Which one among the following nitrogen compounds has the least percentage of nitrogen by mass?

1. Atomic Mass and Molecular Mass Foundations (basic)

To understand how chemicals react and combine, we must first grasp the concept of Atomic Mass and Molecular Mass. Think of atoms as the fundamental building blocks of matter. While some elements like gold or iron can exist as individual atoms, others like hydrogen, oxygen, and nitrogen are more stable when they bond together to form molecules Science, Class VIII, Particulate Nature of Matter, p.115. For instance, a molecule of nitrogen (N₂) consists of two nitrogen atoms held together by a strong triple bond Science, Class X, Carbon and its Compounds, p.60. Each of these atoms has a specific mass, and the sum of these masses gives us the weight of the entire molecule.

The Molecular Mass of a substance is simply the sum of the atomic masses of all the atoms present in its chemical formula. To calculate this, you need to know the 'atomic weight' of each element involved. For example, in a molecule of water (H₂O), we have two hydrogen atoms (atomic mass ≈ 1 each) and one oxygen atom (atomic mass ≈ 16). Thus, the molecular mass of water is (2 × 1) + 16 = 18 units. When we look at more complex substances, like Carbon dioxide (CO₂), we follow the same logic: one carbon atom (12) plus two oxygen atoms (16 × 2), totaling 44 Science, Class X, Carbon and its Compounds, p.61.

Once we know the total molecular mass, we can determine the Mass Percentage of any specific element within that compound. This is a crucial concept in chemistry and industry—it tells us how 'rich' a compound is in a particular nutrient or element. The formula is straightforward: divide the total mass of the specific element by the total molecular mass of the compound, then multiply by 100. This calculation allows us to compare different fertilizers or ores to see which one provides the most of a desired element, such as Nitrogen, for the least amount of total weight.

Sources: Science, Class VIII (NCERT), Particulate Nature of Matter, p.115; Science, Class X (NCERT), Carbon and its Compounds, p.60-61

2. Chemical Formulas and Polyatomic Ions (basic)

In our journey through chemistry, we often encounter groups of atoms that stay together as a single unit, carrying an overall electrical charge. These are known as polyatomic ions. Think of them as "atomic teams"—they bond so tightly that they participate in chemical reactions without breaking apart. A common example is the Ammonium ion (NH₄⁺), which consists of one nitrogen atom and four hydrogen atoms, but functions as a single positive unit Environment, Shankar IAS Academy (10th Ed.), Functions of an Ecosystem, p.20.

To write a chemical formula correctly, the most important rule is electrical neutrality: the total positive charge must exactly balance the total negative charge. We use subscripts to indicate how many of each ion are needed to reach this balance. When we need more than one polyatomic ion in a formula, we must wrap the ion in parentheses before adding the subscript. For example, in Magnesium hydroxide [Mg(OH)₂], the parentheses tell us there are two "Hydroxide teams" for every one Magnesium ion Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.18.

A helpful way to organize these is by their charges. Most polyatomic ions are anions (negatively charged), like Nitrate or Sulphate, while Ammonium is a rare, common cation (positively charged) Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28.

Ion Name	Formula	Charge
Ammonium	NH₄⁺	+1
Hydroxide	OH⁻	-1
Nitrate	NO₃⁻	-1
Carbonate	CO₃²⁻	-2
Sulphate	SO₄²⁻	-2
Phosphate	PO₄³⁻	-3

Sources: Environment, Shankar IAS Academy (10th Ed.), Functions of an Ecosystem, p.20; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.18; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.28

3. The Mole Concept and Molar Mass (intermediate)

To understand the chemistry of the world around us, we need a bridge between the microscopic world of atoms and the macroscopic world of the laboratory. That bridge is the Mole. In simple terms, a mole is just a specific number — 6.022 × 10²³ — known as Avogadro’s Number. Just as a 'dozen' always means 12, a 'mole' of any substance always contains exactly this many particles. The beauty of this concept is that it allows us to weigh out substances on a scale and know exactly how many atoms or molecules we are handling.

The Molar Mass of a substance is the mass of one mole of that substance, expressed in grams per mole (g/mol). Numerically, it is equal to the molecular mass of the substance. For instance, we calculate the molecular mass by adding the atomic masses of all atoms in a formula. As noted in Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66, the atomic mass of Carbon is 12 u and Hydrogen is 1 u. Therefore, the molar mass of Methane (CH₄) is 12 + (4 × 1) = 16 g/mol. This means if you weigh 16 grams of methane, you have exactly one mole of methane molecules.

This concept becomes incredibly practical when we look at Mass Percentage. This tells us what fraction of a compound's total mass comes from a specific element. For example, in a homologous series where compounds differ by a –CH₂– unit, the molar mass increases by exactly 14 g/mol (12 for Carbon + 2 for Hydrogen) with each step Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.67. By knowing the total molar mass and the mass contributed by a specific element, we can calculate the percentage composition: (Total mass of element in 1 mole / Molar mass of compound) × 100. This is how scientists determine the purity of samples or the nutrient value of fertilizers.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.67

4. Chemistry in Agriculture: Nitrogenous Fertilizers (exam-level)

In the world of agriculture, Nitrogen (N) is the most critical nutrient because it is a fundamental building block of amino acids, proteins, and chlorophyll. While the atmosphere is 78% nitrogen, plants cannot 'breathe' it in; they require it in a fixed chemical form, typically as ammonium (NH₄⁺) or nitrate (NO₃⁻) ions. This is where nitrogenous fertilizers come in. In the Indian context, Urea is the 'king' of fertilizers, accounting for nearly 74% of total fertilizer consumption Indian Economy, Nitin Singhania, Agriculture, p.303. It is preferred because of its high nitrogen content and the fact that its prices are regulated by the government to keep it affordable for farmers Indian Economy, Vivek Singh, Subsidies, p.287. To understand the 'strength' of a fertilizer, we look at its Nitrogen mass percentage. This is calculated by taking the total mass of nitrogen atoms in a molecule and dividing it by the molecule's total molar mass. For example, Ammonia (NH₃) has a very high nitrogen density (about 82%) because the molecule is light and mostly made of nitrogen. In contrast, Ammonium Phosphate ((NH₄)₃PO₄), despite having three nitrogen atoms, has a lower nitrogen percentage (around 28%) because the phosphate group (PO₄) is very heavy, 'diluting' the relative mass of the nitrogen. This chemical logic explains why different fertilizers are used for different purposes: urea provides a concentrated nitrogen burst, while complex fertilizers like Diammonium Phosphate (DAP) provide a balanced mix of Nitrogen and Phosphorus. A significant modern breakthrough in this field is Liquid Nano Urea. Unlike conventional granular urea, which has an efficiency of only about 25%, Nano Urea uses nanoparticles (20-50 nm) that can enter the plant directly through the stomata (pores on leaves). This increases nutrient efficiency to a staggering 85-90% Indian Economy, Vivek Singh, Subsidies, p.289. This is crucial because unutilized nitrogen from conventional fertilizers often leaches into groundwater or escapes into the atmosphere as a greenhouse gas, causing environmental degradation.

Fertilizer Type	Approx. Nitrogen %	Key Characteristic
Anhydrous Ammonia	~82%	Highest N-content; gas/liquid form.
Urea	~46%	Most common solid fertilizer; regulated price.
Ammonium Nitrate	~33-35%	Fast-acting; provides both NH₄ and NO₃.
Ammonium Phosphate	~11-28%	Used when both N and P (Phosphorus) are needed.

Sources: Indian Economy by Nitin Singhania, Agriculture, p.303; Indian Economy by Vivek Singh, Subsidies, p.287, 289

5. Environmental Impact of Nitrogen Compounds (intermediate)

Nitrogen is a fundamental building block of life, essential for the synthesis of proteins and DNA. While it makes up approximately 78% of our atmosphere, this gaseous nitrogen (N₂) is chemically inert and inaccessible to most organisms Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.20. However, when humans introduce reactive nitrogen into the environment through synthetic fertilizers (like ammonium nitrate or urea) and sewage, it triggers a cascade of chemical and biological changes known as the Nitrogen Cascade.

One of the most critical chemical transformations occurs in the soil and water through Nitrification. In this process, specialized bacteria like Nitrosomonas convert ammonia (NH₃) into nitrites (NO₂⁻), which are then further oxidized by Nitrobacter into nitrates (NO₃⁻) Environment, Shankar IAS Academy, Functions of an Ecosystem, p.20. While plants readily absorb nitrates, these ions are highly soluble and easily leach into groundwater or wash into water bodies. This leads to Eutrophication—a state of nutrient over-enrichment that causes massive plankton blooms. When these blooms eventually die and sink, bacteria decompose them, consuming nearly all dissolved oxygen in the process. This creates "dead zones" where aquatic life cannot survive Environment, Shankar IAS Academy, Ocean Acidification, p.264.

The impact of nitrogen compounds varies significantly depending on the ecosystem's state, as summarized below:

Parameter	Oligotrophic (Low Nutrient)	Eutrophic (High Nutrient)
Nutrient Flux	Low	High (Excess Nitrogen/Phosphorus)
Oxygen (Bottom Layer)	Present	Absent (Hypoxia)
Water Quality	Good	Poor (Harmful Algal Blooms)

Environment, Shankar IAS Academy, Aquatic Ecosystem, p.36

Beyond the ecosystem, high concentrations of Nitrates in drinking water pose direct risks to human health. They can interfere with the blood's ability to carry oxygen (Methemoglobinemia) and have been linked to long-term issues like stomach cancer due to their conversion into carcinogenic compounds in the digestive tract Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.36.

Sources: Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.20; Environment, Shankar IAS Academy, Functions of an Ecosystem, p.20; Environment, Shankar IAS Academy, Ocean Acidification, p.264; Environment, Shankar IAS Academy, Aquatic Ecosystem, p.36; Environment and Ecology, Majid Hussain, Environmental Degradation and Management, p.36

6. Mass Percentage Composition of Elements (exam-level)

When we look at a chemical formula like H₂O or NH₃, we are seeing the ratio of atoms. However, in practical fields like agriculture or pharmacology, we need to know the Mass Percentage Composition—essentially, how much of the total weight of a substance is contributed by a specific element. This is crucial because, as we've learned, chemical equations must be balanced because mass is conserved and fixed within specific compounds Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3.

To calculate the mass percentage of an element, you follow a simple logic: take the total mass of that element in one mole of the compound and divide it by the total molar mass of the compound. For example, in the environment, we know that nitrogen constitutes nearly 16% by weight of all proteins Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19. This "16%" is a mass percentage that helps scientists estimate protein levels by measuring nitrogen content.

A common pitfall is assuming that more atoms of an element automatically mean a higher mass percentage. This isn't always true! If an element is paired with very heavy "partner" atoms (like Phosphorus or Oxygen), its percentage might actually drop. Look at the comparison below to see how the "weight" of the rest of the molecule affects the percentage of the element we want:

Compound	Element of Interest	Mass of Element	Total Molar Mass	Mass %
Methane (CH₄)	Carbon (C)	~12 g	~16 g	75.0%
Carbon Dioxide (CO₂)	Carbon (C)	~12 g	~44 g	27.3%

Even though both molecules have exactly one carbon atom, the carbon in CO₂ is "diluted" by the much heavier oxygen atoms Physical Geography by PMF IAS, Earths Atmosphere, p.271. This principle is why industrial fertilizers are rated by their nitrogen percentage; a large, complex molecule might have three nitrogen atoms but still be less efficient by weight than a tiny molecule with just one.

Sources: Science, class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.3; Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19; Physical Geography by PMF IAS, Earths Atmosphere, p.271

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of atomic weight and molar mass, this question challenges you to apply proportional reasoning to chemical formulas. The percentage of an element is not just about how many atoms are present, but the ratio of that element's weight to the total weight of the molecule. In this case, even though (NH4)3PO4 has three nitrogen atoms, it is weighed down by the massive phosphate (PO4) group. Phosphorus and four oxygen atoms add a tremendous amount of "molecular baggage," which increases the denominator of your fraction significantly, resulting in a nitrogen percentage of only about 28.2%. Therefore, (A) (NH4)3PO4 is the correct answer.

UPSC often sets traps by including options like NH3 (Ammonia), which actually has the highest percentage of nitrogen (~82.3%) because it is paired with the lightest element, Hydrogen. A common mistake is to pick NH4NO3 because it contains two nitrogen atoms, but the weight of three oxygen atoms keeps its nitrogen content around 35%. As highlighted in NCERT Class 11 Chemistry - Basic Concepts of Chemistry, the key to solving these without tedious calculation is to look for the compound with the heaviest non-nitrogen components. The phosphate group in option A is much heavier than the hydroxide in NH4OH or the lone hydrogens in NH3, making it the clear winner for the "least percentage" category.