Molar Mass of Octane (C₈H₁₈)
Learn how chemists calculate the molar mass of Octane (C₈H₁₈), with a clear formula breakdown, worked steps, and study notes.
Quick answer
The molar mass of Octane (C₈H₁₈) is
114.232g/mol
One mole of Octane therefore has a mass of 114.232 grams—the value you use for stoichiometry and laboratory preparation.
Reviewed for educational accuracy · Accuracy policy
- CAS Registry Number
- 111-65-9
- PubChem CID
- 356
- SMILES
- CCCCCCCC
Step-by-step calculation
Let's find the molar mass of Octane (C₈H₁₈) 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 C₈H₁₈. 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.
- 8 Carbon atoms (C)
- 18 Hydrogen atoms (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.
- Carbon (C) = 12.011 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.
- 8 × 12.011 = 96.088 g/mol (Carbon)
- 18 × 1.008 = 18.144 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:
96.088 + 18.144 = 114.232 g/mol
Step 5 — Final answer
Molar mass of Octane = 114.232 g/mol
That means one mole of Octane (C₈H₁₈) has a mass of about 114.23 grams.
Quick summary
Read the formula → count atoms → look up atomic masses → multiply → add → report g/mol. For C₈H₁₈, the total is 114.232 g/mol.
Common beginner mistakes
- Assuming higher 'octane number' gasoline contains more actual octane molecules — it reflects antiknock performance, not octane content.
- Confusing n-octane with isooctane (2,2,4-trimethylpentane) — same formula and molar mass, very different structure and knock resistance.
- Under-balancing octane combustion by forgetting the fractional/doubled oxygen coefficient (12.5 mol O₂ per mole octane, or 25 mol O₂ per 2 mol octane).
Memory trick
Write the combustion equation doubled (2 C₈H₁₈ + 25 O₂ → 16 CO₂ + 18 H₂O) to avoid fractional coefficients.
Mini practice
Without looking above, list the atoms in C₈H₁₈ 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 Octane, mass needed = 0.100 × 114.232 = 11.423 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 C₈H₁₈.
| Element | Atoms | Atomic mass | Contribution | Mass % |
|---|---|---|---|---|
| C | 8 | 12.011 | 96.088 g/mol | 84.1% |
| H | 18 | 1.008 | 18.144 g/mol | 15.9% |
| Total molar mass | 114.232 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 Octane (C₈H₁₈).
Practice this calculation
Without looking above, write the atom count for C₈H₁₈, then compute the molar mass. Check your answer against 114.232 g/mol.
Next challenge: how many grams are in 0.250 mol of Octane? Multiply 0.250 × 114.232 to get 28.558 g.
Physical and chemical properties
Physical properties
| Appearance | Colorless liquid |
| Color | Colorless |
| Odor | Gasoline-like hydrocarbon odor |
| State (STP) | Liquid |
| Density | 0.703 g/cm³ at 20 °C |
| Melting point | −56.8 °C |
| Boiling point | 125.6 °C |
| Solubility | 0.66 mg/L water at 25 °C (essentially insoluble); miscible with hydrocarbons and most organic solvents |
| Crystal structure | Not commonly crystallized under standard conditions; forms a crystalline solid below −56.8 °C |
Chemical properties
| Classification | Saturated hydrocarbon / alkane |
| Family | Alkanes (eighth homolog, straight-chain) |
| Polarity | Non-polar |
| Geometry | Tetrahedral at each carbon (zig-zag chain conformation) |
| Bond angle | 109.5° |
| Oxidation states | C: −3 to −2 depending on position; H: +1 |
Applications
Industrial uses
- Component of gasoline and other petroleum distillate fuel blends
- Feedstock for catalytic reforming/isomerization to produce higher-octane branched isomers
- Solvent and reference standard in petroleum and petrochemical analysis
- Model compound in combustion kinetics and engine knock research
Laboratory uses
- Gas chromatography retention time and calibration reference
- Teaching example for constitutional isomerism in alkanes
- Combustion enthalpy and calorimetry demonstrations
As a gasoline component, octane combustion contributes CO₂ and, when incomplete, CO and particulate emissions; evaporative losses contribute to volatile organic compound (VOC) emissions from fuel handling and storage.
Preparation and production
n-Octane is obtained by fractional distillation of crude petroleum within the gasoline boiling range, rather than by targeted laboratory synthesis. Isomeric octanes (including isooctane) are produced industrially by catalytic alkylation of isobutane with isobutylene or by isomerization/reforming of straight-chain hydrocarbon feedstocks.
Octane isomers are produced at refinery scale as part of gasoline blending operations, including catalytic reforming, isomerization, and alkylation units specifically designed to maximize branched, high-octane-rating hydrocarbon content.
Important reactions of Octane
2 C₈H₁₈(l) + 25 O₂(g) → 16 CO₂(g) + 18 H₂O(g)
- Reaction type
- Combustion (complete)
- Conditions
- Ignition, excess oxygen (internal combustion engine or open flame)
- Explanation
- Complete combustion of octane releases substantial energy (~5470 kJ/mol), the fundamental reaction powering gasoline engines; incomplete combustion under limited oxygen instead produces CO and soot.
- Products
- Carbon dioxide and water
- Why it matters
- Internal combustion engine power generation, fuel energy content calculations
Related ideas: Combustion · Enthalpy of combustion · Engine chemistry
n-C₈H₁₈ → branched C₈H₁₈ isomers (e.g., isooctane)
- Reaction type
- Catalytic isomerization
- Conditions
- Pt or acidic zeolite catalyst, moderate heat and pressure
- Explanation
- Refinery isomerization units rearrange straight-chain octane into more branched isomers, raising the resulting fuel blend's octane rating and antiknock performance without changing the overall molecular formula.
- Products
- Branched octane isomers
- Why it matters
- Gasoline octane rating improvement in petroleum refining
Related ideas: Isomerization · Catalysis · Petroleum refining
C₈H₁₈ → smaller alkanes + alkenes (catalytic cracking)
- Reaction type
- Catalytic/thermal cracking
- Conditions
- Zeolite catalyst, high temperature (450–550 °C)
- Explanation
- Larger hydrocarbon chains including octane can be cracked into a mixture of shorter alkanes and alkenes, useful both for adjusting the product slate of a refinery and for producing petrochemical feedstocks.
- Products
- Mixture of lighter alkanes and alkenes
- Why it matters
- Petroleum refining product distribution, petrochemical feedstock generation
Related ideas: Cracking · Bond cleavage · Refining processes
C₈H₁₈ + Br₂ → C₈H₁₇Br + HBr
- Reaction type
- Free-radical halogenation
- Conditions
- UV light or heat, radical chain mechanism
- Explanation
- Bromine radicals substitute a hydrogen on octane, giving a mixture of monobrominated products depending on which C–H bond reacts, illustrating regioselectivity trends in radical halogenation of longer alkanes.
- Products
- Bromooctane isomers and hydrogen bromide
- Why it matters
- Illustrates free-radical substitution mechanisms and selectivity in longer-chain alkanes
Related ideas: Free radicals · Substitution reactions · Regioselectivity
History and discovery
Octane and its isomers were characterized as petroleum chemistry matured in the late 19th and early 20th centuries alongside the growth of the automobile industry. The octane rating scale, formalized in the 1920s by Graham Edgar using isooctane and n-heptane as reference fuels, became the standard measure of gasoline antiknock quality still used worldwide today.
Characterized during systematic 19th–20th century petroleum fractionation studies; the octane rating scale itself was established by Graham Edgar in 1926 using isooctane and n-heptane as calibration references.
Interesting facts
- C₈H₁₈ has 18 possible structural isomers, more than double the 9 isomers of heptane (C₇H₁₆), illustrating the rapid combinatorial growth of alkane isomer counts with chain length.
- Isooctane, the fuel industry's octane-rating reference standard, is actually a branched isomer of octane, not 'more octane' in any literal sense.
- The molar mass of octane (114.23 g/mol) means one mole of liquid octane occupies roughly 163 mL, based on its density of about 0.703 g/cm³.
- Diesel and jet fuels contain longer-chain alkanes than gasoline's octane-range hydrocarbons, which is part of why they require different engine designs (compression ignition vs. spark ignition).
Comparison with similar compounds
n-Octane (C₈H₁₈, 114.23 g/mol) and isooctane (2,2,4-trimethylpentane, also C₈H₁₈, 114.23 g/mol) share an identical molar mass but differ dramatically in knock resistance — isooctane defines the top of the 0–100 octane rating scale, while n-octane performs poorly under compression by comparison.
Storage, handling, and safety
Store in tightly sealed, flame-resistant containers away from ignition sources, oxidizers, and open flame, consistent with standard flammable liquid storage practice for petroleum hydrocarbons.
Flammable liquid; avoid ignition sources and static discharge. Use in well-ventilated areas to avoid vapor accumulation. Avoid prolonged skin contact, which can cause defatting and irritation.
Flammable liquid hydrocarbon with low acute toxicity but potential for aspiration hazard if swallowed and vapor inhalation risk in poorly ventilated spaces; handle with standard flammable liquid precautions.
- Flammable liquid and vapor
- Aspiration hazard if swallowed (can enter lungs and cause chemical pneumonitis)
- Vapor inhalation may cause dizziness or respiratory irritation in enclosed spaces
- Environmental hazard to aquatic organisms if released in quantity
Classification: GHS: Flam. Liq. 2, Asp. Tox. 1
Exam notes and student tips
Exam notes
- Molar mass C₈H₁₈ = 8(12.01) + 18(1.008) = 114.22–114.23 g/mol.
- Combustion: 2 C₈H₁₈(l) + 25 O₂(g) → 16 CO₂(g) + 18 H₂O(g) (balance carefully — 25/2 O₂ per mole of octane).
- Octane rating measures antiknock performance relative to isooctane (100) and n-heptane (0) — it is not a direct measure of octane content.
- C₈H₁₈ has 18 structural isomers; only one (n-octane) is the unbranched straight-chain form.
Student tips
- Write the combustion equation doubled (2 C₈H₁₈ + 25 O₂ → 16 CO₂ + 18 H₂O) to avoid fractional coefficients.
- Remember: octane rating compares a fuel's knock resistance to isooctane/heptane blends — memorize 100 = isooctane, 0 = n-heptane.
- Use octane as the go-to example when discussing how many structural isomers a formula can have as chain length grows.
Common mistakes
- Assuming higher 'octane number' gasoline contains more actual octane molecules — it reflects antiknock performance, not octane content.
- Confusing n-octane with isooctane (2,2,4-trimethylpentane) — same formula and molar mass, very different structure and knock resistance.
- Under-balancing octane combustion by forgetting the fractional/doubled oxygen coefficient (12.5 mol O₂ per mole octane, or 25 mol O₂ per 2 mol octane).
Misconceptions
- Premium (higher octane number) gasoline is not inherently 'more powerful' or higher-energy — it simply resists knock better, which matters mainly for high-compression engines designed to use it.
- Isooctane is not a separate element or additive unrelated to octane — it is literally a branched structural isomer of C₈H₁₈.
- n-Octane is not itself a major deliberate gasoline additive prized for its own performance — it is present as one of many hydrocarbons in the gasoline boiling range, with branched isomers preferred for octane rating.
Practice questions
1. Calculate the molar mass of octane (C₈H₁₈).
Show answer
8(12.01) + 18(1.008) = 114.22–114.23 g/mol
2. How many moles of CO₂ are produced from the complete combustion of 228.46 g of octane?
Hint: Use the mole ratio 2 mol octane : 16 mol CO₂, i.e., 8 mol CO₂ per mol octane.
Show answer
228.46 g ÷ 114.23 g/mol = 2.000 mol C₈H₁₈; from 2 C₈H₁₈ + 25 O₂ → 16 CO₂ + 18 H₂O, this yields 16.00 mol CO₂
3. Does a gasoline with an octane rating of 93 contain 93% octane by some measure?
Show answer
No — the octane rating measures resistance to engine knock relative to a reference blend of isooctane (rated 100) and n-heptane (rated 0); it does not directly indicate the percentage of actual octane in the fuel.
4. Why do n-octane and isooctane have the same molar mass but different fuel performance?
Show answer
Both share the molecular formula C₈H₁₈ (114.23 g/mol), but isooctane's highly branched structure resists autoignition under compression far better than n-octane's straight chain, so molecular shape rather than mass governs knock resistance.
Frequently asked questions about Octane
114.23 g/mol.
Chemistry of Octane
The sections above give the number you need for calculations. Here we look more closely at how Octane (C₈H₁₈) behaves chemically—so the molar mass connects to real reactions, properties, and laboratory practice.
Octane (C₈H₁₈) is the eight-carbon straight-chain alkane, n-octane, with molar mass 114.23 g/mol (8 × 12.01 + 18 × 1.008). As a component of gasoline, it is a colorless, flammable liquid that is virtually insoluble in water but fully miscible with other hydrocarbons. C₈H₁₈ has 18 structural isomers (all sharing this same molecular formula), ranging from the straight-chain n-octane to highly branched forms — a fact central to understanding why "octane" as a fuel term is more subtle than a single pure substance.
The famous "octane rating" of gasoline does not measure the amount of n-octane present; rather, it measures a fuel's resistance to premature, uncontrolled ignition (engine knock) under compression, using a scale where 2,2,4-trimethylpentane — commonly called isooctane, a highly branched C₈H₁₈ isomer — is arbitrarily assigned a rating of 100, and n-heptane is assigned 0. A gasoline's octane rating (e.g., 87, 91, 93) reflects how its knock resistance compares to blends of isooctane and heptane, not literal octane content. Branched isomers like isooctane resist knock far better than straight-chain n-octane, which is one reason refineries isomerize and reform straight-chain hydrocarbons into more branched forms to boost octane rating.
n-Octane itself is primarily of interest as a model compound for studying alkane combustion chemistry, as a solvent and reference standard in petroleum analysis, and as one of many components blended into finished gasoline, rather than as a fuel additive valued for its own combustion properties.
C₈H₁₈ follows the general alkane formula CₙH₂ₙ₊₂ with n = 8. The straight-chain isomer, n-octane, has eight carbons connected end-to-end (CH₃–(CH₂)₆–CH₃). Because the formula only specifies atom counts and not connectivity, all 18 possible arrangements of eight carbons and eighteen hydrogens into a saturated, non-cyclic structure share the identical molecular formula C₈H₁₈ and molar mass, despite having different physical properties and combustion behavior.
Like other alkanes, octane is relatively unreactive at room temperature, undergoing combustion, catalytic cracking, and free-radical halogenation as its principal reactions. Industrially, straight-chain octane and similar alkanes are catalytically isomerized and reformed into branched isomers to improve fuel antiknock properties, and can be catalytically cracked into shorter, more volatile hydrocarbons for blending into gasoline or petrochemical feedstocks.
Octane Rating Is Not a Measure of Octane Content
A gasoline's octane rating (e.g., 87 or 93) reflects its resistance to engine knock relative to a reference blend of isooctane (2,2,4-trimethylpentane, rated 100) and n-heptane (rated 0) — it says nothing directly about how much actual octane (of any isomer) is present in the fuel. This is one of the most common misconceptions among consumers about gasoline chemistry.
n-Octane vs. Isooctane: Same Formula, Different Knock Behavior
n-Octane and isooctane are both C₈H₁₈ (114.23 g/mol) but have dramatically different antiknock properties: the highly branched isooctane resists autoignition under compression far better than the straight-chain n-octane, illustrating how molecular branching, not just molecular formula, governs practical fuel performance.
Why Refineries Favor Branched Isomers
Catalytic isomerization and reforming processes convert straight-chain alkanes like n-octane into more branched isomers specifically to raise octane rating and reduce knock tendency, since branched hydrocarbons burn more smoothly and controllably under the high compression ratios used in modern engines.
Constitutional Isomerism in C₈H₁₈
There are 18 distinct structural isomers of C₈H₁₈, differing only in how the eight carbons are branched — a frequently used teaching example for illustrating how rapidly the number of possible isomers grows with chain length in the alkane series.
Octane as a Petroleum Reference Compound
n-Octane serves as a common reference and calibration standard in gas chromatography and petroleum analysis, and as a model alkane in combustion kinetics research aimed at understanding and reducing knock and emissions in internal combustion engines.
Recalculate any formula with the molar mass calculator, compare atoms on the periodic table, or browse more compounds in the hydrocarbon library.
References and further reading
- NIST Chemistry WebBook: Octane thermophysical properties
- PubChem CID 356: n-Octane identifiers and data
- SAE International: Octane rating standards and gasoline antiknock testing