Representation of Compounds
A compound can be written down several different ways, and each way tells you something different.
A compound can be written down several different ways, and each way tells you something different. This subtopic covers the two basic representations — chemical formulas and structural formulas — the law of constant composition that makes those representations meaningful in the first place, the distinction between an empirical formula and a molecular formula, and how to calculate percent composition.
Key Takeaways
A chemical formula gives the types and ratio of atoms in a compound; a structural formula shows their spatial arrangement and bonding.
The law of constant composition (law of definite proportions): every pure sample of a compound has the same elements in the same mass proportions, regardless of source.
The empirical formula is the simplest whole-number ratio of elements; the molecular formula is the actual atom count per molecule. They can be identical (water, H₂O) or related by a whole-number multiplier (glucose: CH₂O empirical, C₆H₁₂O₆ molecular).
Ionic compounds (NaCl, CaCO₃) are represented only by an empirical formula — they don't form discrete molecules (see formula unit, Molecules and Moles).
Percent composition = (mass of element ÷ molar mass of compound) × 100%; calculable from either formula type, and reversible — percent composition plus molar mass lets you determine a compound's molecular formula.
Chemical Formulas vs. Structural Formulas
A chemical formula lists the types and numbers of atoms present in a molecule — for example, H₂O tells you a water molecule contains two hydrogen atoms and one oxygen atom. It gives you the ratio of atoms, nothing more.
A structural formula shows how those atoms are actually arranged in space and how they're bonded to each other — for water, H–O–H, showing that both hydrogens bond to the central oxygen rather than to each other.
Both representations matter because they carry different information. A chemical formula tells you composition; a structural formula tells you three-dimensional shape and bonding pattern, which in turn determines how a compound behaves in a reaction. Being able to read and convert between the two is part of interpreting compounds correctly on the MCAT.
The Law of Constant Composition
The law of constant composition (also called the law of definite proportions) states that any pure sample of a given compound contains the same elements in the same proportion by mass, regardless of where the sample came from or how it was made.
Water is the standard example: every pure sample of H₂O contains exactly two hydrogen atoms for every oxygen atom, and by mass, that works out to about 11.2% hydrogen and 88.8% oxygen — every time, in every sample, from any source. This is what makes compounds predictable: if you know the formula, you know the mass proportions, and that consistency is what allows chemical formulas to mean anything at all.
Empirical Formula vs. Molecular Formula
Two formulas often get used to describe the same compound, and it's important to keep them straight:
The empirical formula gives the simplest whole-number ratio of elements in a compound.
The molecular formula gives the actual number of atoms of each element in one molecule of the compound.
Hydrogen peroxide is a clean example of the two diverging. Its molecular formula is H₂O₂ — each molecule really does contain two hydrogen atoms and two oxygen atoms. But the simplest whole-number ratio of H to O is 1:1, so its empirical formula is HO.
For some compounds, the two formulas are identical. Water is one: the actual atom count per molecule (2 H, 1 O) is already the simplest possible ratio, so both the empirical and molecular formulas are H₂O.
Glucose shows the relationship between the two clearly. Its molecular formula, C₆H₁₂O₆, reduces to a simplest ratio of 1 carbon : 2 hydrogen : 1 oxygen — an empirical formula of CH₂O. The molecular formula is just the empirical formula scaled up by a whole-number multiplier (here, ×6).
Ionic compounds like sodium chloride (NaCl) and calcium carbonate (CaCO₃) are represented only by their empirical formula. As covered in formula units (see Molecules and Moles), ionic compounds don't exist as discrete, countable molecules — they form a continuous crystal lattice — so there's no "molecular formula" to write in the first place. The empirical formula is the only formula that applies.
Percent Composition
The percent composition of an element in a compound is the percentage of the compound's total mass contributed by that one element:
% Composition = (Mass of element in formula ÷ Molar mass of compound) × 100%
Percent composition can be calculated from either the empirical or the molecular formula — the ratio of elements is identical either way, so the result is the same.
The relationship also runs in reverse: given a compound's percent composition and its molar mass, you can determine its molecular formula. The process has three steps:
Convert each element's mass percentage to a mole ratio (treat the percentages as grams per 100 g of compound, then divide each by that element's atomic mass).
Reduce that mole ratio to the simplest whole-number ratio — this gives the empirical formula.
Compare the empirical formula's mass to the compound's actual molar mass to find the whole-number multiplier, then scale the empirical formula up to get the molecular formula.
Worked example: A compound is 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass, with a molar mass of about 180.16 g/mol. What is its molecular formula?
Treating the percentages as grams per 100 g of compound and converting to moles:
Carbon: 40.0 g ÷ 12.011 g/mol ≈ 3.33 mol
Hydrogen: 6.7 g ÷ 1.008 g/mol ≈ 6.65 mol
Oxygen: 53.3 g ÷ 16.00 g/mol ≈ 3.33 mol
Dividing each by the smallest value (3.33) gives a ratio of C : H : O ≈ 1 : 2 : 1, so the empirical formula is CH₂O, with an empirical formula mass of about 30.03 g/mol.
Dividing the actual molar mass by the empirical formula mass: 180.16 ÷ 30.03 ≈ 6. Scaling CH₂O by 6 gives a molecular formula of C₆H₁₂O₆ — glucose.
Common MCAT Mistakes
Assuming a compound's empirical and molecular formulas are always different. They're identical whenever the actual atom count is already the simplest ratio — water (H₂O) is both at once.
Trying to write a molecular formula for an ionic compound. Ionic compounds like NaCl form a continuous crystal lattice, not discrete molecules, so only an empirical formula applies to them.
Forgetting percent composition works in reverse. Given percent composition and molar mass, you can derive the molecular formula — the process runs empirical formula first, then scale up using molar mass.
Mixing up which formula percent composition depends on. Percent composition is identical whether calculated from the empirical or molecular formula, since the ratio of elements doesn't change between the two.
MCAT-Style Concept Check
Question: A compound has an empirical formula of CH₂ and an empirical formula mass of about 14.03 g/mol. If its actual molar mass is about 84.16 g/mol, what is its molecular formula?
A) CH₂
B) C₂H₄
C) C₃H₆
D) C₆H₁₂
Answer: D
Explanation: Dividing the actual molar mass by the empirical formula mass gives the scaling multiplier: 84.16 ÷ 14.03 ≈ 6. Scaling CH₂ by 6 gives C₆H₁₂.
FAQ
What's the difference between a chemical formula and a structural formula?
A chemical formula (like H₂O) gives only the types and ratio of atoms present. A structural formula (like H–O–H) additionally shows how those atoms are bonded and arranged in space — information a chemical formula alone doesn't provide.
What does the law of constant composition mean?
It means any pure sample of a given compound always contains the same elements in the same mass proportions, no matter where the sample came from. This is what makes a chemical formula a reliable, consistent description of a compound.
Can the empirical and molecular formula be the same?
Yes. Water is the standard example — its actual atom count per molecule (2 H, 1 O) is already the simplest whole-number ratio, so its empirical and molecular formulas are both H₂O.
Why don't ionic compounds have a molecular formula?
Ionic compounds like NaCl form a continuous three-dimensional crystal lattice rather than discrete, countable molecules, so there's no single "molecule" to describe. Only the empirical formula — the simplest ratio of ions — applies.
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