Basic Chemical Formulas
| Formula Type | Formula | Name/Description | Explanation |
|---|---|---|---|
| Molecular Formula | NH₃ | Ammonia | Shows the exact number of atoms: 1 nitrogen atom bonded to 3 hydrogen atoms |
| Empirical Formula | NH₃ | Ammonia | Simplest whole number ratio of atoms (same as molecular in this case) |
| Structural Formula | H-N-H with lone pair | Ammonia structure | Shows how atoms are connected; nitrogen has one lone pair of electrons |
| Condensed Formula | NH₃ | Ammonia | Simplified representation showing atom types and quantities |
Ammonia in Aqueous Solutions
| Formula Type | Formula | Name/Description | Explanation |
|---|---|---|---|
| Ammonia in Water | NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ | Base dissociation | Ammonia acts as a weak base, accepting protons from water |
| Ammonium Hydroxide | NH₄OH | Ammonium hydroxide | Traditional representation of ammonia solution (though NH₃·H₂O is more accurate) |
| Ammonium Ion | NH₄⁺ | Ammonium cation | Formed when ammonia accepts a proton (H⁺) |
| Hydroxide Ion | OH⁻ | Hydroxide anion | Released when ammonia acts as a base |
Equilibrium Constants and pH Calculations
| Formula Type | Formula | Name/Description | Explanation |
|---|---|---|---|
| Kb Expression | Kb = [NH₄⁺][OH⁻]/[NH₃] | Base dissociation constant | Measures strength of ammonia as a base; Kb = 1.8 × 10⁻⁵ at 25°C |
| Ka Expression | Ka = [NH₃][H₃O⁺]/[NH₄⁺] | Acid dissociation constant | For ammonium ion acting as acid; Ka = 5.6 × 10⁻¹⁰ at 25°C |
| pKb Formula | pKb = -log(Kb) | Negative logarithm of Kb | pKb = 4.74 for ammonia at 25°C |
| pKa Formula | pKa = -log(Ka) | Negative logarithm of Ka | pKa = 9.25 for ammonium ion at 25°C |
| Kw Relationship | Ka × Kb = Kw = 1.0 × 10⁻¹⁴ | Ion product of water | Relationship between Ka and Kb for conjugate acid-base pairs |
pH and pOH Calculations
| Formula Type | Formula | Name/Description | Explanation |
|---|---|---|---|
| pH of NH₃ Solution | pH = 14 – pOH | pH calculation | After finding pOH from Kb expression |
| pOH from Kb | pOH = ½(pKb – log C) | Weak base approximation | C is the initial concentration of NH₃ |
| Henderson-Hasalbalch | pH = pKa + log([NH₃]/[NH₄⁺]) | Buffer equation | For NH₃/NH₄⁺ buffer systems |
| Ion Concentration | [OH⁻] = √(Kb × C) | Hydroxide concentration | Approximation for weak base solutions |
Industrial and Synthesis Formulas
| Formula Type | Formula | Name/Description | Explanation |
|---|---|---|---|
| Haber Process | N₂ + 3H₂ ⇌ 2NH₃ | Industrial synthesis | High temperature and pressure synthesis of ammonia |
| Catalyzed Haber | N₂ + 3H₂ → 2NH₃ (with Fe catalyst) | Catalyzed synthesis | Iron catalyst increases reaction rate |
| Enthalpy Change | ΔH = -92.4 kJ/mol | Heat of formation | Energy released when forming NH₃ from elements |
Combustion and Decomposition
| Formula Type | Formula | Name/Description | Explanation |
|---|---|---|---|
| Complete Combustion | 4NH₃ + 3O₂ → 2N₂ + 6H₂O | Burning in limited oxygen | Produces nitrogen gas and water |
| Catalytic Oxidation | 4NH₃ + 5O₂ → 4NO + 6H₂O | Ostwald process | First step in nitric acid production |
| Thermal Decomposition | 2NH₃ → N₂ + 3H₂ | High temperature breakdown | Reverse of Haber process |
Physical Property Formulas
| Formula Type | Formula | Name/Description | Explanation |
|---|---|---|---|
| Molar Mass | M = 17.03 g/mol | Molecular weight | Sum of atomic masses: N(14.01) + 3H(1.008) |
| Density (gas, STP) | ρ = 0.771 g/L | Gas density | At standard temperature and pressure |
| Density (liquid) | ρ = 0.682 g/cm³ | Liquid density | At -33.34°C (boiling point) |
| Ideal Gas Law | PV = nRT | Gas behavior | Where n = moles of NH₃ |
Concentration and Molarity
| Formula Type | Formula | Name/Description | Explanation |
|---|---|---|---|
| Molarity | M = moles NH₃ / L solution | Molar concentration | Standard concentration unit |
| Mass Percentage | % = (mass NH₃ / total mass) × 100 | Weight percentage | Common for aqueous solutions |
| ppm Conversion | ppm = (mg NH₃ / L solution) | Parts per million | For dilute environmental samples |
| Normality | N = M × 1 | Normal concentration | For NH₃, normality equals molarity (monoprotic base) |
Key Constants and Values
| Parameter | Value | Units | Conditions |
|---|---|---|---|
| Kb (Base constant) | 1.8 × 10⁻⁵ | – | 25°C |
| pKb | 4.74 | – | 25°C |
| Boiling Point | -33.34 | °C | 1 atm |
| Melting Point | -77.73 | °C | 1 atm |
| Critical Temperature | 132.25 | °C | – |
| Bond Angle (H-N-H) | 106.67 | degrees | Gas phase |
Tips for Students
Understanding Ammonia Formulas:
- Start with the basic NH₃ structure – remember the pyramidal shape due to the lone pair
- Master the equilibrium expressions – Kb and Ka are fundamental for calculations
- Practice pH calculations – use the relationship between Ka, Kb, and Kw
- Learn the Haber process – essential for understanding industrial chemistry
- Connect formulas to applications – fertilizers, cleaning products, refrigeration
Formula Relationships to Remember:
- Ka × Kb = Kw (fundamental relationship)
- pH + pOH = 14 (at 25°C)
- pKa + pKb = 14 (for conjugate pairs)
Frequently Asked Questions (FAQs)
Q. What is the chemical formula of ammonia?
The chemical formula of ammonia is NH₃, which means one nitrogen atom is covalently bonded to three hydrogen atoms. Ammonia has a trigonal pyramidal molecular geometry with a bond angle of approximately 107°. The nitrogen atom has one lone pair of electrons, making ammonia a polar molecule and a weak base. At room temperature, ammonia exists as a colorless gas with a pungent odor.
Q. What is the difference between ammonia (NH₃) and ammonium (NH₄⁺)?
Ammonia (NH₃) is a neutral molecule and acts as a weak base in water. Ammonium (NH₄⁺) is the positively charged ion formed when ammonia accepts a proton (H⁺). The key differences are:
- NH₃ has 3 hydrogen atoms; NH₄⁺ has 4 hydrogen atoms
- NH₃ is neutral; NH₄⁺ carries a +1 charge
- NH₃ is a base; NH₄⁺ is a weak acid
- NH₃ exists as a gas; NH₄⁺ only exists in solution or ionic compounds
The equilibrium reaction is: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
Q. How do you calculate the pH of an ammonia solution?
To calculate the pH of an ammonia solution, follow these steps:
- Write the equilibrium expression: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
- Use the Kb formula: Kb = [NH₄⁺][OH⁻]/[NH₃] where Kb = 1.8 × 10⁻⁵
- Calculate [OH⁻]: [OH⁻] = √(Kb × C), where C is the initial concentration
- Find pOH: pOH = -log[OH⁻]
- Calculate pH: pH = 14 – pOH
Example: For 0.1 M NH₃ solution:
- [OH⁻] = √(1.8 × 10⁻⁵ × 0.1) = 1.34 × 10⁻³ M
- pOH = 2.87
- pH = 14 – 2.87 = 11.13
Q. What is the Kb value of ammonia and what does it mean?
The Kb (base dissociation constant) of ammonia is 1.8 × 10⁻⁵ at 25°C. This value indicates that ammonia is a weak base because:
- Kb is much less than 1, showing incomplete dissociation
- Only a small fraction of NH₃ molecules accept protons in aqueous solution
- The corresponding pKb = 4.74, which confirms weak base behavior
The Kb expression is: Kb = [NH₄⁺][OH⁻]/[NH₃] = 1.8 × 10⁻⁵
For comparison, strong bases like NaOH completely dissociate, while ammonia only partially dissociates in water.
Q. Is ammonia acidic or basic? How does it behave in water?
Ammonia is a weak base (not acidic). When dissolved in water, ammonia accepts protons (H⁺) from water molecules, producing hydroxide ions (OH⁻) that make the solution basic:
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
Important Point:
- Aqueous ammonia solutions have pH values between 11-12 (basic)
- Ammonia acts as a Brønsted-Lowry base (proton acceptor)
- The lone pair of electrons on nitrogen attracts and bonds with H⁺
- Ammonia solutions turn red litmus paper blue
- Commercial ammonia cleaners typically contain 5-10% NH₃ in water
However, ammonium ion (NH₄⁺) can act as a weak acid: NH₄⁺ ⇌ NH₃ + H⁺
Q. What is the Haber process formula and why is it important?
The Haber process formula is: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) + Heat (ΔH = -92.4 kJ/mol)
Process conditions:
- Temperature: 400-500°C
- Pressure: 150-250 atm
- Catalyst: Iron (Fe) with promoters
Importance:
- Produces over 150 million tons of ammonia annually worldwide
- Essential for manufacturing nitrogen-based fertilizers (feeds ~50% of global population)
- Used in producing nitric acid, explosives, and industrial chemicals
- Won Fritz Haber the Nobel Prize in Chemistry (1918)
- The reaction is reversible and exothermic (releases heat)
The process balances temperature (higher speeds reaction but favors reverse) and pressure (higher favors forward reaction) to maximize ammonia yield.
Q. How do you calculate the molar mass of ammonia (NH₃)?
The molar mass of ammonia is calculated by adding the atomic masses of all atoms in the molecule:
Calculation:
- Nitrogen (N): 1 atom × 14.01 g/mol = 14.01 g/mol
- Hydrogen (H): 3 atoms × 1.008 g/mol = 3.024 g/mol
- Total Molar Mass = 14.01 + 3.024 = 17.03 g/mol
Practical applications:
- Converting grams to moles: moles = mass (g) / 17.03 g/mol
- Converting moles to grams: mass (g) = moles × 17.03 g/mol
- Calculating molarity of solutions
- Stoichiometric calculations in chemical reactions
Example: 34.06 g of NH₃ = 34.06 ÷ 17.03 = 2 moles