IB Chemistry HLTopic 3 — Classification of MatterPaper 1 & 2Core skill~11 min read
Formulas of Organic Compounds
Organic chemistry has six different ways of writing down the same molecule, and the exam swaps between them without warning. None of it is hard once you know what each one is hiding — and that is really the whole trick. Each formula shows you less than the one before it, and expects you to fill in the rest.
📘 What you need to know
Empirical formula — the simplest whole-number ratio of the atoms.
Molecular formula — the actual number of each atom in one molecule.
Full structural formula (also called displayed or graphical) — every atom and every bond drawn out as a line.
Condensed structural formula — atoms written in groups, most bonds left out.
Skeletal formula — only the carbon framework, drawn as lines. Carbons and their hydrogens are invisible.
Stereochemical formula — uses wedges and hashes to show the three-dimensional shape around a carbon.
The command word decides which one you give. “State the molecular formula” and “draw the structural formula” want completely different answers.
Empirical and molecular formulas
These two are the easiest pair to tell apart, because only one of them describes a real molecule.
The molecular formula counts atoms. Butane is C4H10 because one butane molecule genuinely contains four carbons and ten hydrogens. The empirical formula throws that information away and keeps only the ratio, so butane becomes C2H5.
Getting from one to the other
empirical formula = molecular formula ÷ highest common factor
So the job is always the same: find the highest number that divides into every subscript, then divide. Glucose, C6H12O6, has a common factor of 6, so its empirical formula is CH2O. Ethanoic acid, C2H4O2, has a common factor of 2, so its empirical formula is also CH2O — and that is exactly why an empirical formula on its own is not enough to identify a compound.
Sometimes they are the same. Butan-2-ol is C4H10O. The highest common factor of 4, 10 and 1 is 1, so nothing divides — the empirical formula is also C4H10O. Students often assume there must be something to cancel and invent a wrong answer. If the common factor is 1, write the formula out again and move on.
Any formula with an odd number in it, or with a single atom of something, is very likely already at its simplest ratio. Check the oxygen count first — if there is only one O, the common factor has to be 1.
Full structural formula
The full structural formula — you will also see it called the displayed formula — shows every single atom and draws every single bond as a line. Nothing is hidden and nothing is assumed. It is slow to draw, which is why the exam usually only asks for it on small molecules.
Below is the same alcohol written four ways, so you can see exactly what each version drops.
Notice what survives every version: the oxygen. Carbons and hydrogens get hidden, but anything that is not carbon or hydrogen is always shown, in every formula type.
Your built-in checking tool: carbon makes exactly four bonds, oxygen two, hydrogen one, nitrogen three. Count the lines coming off every atom in your drawing. If a carbon has only three lines, you have forgotten a hydrogen — and that is a mark gone.
Condensed structural formula
Here you write the atoms in groups and let the reader assume the bonds. Butane goes from a long drawing to CH3CH2CH2CH3, or even CH3(CH2)2CH3 if you want to bracket the repeating part.
Two things do have to be shown, though:
Branches go in brackets. 2-methylbutane is CH3CH(CH3)CH2CH3. The bracketed CH3 is hanging off the carbon written immediately before it.
Double and triple bonds are drawn in. Propene is CH2=CHCH3. If you wrote CH2CHCH3 the reader has no way to know where the double bond is.
🧩 Turning a drawing into a condensed formula
Find the longest chain and start at one end of it.
Walk along one carbon at a time. For each carbon, count the hydrogens attached to that carbon and write it as CH3, CH2, CH or C.
If a carbon carries a branch, write the branch in brackets straight after that carbon.
Keep any double or triple bond as = or ≡ between the two carbons it joins.
Check your hydrogen total against the molecular formula. If they disagree, you dropped one somewhere in step 2.
Skeletal formula
Skeletal formulas look almost empty the first time you see one, and that is the point — they strip out everything predictable so the interesting bits stand out. The rules are short:
Carbon–carbon bonds are plain straight lines, drawn as a zigzag.
Every corner and every free line-end is a carbon atom. No letter C is written.
Hydrogens on carbon are not drawn at all. You work them out by giving each carbon enough to reach four bonds.
Anything that is not carbon, and any hydrogen attached to it, is written — so OH, Cl, NH2 all appear.
This molecule is 2-methylbutane, C5H12. Count the carbons by touching each corner and each loose end with your pen — four along the chain plus one on the branch.
Methane has no skeletal formula, and this catches people out. A skeletal formula is built out of carbon–carbon bonds, and methane only has one carbon, so there is nothing to draw. A single dot would look like a radical, and drawing all four C–H bonds would make it a structural formula instead.
Stereochemical formula
The four formulas above all treat molecules as flat, which is fine most of the time. But a carbon with four different groups on it is a tetrahedron, and there are two ways of arranging those groups that are mirror images of each other. To tell those two apart on a flat page, you need wedges.
The tetrahedral angle here is about 109.5°, because the carbon has four electron domains pushing each other as far apart as they can get in three dimensions.
Only reach for wedges when the question is actually about three-dimensional shape — chiral centres, mirror images, optical isomers. If a question just says “draw the structural formula”, wedges are extra work that earns nothing.
Worked examples
WORKED EXAMPLE
Deduce the molecular and empirical formulas of the ester CH3COOCH2CH3.
Step 1: count the atoms group by groupCH3 gives 1 C and 3 H. CO gives 1 C and 1 O. O gives 1 O. CH2 gives 1 C and 2 H. CH3 gives 1 C and 3 H.C: 1 + 1 + 1 + 1 = 4 H: 3 + 2 + 3 = 8 O: 1 + 1 = 2Step 2: write the molecular formulamolecular formula = C4H8O2Step 3: find the highest common factor of 4, 8 and 2HCF = 2, so divide every subscript by 2C4H8O2 and C2H4OCareful — C2H4O is also the empirical formula of ethanoic acid and of methyl methanoate. An empirical formula never identifies a compound on its own.
WORKED EXAMPLE
A molecule has four carbons in its longest chain, with a methyl group on the second carbon. Write its condensed structural formula and give its molecular formula.
Step 1: walk the main chain, one carbon at a timeCarbon 1 is at the end, so it has 3 H. Carbon 2 carries the branch, so it only has 1 H. Carbons 3 and 4 are a CH2 and an end CH3.Step 2: bracket the branch straight after the carbon it sits onCH3CH(CH3)CH2CH3Step 3: total the atoms, branch includedC: 4 in the chain + 1 in the branch = 5H: 3 + 1 + 3 + 2 + 3 = 12CH3CH(CH3)CH2CH3, which is C5H12This is 2-methylbutane — the same molecule drawn in the skeletal diagram above. Worth checking you get 12 hydrogens both ways.
WORKED EXAMPLE
For CH3CH(OH)CH2CH3: give the molecular formula, the empirical formula, and say how many lines you would draw in its skeletal formula.
Step 1: molecular formulaC: 4 H: 3 + 1 + 1(on the O) + 2 + 3 = 10 O: 1molecular formula = C4H10OStep 2: empirical formulaThe HCF of 4, 10 and 1 is 1, so there is nothing to cancel.empirical formula = C4H10OStep 3: count the lines in a skeletal drawing4 carbons in a row need 3 carbon–carbon lines. Then 1 more line from carbon 2 out to the written OH.C4H10O, C4H10O, and 4 linesThe OH is written out because oxygen is not carbon. Its hydrogen gets shown too, even though the ten on the carbons do not.
Which formula does the question want?
Formula type
What it shows
Example (propan-1-ol)
Typical command word
Empirical
Simplest ratio only
C3H8O
Deduce the empirical formula
Molecular
Real atom counts
C3H8O
State the molecular formula
Full structural
Every atom, every bond
All 12 atoms drawn with 11 bonds
Draw the full structural formula
Condensed structural
Grouped atoms, key bonds
CH3CH2CH2OH
Write the structural formula
Skeletal
Carbon framework plus other atoms
Two-line zigzag ending in OH
Draw the skeletal formula
Stereochemical
Three-dimensional arrangement
Wedges around a chiral carbon
Draw the two enantiomers
💡 Exam tip
Read the command word before you draw anything. Drawing a beautiful full structural formula when the question asked for the molecular formula scores zero.
Count bonds on every atom before you hand it in. Four on carbon, two on oxygen, three on nitrogen, one on hydrogen. This single check catches most structure-drawing errors.
Draw bonds long and clear. If your CH3 hydrogens are crammed against the carbon, an examiner cannot tell how many you meant.
In a condensed formula, keep double and triple bonds visible. CH2=CHCH3, not CH2CHCH3.
When counting carbons on a skeletal formula, touch each vertex with your pen. The two free ends are the ones people forget.
An empirical formula question is a division question. Find the highest common factor first, then divide once. Do not try to do it by eye.
⚠ Common mix-up
Thinking the empirical formula must always be smaller. If the highest common factor is 1, the empirical and molecular formulas are identical. C4H10O stays C4H10O.
Calling a condensed formula a molecular formula. CH3CH2OH is a structural formula; C2H6O is the molecular formula. They contain different information.
Forgetting the free ends of a skeletal formula. A three-line zigzag has four carbons, not three.
Writing hydrogens on a skeletal formula anyway. The only hydrogens that get drawn are the ones on a non-carbon atom, such as the H in OH.
Leaving a carbon with three bonds in a full structural formula. Every carbon needs four lines, no exceptions.
Drawing wedges when nobody asked. Wedges only mean something if the question is about three-dimensional shape.
Trying to give methane a skeletal formula. One carbon means no carbon–carbon bonds, so there is nothing to draw.
Up next: Functional Groups — now that you can read any formula, the next job is spotting the reactive part of the molecule inside it.
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