Molar Mass of Potassium Nitrate (KNO₃)
Learn how chemists calculate the molar mass of Potassium Nitrate (KNO₃), with a clear formula breakdown, worked steps, and study notes · IUPAC name: Potassium nitrate.
Quick answer
The molar mass of Potassium Nitrate (KNO₃) is
101.102g/mol
One mole of Potassium Nitrate therefore has a mass of 101.102 grams—the value you use for stoichiometry and laboratory preparation.
Reviewed for educational accuracy · Accuracy policy
- CAS Registry Number
- 7757-79-1
- PubChem CID
- 24434
- SMILES
- [N+](=O)([O-])[O-].[K+]
Step-by-step calculation
Let's find the molar mass of Potassium Nitrate (KNO₃) together—step by step, as if you are seeing the formula for the first time.
Step 1 — Look at the chemical formula
The formula is KNO₃. Each letter stands for an element. The little number after a letter (the subscript) tells you how many atoms of that element are in one molecule or formula unit.
- 1 Potassium atom (K)
- 1 Nitrogen atom (N)
- 3 Oxygen atoms (O)
Step 2 — Look up each atomic mass
Atomic mass comes from the periodic table. It is the average mass of one mole of atoms of that element, in grams per mole (g/mol). Think of it as the "price tag" for one mole of that element.
- Potassium (K) = 39.098 g/mol
- Nitrogen (N) = 14.007 g/mol
- Oxygen (O) = 15.999 g/mol
Step 3 — Multiply atoms × atomic mass
Why multiply? If one oxygen atom "costs" about 16 g/mol, then two oxygen atoms cost twice as much. Each element's contribution is: number of atoms × atomic mass.
- 1 × 39.098 = 39.098 g/mol (Potassium)
- 1 × 14.007 = 14.007 g/mol (Nitrogen)
- 3 × 15.999 = 47.997 g/mol (Oxygen)
Step 4 — Add the contributions
Why add? The molar mass of the whole compound is simply the total mass of every atom in the formula. Add each element's contribution:
39.098 + 14.007 + 47.997 = 101.102 g/mol
Step 5 — Final answer
Molar mass of Potassium Nitrate = 101.102 g/mol
That means one mole of Potassium Nitrate (KNO₃) has a mass of about 101.10 grams.
Quick summary
Read the formula → count atoms → look up atomic masses → multiply → add → report g/mol. For KNO₃, the total is 101.102 g/mol.
Common beginner mistakes
- Assuming KNO₃ dissolution is exothermic like many salts — it is endothermic, cooling the solution.
- Confusing potassium nitrate (KNO₃, oxidizer) with potassium nitrite (KNO₂, a decomposition product with nitrogen in +3 state).
- Underestimating fire/explosion risk when KNO₃ contacts combustible or reducing materials in bulk.
Memory trick
Remember K, N, O count directly from the formula for quick molar mass recall: 39 + 14 + 48 ≈ 101.
Mini practice
Without looking above, list the atoms in KNO₃ and write one multiplication line for the heaviest element. Then check your work against Step 3.
Real-world example
If a recipe asks for 0.100 mol of Potassium Nitrate, mass needed = 0.100 × 101.102 = 10.110 g. That is how chemists turn a mole amount into a weighable sample.
Atomic contribution table
Each row shows how much mass one element contributes to the total for KNO₃.
| Element | Atoms | Atomic mass | Contribution | Mass % |
|---|---|---|---|---|
| K | 1 | 39.098 | 39.098 g/mol | 38.7% |
| N | 1 | 14.007 | 14.007 g/mol | 13.9% |
| O | 3 | 15.999 | 47.997 g/mol | 47.5% |
| Total molar mass | 101.102 g/mol | 100% | ||
Mass contribution chart
Teal and blue circles alternate like positive and negative ions in a crystal lattice. Formula mass is the mass of one formula unit, not a single molecule.
Download study sheets
Save a printable summary, revision sheet, practice worksheet, or laboratory reference for Potassium Nitrate (KNO₃).
Practice this calculation
Without looking above, write the atom count for KNO₃, then compute the molar mass. Check your answer against 101.102 g/mol.
Next challenge: how many grams are in 0.250 mol of Potassium Nitrate? Multiply 0.250 × 101.102 to get 25.276 g.
Physical and chemical properties
Physical properties
| Appearance | White crystalline solid or powder |
| Color | White |
| Odor | Odorless |
| State (STP) | Solid |
| Density | 2.11 g/cm³ |
| Melting point | 334 °C |
| Boiling point | 400 °C (decomposes) |
| Solubility | 133 g/L water at 0 °C, rising to over 2400 g/L at 100 °C (strongly temperature-dependent) |
| Crystal structure | Orthorhombic (below ~129 °C); trigonal above |
Chemical properties
| Classification | Ionic nitrate salt / oxidizing agent |
| Family | Alkali metal nitrates |
| Polarity | Ionic |
| Geometry | Trigonal planar nitrate anion |
| Oxidation states | K: +1, N: +5, O: −2 |
Applications
Industrial uses
- Oxidizer in black powder (gunpowder) and pyrotechnic/firework formulations
- Chloride-free potassium and nitrogen fertilizer for high-value, chloride-sensitive crops
- Meat curing agent (historically, now often supplemented or replaced by sodium nitrite)
- Heat transfer/thermal energy storage salt component in some solar power installations
Laboratory uses
- Classic oxygen generation demonstration by thermal decomposition
- Endothermic dissolution demonstration for thermochemistry teaching
- Oxidizer component in student-safe combustion and rocket candy (KNO₃/sugar) demonstrations under supervision
Excess nitrate fertilizer runoff contributes to eutrophication of waterways and groundwater nitrate contamination, a concern shared with other nitrate fertilizers.
Nitrate ion is metabolized by plants as an essential nitrogen source; in cured meats, nitrate-derived nitrite contributes to both preservation and formation of nitrosamines under certain cooking conditions, a subject of ongoing dietary research.
Preparation and production
Historically produced by leaching nitrogen-rich organic waste (manure, decaying plant matter) and converting the resulting calcium/ammonium nitrates to potassium nitrate via reaction with potash (potassium carbonate) — the traditional saltpeter "niter bed" process. Modern production reacts potassium chloride with nitric acid or uses double decomposition with sodium nitrate and potassium chloride.
Commercial KNO₃ is produced by reacting potassium chloride with nitric acid, by double decomposition of sodium nitrate (Chile saltpeter) with potassium chloride, or by neutralizing nitric acid with potassium hydroxide or carbonate.
Important reactions of Potassium Nitrate
2 KNO₃(s) → 2 KNO₂(s) + O₂(g)
- Reaction type
- Thermal decomposition
- Conditions
- Heat to ~400 °C
- Explanation
- Moderate heating releases oxygen gas while nitrogen is reduced from +5 (nitrate) to +3 (nitrite); a glowing splint reignites in the evolved gas, demonstrating oxygen generation.
- Products
- Potassium nitrite and oxygen
- Why it matters
- Laboratory oxygen generation demonstrations, understanding oxidizer decomposition
Related ideas: Thermal decomposition · Redox · Oxidizing agents
4 KNO₃(s) + 5 C(s) → 2 K₂CO₃(s) + 3 CO₂(g) + 2 N₂(g)
- Reaction type
- Redox (combustion with carbon)
- Conditions
- Ignition, contact with reducing agent
- Explanation
- A simplified representation of nitrate-carbon combustion illustrating how nitrate's internal oxygen supports rapid oxidation of carbon even without atmospheric air — the operating principle behind black powder's rapid burn rate.
- Products
- Potassium carbonate, carbon dioxide, nitrogen gas
- Why it matters
- Explains gunpowder and pyrotechnic combustion chemistry
Related ideas: Oxidizers · Combustion without air · Pyrotechnics
KNO₃(aq) → K⁺(aq) + NO₃⁻(aq) [no precipitation with common reagents]
- Reaction type
- Solubility / spectator ion illustration
- Conditions
- Aqueous, room temperature
- Explanation
- Both K⁺ and NO₃⁻ are 'always soluble' spectator ions in standard solubility rules — KNO₃ does not precipitate with common test cations or anions, making it a useful inert electrolyte in salt bridges.
- Products
- Hydrated potassium and nitrate ions (fully soluble, no precipitate)
- Why it matters
- Salt bridges in electrochemical cells, solubility rules teaching
Related ideas: Solubility rules · Spectator ions · Electrochemistry
KNO₃(s) + H₂O(l) → K⁺(aq) + NO₃⁻(aq) [ΔH > 0]
- Reaction type
- Endothermic dissolution
- Conditions
- Room temperature water
- Explanation
- Dissolving KNO₃ absorbs heat from the surroundings, noticeably cooling the solution — a favorite thermochemistry demonstration of an endothermic yet spontaneous process (entropy-driven).
- Products
- Hydrated potassium and nitrate ions
- Why it matters
- Thermochemistry demonstrations, historical cold-pack chemistry
Related ideas: Enthalpy of solution · Entropy · Spontaneity
History and discovery
Saltpeter was known in ancient China by at least the 9th century CE and became essential to gunpowder formulations that spread across Asia, the Middle East, and Europe by the medieval period. Niter beds — controlled composting operations designed to maximize nitrate production — were a major industrial-scale chemical process before synthetic nitrate fixation (Haber–Bosch, Ostwald) made KNO₃ production far more efficient in the 20th century.
Known and used in China from at least the 9th century CE for early pyrotechnic and incendiary formulations; its role as gunpowder's oxidizer was refined through medieval Islamic and European alchemy and chemistry.
Interesting facts
- The word 'saltpeter' derives from Latin sal petrae, 'salt of stone,' referring to nitrate efflorescence historically scraped from cave walls and stable floors rich in decomposing organic matter.
- KNO₃'s molar mass 101.10 g/mol is close to that of many organic solvents, but its ionic, oxidizing chemistry is entirely different.
- Gunpowder's basic 75:15:10 (KNO₃:charcoal:sulfur) recipe has remained essentially unchanged for centuries since its widespread adoption.
- Dissolving KNO₃ in water is used in classroom thermochemistry to demonstrate an endothermic process that nonetheless occurs spontaneously, driven by entropy increase.
Comparison with similar compounds
KNO₃ (101.10 g/mol) provides chloride-free potassium and nitrate for sensitive crops, whereas potassium chloride (74.55 g/mol, muriate of potash) is cheaper but introduces chloride that some crops tolerate poorly.
Storage, handling, and safety
Store as a dry, stable oxidizer away from combustible and reducing materials (fuels, sulfur, organic matter, metal powders), heat sources, and open flame, since mixtures with such materials can become highly flammable or explosive.
Not acutely toxic in typical handling, but strong oxidizing behavior means it must never be mixed with combustibles outside of controlled pyrotechnic formulations. Avoid generating dust; wash hands after handling.
A strong oxidizer that intensifies fires and can form explosive mixtures with fuels, reducing agents, or organic materials; low acute toxicity to humans in isolation but hazardous in combination with combustibles.
- Oxidizer — intensifies fires and can cause or contribute to explosions when mixed with fuels
- Dust may irritate eyes and respiratory tract
- Excessive ingestion can cause methemoglobinemia via nitrate/nitrite metabolism
Classification: GHS: Ox. Solid 3
Exam notes and student tips
Exam notes
- Molar mass KNO₃ = 39.10 + 14.01 + 3(16.00) = 101.11 g/mol (commonly rounded to 101.10 g/mol).
- Thermal decomposition: 2 KNO₃(s) → 2 KNO₂(s) + O₂(g) on moderate heating.
- Nitrogen oxidation state in KNO₃ (nitrate) is +5, the same as in HNO₃.
- Solubility increases sharply with temperature — a classic recrystallization/solubility curve example in general chemistry.
Student tips
- Remember K, N, O count directly from the formula for quick molar mass recall: 39 + 14 + 48 ≈ 101.
- Link gunpowder's rapid burning to the internal oxygen supply from nitrate decomposition, unlike ordinary combustion relying on atmospheric O₂.
- Use KNO₃'s steep solubility-vs-temperature curve as a model system for recrystallization purification problems.
Common mistakes
- Assuming KNO₃ dissolution is exothermic like many salts — it is endothermic, cooling the solution.
- Confusing potassium nitrate (KNO₃, oxidizer) with potassium nitrite (KNO₂, a decomposition product with nitrogen in +3 state).
- Underestimating fire/explosion risk when KNO₃ contacts combustible or reducing materials in bulk.
Misconceptions
- KNO₃ itself does not explode on its own — it is an oxidizer that dramatically accelerates combustion of fuels/reducing agents mixed with it.
- Saltpeter is not the same substance as Chile saltpeter (sodium nitrate, NaNO₃) — both are nitrate salts but with different cations and slightly different properties.
- Curing meat with nitrate/nitrite is not simply 'adding preservative flavor' — it involves specific chemistry (nitric oxide-myoglobin binding) responsible for color and antimicrobial action.
Practice questions
1. Calculate the molar mass of KNO₃.
Show answer
39.10 + 14.01 + 3(16.00) = 101.11 g/mol (≈ 101.10 g/mol)
2. How many grams of O₂ are produced by fully decomposing 202.2 g of KNO₃ via 2 KNO₃ → 2 KNO₂ + O₂?
Hint: Divide the KNO₃-to-O₂ mole ratio by 2 from the balanced equation.
Show answer
202.2 g ÷ 101.10 g/mol = 2.00 mol KNO₃; ratio 2 KNO₃ : 1 O₂, so 1.00 mol O₂ = 32.00 g
3. Why does dissolving KNO₃ in water make the solution feel cold?
Show answer
The dissolution process is endothermic (ΔH > 0); it absorbs heat from the surrounding water and container, lowering the temperature even though the process occurs spontaneously due to increased entropy.
4. What is the oxidation state of nitrogen in KNO₃ compared to KNO₂?
Show answer
Nitrogen is +5 in KNO₃ (nitrate) and +3 in KNO₂ (nitrite) — a two-unit reduction occurs during thermal decomposition.
Frequently asked questions about Potassium Nitrate
101.10 g/mol.
Chemistry of Potassium Nitrate
The sections above give the number you need for calculations. Here we look more closely at how Potassium Nitrate (KNO₃) behaves chemically—so the molar mass connects to real reactions, properties, and laboratory practice.
Potassium nitrate (KNO₃) has a molar mass of 101.10 g/mol (K 39.10 + N 14.01 + 3 × 16.00), an ionic salt consisting of K⁺ cations and planar, resonance-stabilized NO₃⁻ anions. Known historically as saltpeter, it is a white crystalline solid that dissolves readily in water with a strongly endothermic heat of solution — a property exploited in classic "instant cold pack" and freezing-point-depression demonstrations, since dissolving KNO₃ noticeably cools its surroundings.
KNO₃ is best known as the oxidizer component of black powder (gunpowder), traditionally 75% KNO₃, 15% charcoal, and 10% sulfur by mass; the nitrate ion supplies oxygen internally, allowing the mixture to combust rapidly even without external air. This same oxidizing ability makes KNO₃ useful, in dramatically diluted and controlled contexts, for supporting combustion in fireworks, tree stump removal formulations, and curing salts for meat preservation, where it also inhibits bacterial growth (notably Clostridium botulinum).
Agriculturally, KNO₃ is a premium fertilizer that supplies both essential macronutrients potassium and nitrogen in a chloride-free form, making it especially valued for chloride-sensitive crops such as tobacco, potatoes, and many fruits and vegetables. Because both ions are plant nutrients, essentially the entire compound is agriculturally useful, unlike fertilizers that include non-nutrient counter-ions.
KNO₃ contains one K⁺ ion ionically paired with one NO₃⁻ ion. The nitrate ion has a trigonal planar structure with nitrogen at +5 oxidation state, bonded to three oxygens that are equivalent due to resonance delocalization of the negative charge. The 1:1 cation-to-anion ratio reflects the single positive and single negative charge of the two ions.
KNO₃ is a powerful oxidizer, especially when heated or mixed with reducing agents like carbon or sulfur, releasing oxygen as it decomposes: 2 KNO₃ → 2 KNO₂ + O₂ (moderate heating) or further to K₂O and nitrogen oxides at higher temperatures. It undergoes straightforward double displacement reactions in solution and is the classic source of O₂ gas in laboratory demonstrations of oxygen generation from heated solid oxidizers, alongside KClO₃ and KMnO₄.
Gunpowder Chemistry
Black powder's rapid combustion depends on KNO₃ supplying its own oxygen internally as it decomposes, rather than relying solely on atmospheric O₂. The approximate reaction 2 KNO₃ + S + 3 C → K₂S + N₂ + 3 CO₂ (simplified) illustrates how the nitrate's oxygen oxidizes both sulfur and carbon almost instantaneously, generating the rapidly expanding gases responsible for gunpowder's explosive force.
Endothermic Dissolution and Cold Packs
Unlike most ionic solids, KNO₃ dissolves in water with a positive (endothermic) enthalpy of solution, noticeably cooling the solution — a favorite classroom demonstration of the distinction between exothermic and endothermic dissolution, and the historical basis of some early instant cold packs.
Chloride-Free Fertilizer
KNO₃ delivers both potassium and nitrate — two essential plant macronutrients — without introducing chloride ion, which can harm chloride-sensitive crops like tobacco, citrus, and potatoes at high concentrations. This makes it a premium, if more expensive, alternative to potassium chloride (muriate of potash) fertilizers.
Meat Curing and Historical Preservation
Saltpeter has been used for centuries to cure meats such as bacon and corned beef; nitrate is reduced (often via bacterial or added nitrite intermediates) to nitric oxide, which reacts with myoglobin to produce the characteristic pink color of cured meat while also suppressing Clostridium botulinum spore germination.
Oxygen Generation and Combustion Demonstrations
Heating solid KNO₃ releases oxygen gas as it decomposes to potassium nitrite, a standard laboratory demonstration of how ionic oxidizers can support or intensify combustion (a glowing splint reignites in the evolved O₂) — directly paralleling KClO₃'s more vigorous, catalyzed oxygen-release chemistry.
Recalculate any formula with the molar mass calculator, compare atoms on the periodic table, or browse more compounds in the salt library.
References and further reading
- NIST Chemistry WebBook: KNO₃ thermodynamic and solubility data
- PubChem CID 24434: Potassium nitrate identifiers and properties
- International Fertilizer Association: Chloride-free fertilizer applications