Concentration

Concentration is a measure of the amount of one specified element, substance or other defined item within a mixture.

The concentration of something may be measured in all states of matter, and may be measured and quantified in a number of ways.

The concentration may be quantified as the mass of the compound measured as a proportion of the total mass. This may also be applied with volume and number of particles (see mole). Potentially mixtures of the methods may also be used.

Expressions of concentration

• Proportion of mass units
  • percentage (e.g. 50% w/w or weight for weight). For example if substance a has a mass of 50g in a total mixture mass of 100g, then substance A has a concentration of 50% w/w in that mixture.
• Proportion of volume units
• Number of particles units
  • mmol
  • moles A standard usage is the number of moles of substance A in a mixture of X litres, the units is thus m dm^-3
• Mixed method units
  • ppm?
  • ppb?
  • ppt?

Why use different measures of concentration?

The reason that you might want to use different measures are for use in different circumstances. For example, if you are performing an experiment such that the charge of a solution is important, then knowing the mass proportion of the ions in the solution is irrelevant, as different ions will have different mass, but may have the same charge. Thus, you would want to know the molar concentration of the solution, as molarity takes into account the [atomic mass]? of the substance.