1. pH in Aqueous Solution

Pure liquid water ionizes according to the equilibrium below.

ionwater.jpg

In pure water, the concentrations of both [H+] and [OH-] are equal to 1 x 10-7.

pH is defined as the negative logarithm (base 10) of the concentration of H+. So in pure water

pHeq.jpg

An acid or base is anything that changes [H+] and pH when added to water.

 

A strong acid or base ionizes completely in aqueous solution.

 

Hydrochloric acid is an example of a strong acid. Adding a mole of HCl to water produces a mole of H+ (and a mole of Cl-), so calculating pH changes is straightforward.

A 0.1 molar solution of HCl has a pH = -log(0.1) = 1, a 0.01 molar solution has a pH = -log(0.01) = 2, and so forth.

Hydrochloric acid is an essential component of gastric acid, which has a normal pH of 1.5 to 3.5.

 

A weak acid or base does not ionize completely in aqueous solution.

 

Ionization of a weak acid (HA) is characterized by its dissociation constant (Ka).

Kaeq.jpg

The value of Ka is proportional to the strength of the acid. A larger Ka means more dissociation, higher [H+], and lower pH.

A- is known as the conjugate base of the weak acid HA.

pKa = -log(Ka), analogous to pH.

Ionization of a weak base is described in the same manner using Kb and pKb.

 

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