Analysis: The determination of the composition of a substance.
Composition: The composition of a substance or of a mixture can be expressed in many ways.
In elemental analysis it is usually expressed as the percentage by mass of each element in the substance.
For solutions or mixtures common terms are concentration, mole fraction, molality, percentage composition, and parts per million.
Concentration: Symbol c, the "amount" of solute per unit volume of the solution.
The "amount" may be given as mass, and , where m is the mass of solute and V the volume of the solution (common unit g L−1), or using the formal definition of amount of substance as , (common unit mol L−1).
For a given solute, concentration in one set of units may be converted to the other using
m = n M or
The term molarity, symbol M, is often used by analytical chemists for concentration
in units of mol L−1.
Example The molarity of the above NaCl solution is 0.470; it is a 0.470 M solution of NaCl;
in speech it may be described as a 0.470 molar solution.
Here molar means mol L−1, not per mole.
This is an example of chemists having two different meanings for the same word.
The meaning is made clear by the context in which the word is used.
Mole fraction: Symbol x, the amount of a substance or entity (molecules, atoms or ions) in a mixture as a fraction of the total amount of substances or entities in the mixture.
Molality: The amount of solute per kilogram of solvent. (unit: mol kg−1)
Percentage composition: There are various forms of this.
w/w is percentage by mass of the substance relative to the total mass.
%w/V is the mass of substance in grams divided by the volume of the solution in millilitres and multiplied by 100. (Chemists here use w for mass, originating from the loose use of the word weight.)
V/V is the percentage by volume of the substance relative to the total volume.
Parts per million: Symbol ppm: a ratio.
For a gaseous mixture, the mole ratio, (mol/mol).
For solutions, normally calculated from mg of substance per kg of solution, but for dilute aqueous solutions as mg of substance per litre of solution.
For solids, the mass ratio.