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The self-ionization of water (also autoionization of water, autoprotolysis of water, autodissociation of water, or simply dissociation of water) is an ionization reaction in pure water or in an aqueous solution, in which a water molecule, H 2 O, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH −.
It is usually indicated by the Greek symbol α. More accurately, degree of dissociation refers to the amount of solute dissociated into ions or radicals per mole. In case of very strong acids and bases, degree of dissociation will be close to 1. Less powerful acids and bases will have lesser degree of dissociation.
The dissociation constant of water is denoted K w: = [+] [] The concentration of water, [H 2 O], is omitted by convention, which means that the value of K w differs from the value of K eq that would be computed using that concentration.
The degree of dissociation α (also known as degree of ionization), is a way of representing the strength of an acid. It is defined as the ratio of the number of ionized molecules and the number of molecules dissolved in water. It can be represented as a decimal number or as a percentage.
i = 1 for sugar in water; i = 1.9 for sodium chloride in water, due to the near full dissociation of NaCl into Na + and Cl − (often simplified as 2) i = 2.3 for calcium chloride in water, due to nearly full dissociation of CaCl 2 into Ca 2+ and 2Cl − (often simplified as 3)
When solute particles neither dissociate nor associate in solution, i equals 1 (e.g. glucose in water). The value of i is the actual number of particles in solution after dissociation divided by the number of formula units initially dissolved in solution and means the number of particles per formula unit of the solute when a solution is dilute.
Acid dissociation constants are also essential in aquatic chemistry and chemical oceanography, where the acidity of water plays a fundamental role. In living organisms, acid–base homeostasis and enzyme kinetics are dependent on the p K a values of the many acids and bases present in the cell and in the body.
ν i is the number of ions i in the formula unit of the electrolyte (e.g. 2 and 1 for Na + and SO 2− 4 in Na 2 SO 4). Kohlrausch's evidence for this law was that the limiting molar conductivities of two electrolytes with two different cations and a common anion differ by an amount which is independent of the nature of the anion.