What Is the Molar Mass of Urea (NH2CONH2)?

Urea molecular formula: NH2CONH2 (or CH4N2O)

Structural formula: H2N — C(=O) — NH2

IUPAC name: Diaminomethanone (or carbamide) Common name: Urea

Urea has a planar structure. The central carbon is doubly bonded to oxygen (carbonyl group, C=O) and singly bonded to two nitrogen atoms, each bearing two hydrogen atoms. The nitrogen atoms have partial double bond character due to resonance with the carbonyl group, making all N-C bonds equivalent. Urea is classified as an amide (diamide of carbonic acid).

Molecular formula of urea: CH4N2O

Step-by-step calculation:

• Carbon (C): 1 atom × 12 g/mol = 12 g/mol
• Hydrogen (H): 4 atoms × 1 g/mol = 4 g/mol
• Nitrogen (N): 2 atoms × 14 g/mol = 28 g/mol
• Oxygen (O): 1 atom × 16 g/mol = 16 g/mol

Total molar mass = 12 + 4 + 28 + 16 = 60 g/mol

Atomic masses used: C = 12, H = 1, N = 14, O = 16.

Counting atoms in NH2CONH2:

• N: 2 (one in each NH2 group)
• H: 4 (two in each NH2 group)
• C: 1
• O: 1 (the C=O) Confirming CH4N2O → molar mass 60 g/mol.

Physical properties of urea:

• Appearance: White crystalline solid
• Odour: Odourless
• Melting point: 133°C
• Solubility: Highly soluble in water (108 g/100 mL at 20°C) and polar solvents
• Density: 1.32 g/cm³
• Nitrogen content: 46.7% by mass (the highest of any solid nitrogen fertiliser)
• Aqueous solution: Slightly basic (can hydrolyse to release NH3)

Urea is hygroscopic (absorbs moisture from air) and forms hydrogen bonds readily due to its NH2 and C=O groups.

Key chemical properties of urea:

1. Hydrolysis: Urea hydrolyses in the presence of the enzyme urease or in acidic/basic conditions: NH2CONH2 + H2O → CO2 + 2NH3 This is how urea is broken down in soil and the body.

2. Wöhler Synthesis (1828): Urea was first synthesised by heating ammonium cyanate: NH4OCN → NH2CONH2 This was the first synthesis of an organic compound from an inorganic salt, disproving the vitalist theory.

3. Biuret reaction: Urea reacts with NaOH and CuSO4 (Biuret reagent) to give a violet colour if biuret is present (formed by condensation of two urea molecules at high temperature).

4. Reaction with formaldehyde: Forms urea-formaldehyde resins used in plastics and adhesives.

Urea is one of the most important industrial chemicals:

1. Fertiliser: The largest use — urea is the most widely used nitrogen fertiliser globally, providing 46.7% nitrogen by weight. Applied to soil, it hydrolyses to give ammonium and nitrate ions used by plants.

2. Urea-formaldehyde resins: Used in wood adhesives (plywood, MDF), foam insulation, and moulded plastics.

3. Animal feed: Used as a protein supplement (non-protein nitrogen source) in ruminant animal diets.

4. Pharmaceuticals: Used in skin creams and lotions as a moisturiser and keratolytic agent (softens hardened skin).

5. Automotive industry: Used in SCR (Selective Catalytic Reduction) systems as AdBlue — injected into diesel exhaust to convert NOx into N2 and H2O, reducing air pollution.

6. Biological importance: Urea is the primary nitrogenous waste product in mammals. The liver converts ammonia (from amino acid metabolism) to urea via the urea cycle, which is then excreted by the kidneys in urine.