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The amount fraction of component $i$ is $x_i=n_i/{\sum }_j{n}_j$, where $n_i$ is its amount of substance. It is dimensionless and the fractions sum to one for a complete composition specification.

For an ideal gas mixture at common temperature and pressure, amount fraction equals partial-pressure fraction. It generally differs from mass fraction because different species have different molar masses. For one mole each of hydrogen and oxygen molecules, both amount fractions are one half, although their masses are very different. The composition basis must therefore accompany a quoted concentration.

A binary mixture is a mixture with two specified chemical components. Water mixed with ethanol is a familiar example. “Binary” counts the components used to describe composition, rather than the number of thermodynamic phases: two components can form one homogeneous mixture or coexist in distinct thermodynamic phases.^[ref]

For two nonreacting components with amounts of substance $n_A,n_B$, the amount fractions satisfyOnly one composition coordinate is independent. In an original preparation using1mole of ethanol and3moles of water, $x_{\mathrm{ethanol}}=1/4$. Using rounded molar masses46.07and18.015grams per mole gives the ethanol mass fractionThus25percent by amount of substance and about46percent by mass describe the same preparation. A composition percentage without its basis is ambiguous. The calculation uses component masses, without assuming additive liquid volume.

Hexane and water illustrate why component count and phase count differ. Their limited mutual solubility permits two liquid thermodynamic phases, each containing some of the other component. A primary compiled experiment equilibrated the liquids by overnight shaking, sampled the aqueous portion with a microsyringe, and measured dissolved hydrocarbon by gas chromatography with flame-ionization detection.^[ref] The liquid immiscibility article develops a lever rule calculation distinguishing each phase's composition from the overall composition.

A binary mixture is not automatically an ideal solution. Interactions between unlike molecules can change activities, mixing enthalpy and phase stability. Stating temperature, pressure and the composition basis is therefore part of specifying the system, rather than optional experimental detail.

Concentration quantifies how much of a named constituent is present relative to a specified sample measure. In chemical terminology, volumetric concentrations use the final mixture volume: mass concentration is mass per volume and molar concentration is amount of substance per volume. Mass fraction and amount fraction instead compare a constituent with the corresponding total quantity in the mixture.^[ref]

The distinction matters in transport models. If an isolated sample expands without exchanging constituents, its volumetric concentration decreases even though its composition fractions stay fixed. A scalar called concentration in an advection model may represent a fraction instead, so its definition must be stated.

Conservation applies to the transported constituent amount. Reactions, diffusion and exchange then determine how the chosen concentration changes relative to its carrier and denominator.

Parts per million, written ppm, expresses a dimensionless ratio multiplied by $10^6$. A mass fraction of $3\times 10^{-6}$ is 3 ppm by mass; an amount fraction of the same numerical value is 3 ppm by amount of substance. The two compare different quantities and need not describe the same mixture composition.

A concentration written in milligrams per litre is a mass concentration, not automatically a parts-per-million ratio. Its approximate numerical equality to ppm by mass for a dilute aqueous sample uses an additional mass density approximation of $1kg/L$. Stating the basis prevents a unit conversion from silently becoming an assumption about the sample.