Molar Mass of Potassium Hydroxide (KOH)
Learn how chemists calculate the molar mass of Potassium Hydroxide (KOH), with a clear formula breakdown, worked steps, and study notes · IUPAC name: Potassium hydroxide.
Quick answer
The molar mass of Potassium Hydroxide (KOH) is
56.105g/mol
One mole of Potassium Hydroxide therefore has a mass of 56.105 grams—the value you use for stoichiometry and laboratory preparation.
Reviewed for educational accuracy · Accuracy policy
- CAS Registry Number
- 1310-58-3
- PubChem CID
- 14797
- SMILES
- [OH-].[K+]
Step-by-step calculation
Let's find the molar mass of Potassium Hydroxide (KOH) 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 KOH. 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 Oxygen atom (O)
- 1 Hydrogen atom (H)
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
- Oxygen (O) = 15.999 g/mol
- Hydrogen (H) = 1.008 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 × 15.999 = 15.999 g/mol (Oxygen)
- 1 × 1.008 = 1.008 g/mol (Hydrogen)
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 + 15.999 + 1.008 = 56.105 g/mol
Step 5 — Final answer
Molar mass of Potassium Hydroxide = 56.105 g/mol
That means one mole of Potassium Hydroxide (KOH) has a mass of about 56.11 grams.
Quick summary
Read the formula → count atoms → look up atomic masses → multiply → add → report g/mol. For KOH, the total is 56.105 g/mol.
Common beginner mistakes
- Confusing KOH (56.11 g/mol) with NaOH (40.00 g/mol) in stoichiometry — different molar masses despite similar chemistry.
- Assuming KOH and NaOH produce identical soap textures — KOH-based soaps are characteristically softer or liquid.
- Using KOH molar mass when the question specifies moles of OH⁻ from a diprotic base like Ca(OH)₂.
Memory trick
Round KOH's molar mass to about 56 for quick titration calculations.
Mini practice
Without looking above, list the atoms in KOH 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 Hydroxide, mass needed = 0.100 × 56.105 = 5.611 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 KOH.
| Element | Atoms | Atomic mass | Contribution | Mass % |
|---|---|---|---|---|
| K | 1 | 39.098 | 39.098 g/mol | 69.7% |
| O | 1 | 15.999 | 15.999 g/mol | 28.5% |
| H | 1 | 1.008 | 1.008 g/mol | 1.8% |
| Total molar mass | 56.105 g/mol | 100% | ||
Mass contribution chart
Count every atom in this formula, multiply by atomic mass, then add. That total is the molar mass used in lab weighing.
Download study sheets
Save a printable summary, revision sheet, practice worksheet, or laboratory reference for Potassium Hydroxide (KOH).
Practice this calculation
Without looking above, write the atom count for KOH, then compute the molar mass. Check your answer against 56.105 g/mol.
Next challenge: how many grams are in 0.250 mol of Potassium Hydroxide? Multiply 0.250 × 56.105 to get 14.026 g.
Physical and chemical properties
Physical properties
| Appearance | White pellets, flakes, or lumps (solid); colorless solution |
| Color | White (solid) |
| Odor | Odorless |
| State (STP) | Solid (deliquescent) |
| Density | 2.04 g/cm³ (solid) |
| Melting point | 360 °C |
| Boiling point | 1,327 °C |
| Solubility | 1,120 g/L water at 20 °C (highly soluble, exothermic); soluble in ethanol and methanol |
| Crystal structure | Orthorhombic (room temperature form) |
Chemical properties
| Classification | Strong base / alkali metal hydroxide |
| Family | Group 1 hydroxide (alkali hydroxide) |
| Basicity | Strong base (fully dissociates in water) |
| Polarity | Ionic compound; OH⁻ highly polar |
| Oxidation states | K: +1, O: −2, H: +1 |
Applications
Industrial uses
- Soft soap, shaving soap, and liquid hand soap manufacturing (saponification)
- Biodiesel production (transesterification catalyst, favored for alcohol solubility)
- Alkaline battery and fuel cell electrolyte
- Potassium salt manufacturing (e.g., potassium carbonate, potassium phosphates)
Laboratory uses
- Strong base titrant, particularly in nonaqueous or alcohol-based titrations
- Karl Fischer and saponification value determinations
- Preparation of potassium salts and buffer components
Used in some flue gas scrubbing and wastewater neutralization applications; requires careful disposal given strong alkalinity and aquatic toxicity at high concentration.
Not used biologically; highly destructive to tissue on contact, similar in mechanism to NaOH but often perceived as slightly more aggressive.
Preparation and production
Industrially produced by electrolysis of potassium chloride brine (chlor-alkali process), yielding KOH, hydrogen, and chlorine as co-products. Historically prepared by leaching wood ash (potassium carbonate) and treating with calcium hydroxide (causticization): K₂CO₃ + Ca(OH)₂ → 2 KOH + CaCO₃.
Global KOH production is smaller than NaOH but substantial, driven by demand for soft soaps, biodiesel catalysis, and battery electrolyte manufacturing; membrane cell electrolysis is the dominant modern production technology.
Important reactions of Potassium Hydroxide
KOH(aq) + HCl(aq) → KCl(aq) + H₂O(l)
- Reaction type
- Acid–base neutralization
- Conditions
- Aqueous, equimolar
- Explanation
- Classic strong acid–strong base reaction producing potassium chloride and water with a large, favorable neutralization enthalpy.
- Products
- Potassium chloride and water
- Why it matters
- Titrations, pH adjustment, salt production
Related ideas: Neutralization · Titration · Stoichiometry
Fat + 3 KOH → Glycerol + 3 R-COO⁻K⁺ (soft soap)
- Reaction type
- Saponification (hydrolysis)
- Conditions
- Heat, aqueous or alcoholic KOH
- Explanation
- Triglyceride ester bonds hydrolyze to glycerol and potassium salts of fatty acids, producing a soft, often liquid soap distinct from the hard bar soaps made with NaOH.
- Products
- Glycerol and potassium soap (soft/liquid soap)
- Why it matters
- Soft soap and liquid soap manufacturing, shaving cream production
Related ideas: Ester hydrolysis · Organic chemistry · Colloids
2 KOH(aq) + CO₂(g) → K₂CO₃(aq) + H₂O(l)
- Reaction type
- Acid–base (with acidic oxide)
- Conditions
- Aqueous KOH, atmospheric or supplied CO₂
- Explanation
- Potassium hydroxide absorbs carbon dioxide to form potassium carbonate, gradually converting stored KOH if left exposed to air.
- Products
- Potassium carbonate and water
- Why it matters
- CO₂ scrubbing, understanding KOH storage degradation
Related ideas: Acidic oxides · Gas scrubbing · Carbonate formation
2 Al(s) + 2 KOH(aq) + 6 H₂O(l) → 2 K[Al(OH)₄](aq) + 3 H₂(g)
- Reaction type
- Redox with amphoteric metal
- Conditions
- Hot concentrated KOH solution
- Explanation
- Aluminum metal, being amphoteric, dissolves in hot concentrated KOH to form soluble potassium aluminate with evolution of hydrogen gas.
- Products
- Potassium aluminate and hydrogen gas
- Why it matters
- Amphoteric metal reactivity demonstrations, drain cleaner chemistry
Related ideas: Amphoterism · Redox reactions · Gas evolution
History and discovery
Potash production from wood ash dates back to ancient civilizations and remained the dominant alkali source through the early Industrial Revolution. The development of electrolytic chlor-alkali technology in the late 19th and early 20th centuries, applied to potassium chloride brine, gradually replaced wood-ash-derived potash with directly electrolyzed potassium hydroxide as the primary industrial production method.
Long known in impure form via wood-ash leaching; recognized as a distinct compound and produced in pure form following advances in 18th–19th century inorganic and electrochemistry.
Interesting facts
- The name 'potash' comes from the historical practice of leaching alkali from wood ashes collected in pots.
- KOH's molar mass (56.11 g/mol) is notably higher than NaOH's (40.00 g/mol) due to potassium's greater atomic mass relative to sodium.
- Traditional soft soaps and modern liquid hand soaps are almost always potassium-based, while bar soaps are typically sodium-based.
- KOH's superior solubility in alcohols makes it the preferred catalyst for many small-scale and homebrew biodiesel production setups.
Comparison with similar compounds
KOH (56.11 g/mol) and NaOH (40.00 g/mol) are both strong bases with nearly identical acid–base behavior, but KOH is considerably more soluble in alcohols and produces softer, more liquid soaps on saponification, while NaOH dominates hard bar soap and large-scale industrial alkali applications.
Storage, handling, and safety
Store in airtight, corrosion-resistant containers (HDPE or similar). Keep dry — pellets and flakes absorb atmospheric CO₂ and moisture, converting surface material to potassium carbonate over time. Separate from acids and reactive metals.
Extremely corrosive. Wear chemical-resistant gloves, splash goggles, and a face shield. Always add KOH slowly to water, never the reverse, due to the strongly exothermic dissolution. Treat any solution as hazardous regardless of apparent dilution.
Severe corrosive hazard to skin, eyes, and respiratory tract, comparable to or slightly exceeding sodium hydroxide in tissue-damaging potential. Generates significant heat when dissolved.
- Severe chemical burns, potentially deeper than initially apparent
- Serious eye damage risk, including permanent injury
- Exothermic dissolution can cause splattering and boiling
- Reacts with amphoteric metals (Al, Zn) to release flammable hydrogen gas
Classification: GHS: Skin Corr. 1A, Eye Dam. 1
Exam notes and student tips
Exam notes
- Molar mass KOH = 39.10 + 16.00 + 1.008 = 56.11 g/mol.
- Strong base: 1 mol KOH neutralizes 1 mol monoprotic acid (e.g., HCl).
- For H₂SO₄: 2 mol KOH required per mol acid (diprotic).
- KOH absorbs CO₂: 2 KOH + CO₂ → K₂CO₃ + H₂O, relevant to storage and titration standardization.
Student tips
- Round KOH's molar mass to about 56 for quick titration calculations.
- Link KOH's alcohol solubility advantage directly to biodiesel transesterification questions.
- Remember 'soft soap = potassium, hard soap = sodium' as a quick mnemonic for saponification product differences.
Common mistakes
- Confusing KOH (56.11 g/mol) with NaOH (40.00 g/mol) in stoichiometry — different molar masses despite similar chemistry.
- Assuming KOH and NaOH produce identical soap textures — KOH-based soaps are characteristically softer or liquid.
- Using KOH molar mass when the question specifies moles of OH⁻ from a diprotic base like Ca(OH)₂.
Misconceptions
- KOH is not simply 'weaker' or 'stronger' than NaOH as a base — both are effectively fully dissociated strong bases; differences lie in solubility and practical applications, not intrinsic base strength.
- Potash historically referred to impure potassium carbonate/hydroxide mixtures from wood ash, not necessarily pure KOH.
- KOH pellets exposed to air do not remain pure — they progressively carbonate, altering titration standardization if not accounted for.
Practice questions
1. Calculate the molar mass of KOH.
Show answer
39.10 + 16.00 + 1.008 = 56.11 g/mol
2. How many grams of KOH are needed to neutralize 0.250 mol of HCl?
Show answer
0.250 mol HCl needs 0.250 mol KOH; 0.250 × 56.11 = 14.03 g
3. What pH is expected for a 0.010 M KOH solution?
Show answer
pOH = −log(0.010) = 2, so pH = 14 − 2 = 12 (strong base, complete dissociation assumed)
4. Why is KOH often preferred over NaOH for making liquid soap?
Show answer
Potassium fatty acid salts are more soluble and remain soft or liquid at room temperature, unlike the harder sodium salts formed with NaOH.
Frequently asked questions about Potassium Hydroxide
56.11 g/mol.
Chemistry of Potassium Hydroxide
The sections above give the number you need for calculations. Here we look more closely at how Potassium Hydroxide (KOH) behaves chemically—so the molar mass connects to real reactions, properties, and laboratory practice.
Potassium hydroxide (KOH) is a strong base with molar mass 56.11 g/mol (K 39.10 + O 16.00 + H 1.008), dissociating completely in water into K⁺ and OH⁻ ions to give strongly alkaline solutions. Like its sodium counterpart, KOH is deliquescent and dissolves in water with a substantial exothermic heat of solution, and it rapidly absorbs atmospheric carbon dioxide and moisture if left exposed, gradually converting to potassium carbonate on the surface of stored pellets or flakes.
Historically called caustic potash to distinguish it from caustic soda (NaOH), potassium hydroxide has been prepared since antiquity by leaching wood ash — a process that gave "potash" (pot ashes) its name and provided the primary industrial source of alkali before the large-scale chlor-alkali electrolysis processes of the modern era. Today essentially all KOH is produced by electrolyzing potassium chloride brine, mirroring the chlor-alkali process used for NaOH but yielding potassium hydroxide, hydrogen, and chlorine instead.
KOH's most distinctive practical niche relative to NaOH is soap chemistry: saponifying fats and oils with potassium hydroxide produces potassium salts of fatty acids that remain soft, paste-like, or even liquid at room temperature — the basis of traditional soft soaps, shaving soaps, and liquid hand soaps — whereas the corresponding sodium salts from NaOH saponification are typically hard, solid bar soaps. KOH is also the standard electrolyte in alkaline batteries and many fuel cells, where its high ionic conductivity and chemical stability toward the battery's zinc and manganese dioxide electrodes make it far more effective than neutral salt electrolytes.
KOH consists of K⁺ and OH⁻ ions in a 1:1 ratio, structurally analogous to NaOH but with the larger, more polarizable potassium cation replacing sodium. In the solid state it exists as an ionic lattice; in solution hydroxide ion is the active base species responsible for its strongly alkaline chemistry.
KOH is a strong Brønsted base and Lewis base, reacting completely with acids in neutralization reactions and precipitating insoluble metal hydroxides from salt solutions. It saponifies esters (including fats and oils) to form potassium carboxylate salts (soft soap) and glycerol, absorbs atmospheric CO₂ to form potassium carbonate, and reacts with amphoteric metals such as aluminum and zinc to release hydrogen gas. Molten KOH is even more aggressively corrosive than the aqueous solution, attacking glass and many metals.
Soft soap and liquid soap chemistry vs. NaOH
Saponifying fats or oils with potassium hydroxide produces potassium salts of fatty acids that are markedly softer, more soluble, and often paste-like or liquid at room temperature, in contrast to the firm bar soaps produced by sodium hydroxide saponification. This difference — rooted in the larger, more polarizable, and more weakly ion-paired potassium cation disrupting fatty acid salt crystal packing — is why traditional 'soft soap,' shaving creams, and modern liquid hand soaps are typically potassium-based formulations.
KOH vs. NaOH: practical and chemical distinctions
Although both are strong Group 1 hydroxides with very similar acid–base chemistry, KOH is notably more soluble in alcohols than NaOH, making it preferred in biodiesel production (transesterification catalysis) and titrations in nonaqueous or mixed solvents, while NaOH remains the more common and less expensive choice for large-scale industrial applications like pulp processing and hard soap manufacture.
Alkaline battery and fuel cell electrolyte
Concentrated potassium hydroxide solution serves as the electrolyte in most modern alkaline batteries (zinc–manganese dioxide cells) and in classic alkaline fuel cells, chosen for its high ionic conductivity, chemical compatibility with the electrode materials, and stability across a wide range of operating conditions compared to neutral salt electrolytes.
Potash: from wood ash to industrial electrolysis
The name 'potash' originates from the historical method of leaching wood ashes in iron pots to extract soluble potassium carbonate and hydroxide, which served as the main industrial alkali source for centuries before large-scale electrolytic chlor-alkali technology made KOH production from potassium chloride brine the dominant modern route.
Aggressive tissue and material corrosivity
KOH is, if anything, slightly more corrosive per unit concentration than NaOH toward biological tissue and many materials, rapidly hydrolyzing proteins and lipids on contact; this same aggressive chemistry underlies its use in chemical hair relaxers, drain cleaners, and oven cleaners, always requiring careful protective handling.
Recalculate any formula with the molar mass calculator, compare atoms on the periodic table, or browse more compounds in the base library.
References and further reading
- PubChem CID 14797: Potassium hydroxide compound data
- NIST Chemistry WebBook: Thermodynamic properties
- ILO Encyclopaedia: Occupational safety for caustic alkalis