The molecular formula for butane is C4H10. That answer is correct as written. If your instructor also wants the condensed structural formula, the standard straight-chain form is CH3CH2CH2CH3. Molecular formulas count atoms; structural formulas show how those atoms connect. Butane's four carbon atoms can also connect as a branched isomer, so the formula alone does not pin down one molecule. This distinction trips up more students than the formula itself.

Why C4H10 and Not Something Else

A molecular formula answers one question: how many atoms of each element are in one molecule. For butane, the count is four carbon atoms and ten hydrogen atoms.

Carbon has four valence electrons, so each carbon forms four bonds. Hydrogen has one valence electron, so each hydrogen forms one bond. In an alkane, every carbon-carbon bond is single and every remaining valence sits on a hydrogen.

Work the arithmetic. Four carbons making a straight chain have three C-C bonds. Those bonds consume 3 of each carbon's 4 valence slots, leaving a total of 10 open slots (4 carbons × 4 slots − 2 electrons per bond × 3 bonds = 10). Each hydrogen fills one slot. That gives H = 2n + 2 for n carbons, or 2(4) + 2 = 10.

The 2n + 2 rule works for every acyclic, saturated hydrocarbon. Hexane (C6H14), propane (C3H8), and methane (CH4) all follow it. Any ring or double bond removes two hydrogens from the count.

This is why butane cannot be C4H9 or C4H11. A molecule with nine hydrogens would have an unpaired electron, which is a radical, not a stable alkane. A molecule with eleven hydrogens would exceed carbon's bonding capacity. The 4:10 ratio is not arbitrary; it is forced by valence.

One Formula, Two Molecules

C4H10 corresponds to two distinct constitutional isomers. The straight chain, n-butane, connects carbon 1 to 2 to 3 to 4. The branched form, isobutane (2-methylpropane), places three carbons on a central carbon.

Both molecules have identical molecular formulas. They differ in connectivity, which makes them structural isomers. The formula C4H10 gives you the atom count but leaves the skeleton ambiguous.

This ambiguity is central to chemistry notation. Runners of molecular formulas report composition. A separate notation, the condensed structural formula or SMILES string, reports architecture.

n-Butane condensed formula: CH3CH2CH2CH3 Isobutane condensed formula: CH(CH3)3 or (CH3)3CH n-Butane SMILES: CCCC Isobutane SMILES: CC(C)C

The isomer distinction has measurable consequences. n-Butane boils at −0.5 °C (31 °F) and isobutane boils at −11.7 °C (10.9 °F). That 11 °C gap changes how each behaves as a fuel and refrigerant.

Quantified Context: Boiling Points and Beyond

Boiling point is the cleanest number for separating the isomers. n-Butane's −0.5 °C boiling point means it boils just below the freezing point of water. Isobutane's −11.7 °C means it boils well below that, and it stays liquid only under modest pressure or at cold temperatures.

These values are not trivia. They set practical thresholds. A butane lighter holds fuel under pressure at room temperature because the fuel would boil off instantly at atmospheric pressure. A cold-weather lighter fails because the internal pressure drops and fuel stops vaporizing.

Melting points diverge similarly: n-butane melts at −138 °C (−216 °F) and isobutane at −159.6 °C (−255 °F). Density and vapor pressure also differ. The pattern holds across the entire isomer set: identical formulas, different physical properties.

The molecular mass is the same for both isomers: 58.12 g/mol. Any method that measures only molar mass, such as basic mass spectrometry, cannot distinguish the two. You need a technique that probes structure, like infrared spectroscopy or nuclear magnetic resonance.

Where the Formula Falls Short

A molecular formula is necessary but not sufficient. It is a census, not a map. Several parts of a chemistry answer depend on structure when the prompt gave you only a formula.

Count hydrogen environments. n-Butane has two distinct hydrogen environments, the terminal methyl groups and the internal methylenes. Isobutane has two as well, but in a different pattern. In proton NMR, n-butane shows two signals and isobutane shows two, with different chemical shifts and splitting.

Predict combustion. Both isomers combust to carbon dioxide and water: 2 C4H10 + 13 O2 → 8 CO2 + 10 H2O. The balanced equation hides the isomer difference, because it tracks atoms, not connectivity. Per mole, both release similar energy, but isobutane has a slightly higher enthalpy of combustion.

Predict a permanent gas. At 25 °C and 1 atm, n-butane is a gas, but it is close enough to its boiling point that small changes matter. Isobutane at the same conditions is clearly a gas. If a problem asks about the "physical state of butane," the answer depends on which isomer and which temperature.

Read the instruction. "Express your answer as a chemical formula" asks for letters and subscripts, not a skeleton. A student who writes CH3CH2CH2CH3 in a field requesting a formula may lose points for over-answering.

Common Misconception About "Which Butane"

The most frequent error is treating butane as thermodynamically singular. There is no single stable form of C4H10 in nature that excludes the other. Both isomers exist on Earth, and natural gas and petroleum contain mixtures.

A second misconception confuses butane with butene. Butene has the formula C4H8 because it contains one carbon-carbon double bond. Butane's single bonds allow the full complement of ten hydrogens. Swap a double bond in and you lose two hydrogens. The distinction between alkane and alkene formulas is exactly this two-hydrogen difference.

A third error is assuming the formula tells you how the molecule will react. On paper, both isomers are saturated and relatively unreactive toward many reagents. The branching still affects reaction rates and products in combustion and halogenation, because branched radicals behave differently from linear ones.

Practical Actions for Students

If the problem asks for a molecular formula, write exactly C4H10 and stop. The four and the ten are subscripted in typeset form, but many online systems accept plain-text C4H10.

If the problem asks for the condensed structural formula, give CH3CH2CH2CH3 for n-butane or CH(CH3)3 for isobutane. State which isomer you are drawing.

If you are unsure whether the question wants formula or structure, re-read the verb. "Express as a chemical formula" is the formula. "Draw the structure" is the structure.

Check your subscript placement. A system that accepts "C4H10" and rejects "C4 H10" with a space is common. Test the input format before submitting.

Verify the balanced combustion equation if the problem extends to reactivity. Count carbons and hydrogens on both sides, then balance oxygen last.

When the Formula Is Not Enough

Some users of butane need structural information, not just composition. A formulation scientist selecting a propellant needs to know which isomer is used. An HVAC technician working with R-600a refrigerant needs to know the boiling behavior at operating temperatures. The molecular formula alone cannot answer those questions.

There are boundary conditions to state plainly. The 2n + 2 rule applies only to acyclic alkanes. It fails for cycloalkanes (CnH2n) and alkenes (CnH2n). Butane's formula is correct precisely because it is acyclic and fully saturated.

The formula also predicts nothing about the substance's safety profile. Butane is a highly flammable gas at room temperature, and both isomers share that hazard. The formula does not encode flammability, toxicity, or storage requirements. Those properties come from the full molecular description and from regulatory data, not from atom counting alone.

A limitation worth stating: the molecular formula for butane is an exact answer to a narrow question. Extend it only as far as the question requires.