Which one of the following contains maximum percentage of nitrogen by mass?

1. Atomic Mass and Molar Mass Foundations (basic)

To understand the composition of any substance—whether it is the carbon compounds in our bodies or the fertilizers used in our fields—we must first learn how to measure the mass of atoms and molecules. Since individual atoms are far too small to weigh on a standard scale, scientists use a relative mass scale. The standard reference for this scale is the Carbon-12 atom. The mass of an atom is measured in Unified Atomic Mass Units (u).

When atoms join together to form molecules, we calculate the Molecular Mass by simply adding up the atomic masses of all the atoms present in that molecule. For instance, as highlighted in Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66, the atomic mass of Carbon (C) is 12 u and Hydrogen (H) is 1 u. Therefore, a molecule like Methane (CH₄) has a molecular mass of 16 u (12 + 4×1). This foundational math allows us to compare different substances by their weight and density.

In practical chemistry, we use the Mole concept to bridge the gap between the microscopic world of atoms and the macroscopic world of grams. The Molar Mass is defined as the mass of one mole (6.022 × 10²³ particles) of a substance. The beauty of this system is that the numerical value of the molar mass in grams per mole (g/mol) is identical to the molecular mass in units (u). For example, if Water (H₂O) has a molecular mass of 18 u, its molar mass is exactly 18 g/mol. This conversion is crucial for calculating the percentage of specific elements, such as nitrogen, within a larger compound.

Element	Symbol	Approx. Atomic Mass (u)
Hydrogen	H	1
Carbon	C	12
Nitrogen	N	14
Oxygen	O	16

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.66; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59

2. Mass Percentage Composition of Elements (basic)

When we study chemistry, we often need to know more than just which elements are in a compound; we need to know how much of each element is present. This is where Mass Percentage Composition comes in. Every pure chemical compound has a fixed composition by mass, meaning the elements are always present in the same proportions regardless of the source of the sample Science, Class VIII (NCERT), Nature of Matter: Elements, Compounds, and Mixtures, p.131. If you change these proportions, you are no longer dealing with the same substance.

To calculate the mass percentage of an element, we use a simple two-step process. First, we determine the molar mass of the entire compound by adding up the atomic masses of all the atoms present in its chemical formula Science, Class X (NCERT), Carbon and its Compounds, p.67. Second, we divide the total mass of the specific element we are interested in by the molar mass of the whole compound and multiply by 100. The formula looks like this:

Mass % of element = (Mass of that element in 1 mole of compound / Molar mass of the compound) × 100

Consider Water (H₂O) as an example. The molar mass is approximately 18 g/mol (2 g from Hydrogen + 16 g from Oxygen). To find the mass percentage of Oxygen, we take its mass (16 g) divided by the total mass (18 g), which gives us roughly 88.8%. This tells us that in any given mass of pure water, nearly 89% of that mass is contributed by oxygen atoms. This concept is vital for scientists and engineers when they need to check the purity of a sample or determine the efficiency of a fertilizer or an ore.

Sources: Science, Class VIII (NCERT), Nature of Matter: Elements, Compounds, and Mixtures, p.131; Science, Class X (NCERT), Carbon and its Compounds, p.67

3. Nitrogen: A Vital Macro-Nutrient for Plants (basic)

In the world of plant biology, nutrients are the essential chemical elements required for growth and reproduction. These are broadly categorized into two groups based on the quantity required: macro-nutrients (needed in large amounts) and micro-nutrients (needed in trace amounts). Nitrogen (N) stands as the most critical macro-nutrient. Along with Phosphorus (P) and Potassium (K), it forms the "NPK" trio that drives global agriculture. As noted in Indian Economy, Nitin Singhania, Agriculture, p.302, while there are six primary macro-nutrients (including Calcium, Magnesium, and Sulphur), Nitrogen is the primary engine of plant vitality.

The paradox of Nitrogen is that although it makes up about 78% of our atmosphere, most living organisms cannot use it in its gaseous form (N₂). It must be "fixed" into a usable form like nitrates or ammonia. In nature, this is achieved by nitrogen-fixing bacteria often found in the root nodules of legumes like beans and pulses Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.20. Biologically, Nitrogen is indispensable because it is a fundamental building block of amino acids, which form proteins, and is a key component of chlorophyll, the molecule responsible for photosynthesis.

To support high-yield farming, we supplement soil with chemical fertilizers. These are industrially manufactured substances that release nutrients almost immediately Environment, Shankar IAS Academy, Agriculture, p.363. Among these, Urea (NH₂CONH₂) is the most widely used solid nitrogenous fertilizer. It is highly valued because of its high nitrogen concentration—approximately 46.67% by mass. Recently, technology has evolved toward Liquid Nano Urea, which uses nanoparticles to increase nutrient absorption efficiency to over 80%, compared to just 25% for conventional granular urea Indian Economy, Vivek Singh, Subsidies, p.289.

Nutrient Category	Examples	Role in Plant
Macro-nutrients	Nitrogen (N), Phosphorus (P), Potassium (K)	Structural growth, energy transfer, protein synthesis.
Micro-nutrients	Iron (Fe), Zinc (Zn), Boron (B), Copper (Cu)	Enzyme activation, metabolic catalysts.

Sources: Indian Economy, Nitin Singhania, Agriculture, p.302; Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.20; Environment, Shankar IAS Academy, Agriculture, p.363; Indian Economy, Vivek Singh, Subsidies, p.289

4. Classification of Nitrogenous Fertilizers (intermediate)

Nitrogen is a critical macro-nutrient required by plants for the synthesis of chlorophyll, amino acids, and proteins Indian Economy, Nitin Singhania, Agriculture, p.302. While our atmosphere is nearly 78% nitrogen, plants cannot absorb it directly in its elemental form (N₂). Instead, they rely on nitrogenous fertilizers which provide nitrogen in forms that are chemically "fixed" and bioavailable.

We classify these fertilizers based on the specific chemical form in which the nitrogen atoms are present. This classification is vital because it determines how quickly the plant can absorb the nutrient and how easily the fertilizer might be lost to the environment through leaching (washing away by water) or denitrification (conversion back to gas by bacteria) Environment, Shankar IAS Academy, Functions of an Ecosystem, p.20.

Class	Form of Nitrogen	Characteristics	Examples
Nitrate Fertilizers	Nitrate ion (NO₃⁻)	Highly mobile in soil; ready for immediate plant uptake; prone to leaching.	Sodium Nitrate, Calcium Nitrate
Ammoniacal Fertilizers	Ammonium ion (NH₄⁺)	Resistant to leaching as they bind to soil particles; slower availability.	Ammonium Sulphate, Ammonium Chloride
Ammoniacal & Nitrate	Both NH₄⁺ and NO₃⁻	Provides both immediate and sustained nitrogen supply.	Ammonium Nitrate, Calcium Ammonium Nitrate (CAN)
Amide Fertilizers	Amide group (-NH₂)	Organic-based; must be converted to ammonium by soil microbes before uptake.	Urea [NH₂CONH₂]

In terms of efficiency and concentration, we often calculate the Nitrogen Percentage by mass. This is done by taking the total atomic mass of nitrogen in one molecule and dividing it by the total molar mass of the compound. For instance, Urea is highly favored in agriculture because it is an amide fertilizer with a very high nitrogen concentration—approximately 46% by weight—making it cost-effective for transport and application Environment, Shankar IAS Academy, Agriculture, p.362.

Sources: Indian Economy, Nitin Singhania, Agriculture, p.302; Environment, Shankar IAS Academy, Functions of an Ecosystem, p.20; Environment, Shankar IAS Academy, Agriculture, p.362

5. The Chemistry and Economics of Urea (intermediate)

At its heart, Urea (NH₂CONH₂) is a simple organic compound that serves as the world’s most important nitrogenous fertilizer. From a chemical perspective, urea is highly valued because of its high nitrogen content. To calculate this, we look at its molar mass: Carbon (12), Oxygen (16), two Nitrogens (28), and four Hydrogens (4) sum up to 60 g/mol. Since nitrogen accounts for 28g of that total, urea consists of approximately 46.67% nitrogen. While some other compounds like ammonium cyanide have a higher theoretical percentage, urea remains the gold standard for agriculture because it is stable, solid, and safe to handle. As we see in fundamental chemistry, atoms like nitrogen and carbon achieve stability through covalent bonding to reach a noble gas configuration Science Class X NCERT, Carbon and its Compounds, p.60.

However, conventional urea faces two major challenges: environmental leaching and economic diversion. When farmers apply standard urea, much of it dissolves too quickly, washing away into groundwater (leaching) or escaping into the atmosphere as gas. To solve this, the Government of India mandated Neem Coated Urea (NCU). The neem oil acts as a natural nitrification inhibitor, slowing down the rate at which nitrogen is released into the soil Nitin Singhania, Agriculture, p.361. Economically, this coating makes urea unfit for industrial use (like in plywood or explosives), ensuring that subsidized fertilizer actually reaches the farmers' fields rather than being diverted to factories Vivek Singh, Subsidies, p.288.

The latest frontier in this field is Liquid Nano Urea. Unlike bulky 50kg bags of conventional urea, Nano Urea comes in small bottles and contains nitrogen in the form of nanoparticles (20-50 nm). While it contains only 4% nitrogen by volume, its efficiency is a staggering 85-90%, compared to just 25% for conventional urea Vivek Singh, Subsidies, p.289. This is because the tiny particles can enter the plant directly through the stomata (pores) on leaves and are stored in the vacuoles for slow release, minimizing waste and environmental damage.

Feature	Conventional Urea	Liquid Nano Urea
Nitrogen Efficiency	Low (~25%)	High (85-90%)
Application	Soil application (spreading)	Foliar spray (on leaves)
Environmental Impact	High leaching and gas loss	Minimal waste; stored in vacuoles

Sources: Science Class X NCERT, Carbon and its Compounds, p.60; Indian Economy, Nitin Singhania, Agriculture, p.361; Indian Economy, Vivek Singh, Subsidies, p.288-289

6. Comparing Nitrogen Content in Chemical Compounds (exam-level)

Nitrogen is the most abundant gas in our atmosphere, making up roughly 78.08% of the air by volume Physical Geography by PMF IAS, Earths Atmosphere, p.271. However, in its gaseous form (N₂), it is relatively inert and cannot be used directly by plants. It must be "fixed" into compounds like ammonia, nitrites, or nitrates to become a building block for proteins, which constitute nearly 16% by weight of all living tissue Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19. In the context of agriculture and chemistry, we often need to compare how "rich" different compounds are in nitrogen to determine their efficiency as fertilizers. To compare the nitrogen content of different chemical compounds, we use the principle of percentage composition by mass. This is calculated using the formula: (Total mass of Nitrogen atoms / Total Molar Mass of the compound) × 100. For example, in Urea [NH₂CONH₂], there are two nitrogen atoms. Since the atomic mass of nitrogen is approximately 14, the total mass of nitrogen in one mole of urea is 28g. Given that the total molar mass of Urea is 60g/mol, the nitrogen content is (28/60) × 100, which is approximately 46.67%. This high concentration makes Urea the most widely used solid nitrogenous fertilizer in India Indian Economy, Vivek Singh, Subsidies, p.287. While some compounds like Ammonium Cyanide (NH₄CN) may have a higher theoretical percentage of nitrogen (approx. 63.6%), they are often unstable or toxic, making them unsuitable for practical use. For UPSC purposes, we focus on stable, common organic and inorganic compounds. Comparing Urea to other fertilizers highlights why it is a staple in the N:P:K (Nitrogen, Phosphorus, Potassium) ratio required for soil health Indian Economy, Vivek Singh, Subsidies, p.287.

Compound	Formula	Total N Mass / Molar Mass	Approx. N Content (%)
Urea	NH₂CONH₂	28 / 60	46.7%
Ammonium Nitrate	NH₄NO₃	28 / 80	35.0%
Ammonium Carbonate	(NH₄)₂CO₃	28 / 96	29.2%
Ammonium Sulfate	(NH₄)₂SO₄	28 / 132	21.2%

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.271; Environment, Shankar IAS Academy, Functions of an Ecosystem, p.19; Indian Economy, Vivek Singh, Subsidies, p.287

7. Solving the Original PYQ (exam-level)

Now that you have mastered the concepts of atomic mass and molecular weight, this question provides the perfect opportunity to apply those building blocks through percentage composition. In chemistry, the 'richness' of an element in a compound isn't just about how many atoms are present, but the ratio of that element's mass to the total mass of the molecule. To solve this, you must recall or derive the chemical formulas, calculate the total molar mass for each, and then divide the mass of the nitrogen atoms by that total. This analytical process is a staple of the NCERT Class 11 Chemistry curriculum, which serves as the foundation for UPSC General Science.

Walking through the reasoning, we see that Urea (NH₂CONH₂) has a molar mass of 60 g/mol and contains two nitrogen atoms (28g), yielding a high concentration of approximately 46.67%. When you compare this to Ammonium nitrate or Ammonium carbonate, you'll notice that the presence of more oxygen and carbon atoms increases the denominator of our fraction, effectively diluting the nitrogen percentage to 35% and 29% respectively. Therefore, among stable, common substances, (A) Urea is the most nitrogen-dense option, which is exactly why it is the most preferred solid nitrogenous fertilizer in Indian agriculture.

A common trap in this question is Ammonium cyanide. While a purely mathematical calculation suggests it has a higher nitrogen percentage (roughly 63%), it is a highly unstable and toxic compound. UPSC often frames these questions within the practical context of General Science; they are looking for the most nitrogen-rich compound among those used in standard industrial or agricultural applications. By focusing on Urea, you are choosing the compound that is both mathematically high and chemically stable, which is the hallmark of a well-prepared candidate who understands applied chemistry over abstract numbers.