sulphuric acid (H₂SO₄), which has two replaceable hydrogen ions, the equivalent weight is exactly half of its molecular weight (98/2 = 49). Understanding these concentrations is vital not just in the lab, but also in environmental science; for instance, the accumulation of soluble salts in soil (salinity) can render fertile agricultural land useless, a common issue in arid regions of Punjab and Haryana Geography of India, Majid Husain, Agriculture, p.67.

Sources: Science, Class VIII (NCERT), Electricity: Magnetic and Heating Effects, p.55; Geography of India, Majid Husain, Agriculture, p.67; Science, Class VIII (NCERT), The Amazing World of Solutes, Solvents, and Solutions, p.136

5. Concentration Terms: Molarity and Normality (intermediate)

In chemistry, understanding how much of a substance is present in a solution is vital, but how we measure that quantity depends on what we want to do with it. Molarity (M) is the most common unit; it measures the moles of solute per liter of solution. It tells us the sheer number of molecules or particles present. However, when we perform reactions like titrations—where we balance the strength of an acid against a base Science, class X, Carbon and its Compounds, p.73—we need to know the 'reactive power' of those particles. This is where Normality (N) becomes essential.

The relationship between the two is governed by the n-factor (also called the valency factor). Think of Molarity as the number of people in a room, and Normality as the total number of hands available to do work. If every person has two hands, the 'Normality' is double the 'Molarity.' Mathematically, we express this as: Normality = Molarity × n-factor. This concept is foundational to Modern Indian Chemistry, a field pioneered by figures like Acharya Prafulla Chandra Ray Science-Class VII, Exploring Substances: Acidic, Basic, and Neutral, p.17, who emphasized the precision required in pharmaceutical and chemical research.

To calculate the n-factor, we look at the specific role of the substance in a reaction:

• Acids: The n-factor is the number of replaceable H⁺ ions (basicity). For H₂SO₄, n = 2; for HCl, n = 1.
• Bases: The n-factor is the number of replaceable OH⁻ ions (acidity). For NaOH, n = 1; for Ca(OH)₂, n = 2.
• Salts: The n-factor is the total positive or negative charge. For example, in CaCl₂, the Calcium ion has a charge of +2, so the n-factor is 2. In NaCl, the charge is +1, so the n-factor is 1.

Feature	Molarity (M)	Normality (N)
Definition	Moles of solute per liter	Gram equivalents per liter
Focus	Number of particles	Reactive capacity (H⁺, OH⁻, or charge)
Formula	M = Moles / Volume (L)	N = Molarity × n-factor

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.73; Science-Class VII . NCERT(Revised ed 2025), Exploring Substances: Acidic, Basic, and Neutral, p.17; Science , class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.14

6. Understanding the n-factor (Valency Factor) (exam-level)

To master stoichiometry and chemical reactions, we must look beyond just how much a molecule weighs and focus on how much 'reactive power' it packs. This is where the n-factor (or valency factor) comes in. Essentially, the n-factor is a numerical value that represents the number of electrons lost or gained, or the number of replaceable hydrogen/hydroxyl ions in a substance. It allows us to calculate the Equivalent Weight of a substance using the formula: Equivalent Weight = Molar Mass / n-factor.

The calculation of the n-factor depends on the nature of the substance:

Substance Type	Criteria for n-factor	Example
Acids	Basicity: Number of replaceable H⁺ ions.	H₂SO₄ has 2 replaceable H⁺ ions, so n = 2.
Bases	Acidity: Number of replaceable OH⁻ ions.	Ca(OH)₂ (slaked lime) has 2 OH⁻ ions, so n = 2.
Salts	Total Charge: Magnitude of total positive or negative charge.	In AlCl₃, the Al³⁺ ion has a charge of +3, so n = 3.

As we see in laboratory experiments Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29, different salts like Sodium Carbonate (Na₂CO₃) or Sodium Chloride (NaCl) behave differently based on their ionic makeup. For a salt like NaCl, the sodium ion carries a +1 charge and the chloride ion a -1 charge; thus, the n-factor is 1. Consequently, for NaCl, the equivalent weight is exactly equal to its molecular weight. However, for Na₂CO₃, the two sodium ions (2 × Na⁺) create a total positive charge of 2, making the n-factor 2 and the equivalent weight half of its molar mass.

Sources: Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.29; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28

7. Concept of Equivalent Weight (exam-level)

In our previous discussions, we explored how the weight of an object is the force with which Earth pulls it Science, Class VIII NCERT, Exploring Forces, p.72. However, in the laboratory, we often need to go beyond simple mass. While we use grams and kilograms for everyday goods Science, Class VIII NCERT, Exploring Forces, p.75, chemistry requires a measurement of reactive capacity. This is where the Equivalent Weight comes in. It is defined as the mass of a substance which combines with or displaces a fixed quantity of another substance (standardized as 1.008 parts of Hydrogen or 8 parts of Oxygen).

The calculation of equivalent weight relies on a bridge called the n-factor (also known as the valency factor). Just as ancient civilizations like the Harappans developed standardized weights for commerce History, class XI (Tamilnadu state board), Early India, p.12, chemists use the n-factor to standardize reactions. The relationship is expressed as:

Equivalent Weight = Molar Mass / n-factor

The n-factor varies depending on the nature of the substance:

• Acids: The n-factor is the basicity (the number of replaceable H⁺ ions). For H₂SO₄, basicity is 2, so the equivalent weight is M/2 (98/2 = 49).
• Bases: The n-factor is the acidity (the number of replaceable OH⁻ ions). For NaOH, it is 1.
• Salts: The n-factor is the total magnitude of positive or negative charge. For example, in NaCl, the charge on Na⁺ is 1, so the equivalent weight equals the molecular weight (M/1).

Understanding this distinction is vital for titration and stoichiometry. For instance, while we can calculate the molecular mass of organic compounds like methanol (CH₃OH) or ethanol (C₂H₅OH) by summing their atomic masses Science, class X NCERT, Carbon and its Compounds, p.67, their equivalent weight in a reaction will depend entirely on how many electrons or ions they exchange during that specific process.

Substance	Type	n-factor Calculation	Equivalent Weight
HCl	Acid	1 replaceable H⁺	M / 1
Ca(OH)₂	Base	2 replaceable OH⁻	M / 2
AlCl₃	Salt	Total positive charge (Al³⁺) = 3	M / 3
Na₂CO₃	Salt	Total positive charge (2 × Na⁺) = 2	M / 2

Sources: Science, Class VIII NCERT, Exploring Forces, p.72, 75; History, class XI (Tamilnadu state board), Early India: From the Beginnings to the Indus Civilisation, p.12; Science, class X NCERT, Carbon and its Compounds, p.67

8. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental concepts of molar mass and valency factors, this question serves as the perfect application of the core relationship: Equivalent Weight = Molecular Weight / n-factor. To identify a compound where these two values are identical, you are essentially hunting for a substance with an n-factor of 1. This requires you to integrate your knowledge of chemical bonding and ionic dissociation to determine the total charge or replaceable ions for each species provided.

Let’s walk through the reasoning for the correct choice, NaCl (Sodium Chloride). As a salt, its n-factor is determined by the total magnitude of the positive or negative charge. When NaCl dissociates, it yields one Na⁺ ion and one Cl^- ion, meaning the total positive charge is exactly 1. By applying our formula, Molecular Weight / 1, we find that the values are equal, making Option (D) the definitive answer. This step-by-step breakdown from formula to ionic state is a vital skill for the UPSC General Science section.

It is equally important to recognize the traps in the other options. For instance, H₂SO₄ is a dibasic acid with an n-factor of 2, meaning its equivalent weight is only half of its molecular weight (98/2 = 49). Similarly, CaCl₂ features a Calcium ion with a 2+ charge, also resulting in an n-factor of 2. UPSC often includes typographical errors like "Na^C^" to test your composure; however, by focusing on the valency factor as explained in mlsu.ac.in/econtents/1844_SPOT%20I.pdf, you can confidently eliminate incorrect distractors and focus on the underlying chemical principles.