pH Formula

pH Formula

Definition: pH is a chemical scale invented by the Danish chemist Soren Sorensen for defining the acidity of a substance. Note that, there are different definitions for the term "acid", according to the Bronsted-Lowry theory, acid is a substance that can donate protons H+ in solution. On the other hand, Lewis theory postulates an acid is a substance that can accept electrons for forming covalent bonds.

Mathematically, it is defined as the decimal logarithm of the reciprocal of the H3O+ concentration in a solution. pH scale define as 0 the maximum of acidity and 14 the minimal of acidity (or also the maximum of basicity). Thus, substances with a pH between 0-7 are called acids and when the pH is between 8-14, the substances are called bases.

General formula: The formula for calculating the pH of a solution is:

pH = - log [H3O+]

also note that, pOH is used for defining the basicity of a substance and it is calculated;

pOH = - log [OH-] or alternatively, pH + pOH = 14

where, [H3O+] is the concentration of hydronium ion in moles per litro.

Uses: Today the pH is one of the most used parameters for determining the quality of water. It is par f the physical chemistry analysis running for quality control. Moreover, the pH is also extremely needed for knowing if a reaction can occur or not, pH of a solutions is fundamental for producing or isolating substances in a reaction, for example, if the pH of a solution is 9 and the product to be formed is an carboxylic acid, it cannot be recovery because an acid cannot exist at pH higher than 7. pH is also used for controlling environmental (pH of seas, lakes, soils, etc) and health factors, for example, the pH of body fluids is between pH 6-7.5. In a general way, it can be measured using pH universal indicators or pHmeters equipments.

Examples: A solution of HCl in water as a concentration of 0.0050 mol/L. Calculate the pH of this solution.

Note that HCl is a strong acid, so that the dissociation of HCl in solution is complete:

HCl + H2O → H3O+ + Cl-

And the concentration of HCl, remains the same for the concentration of H3O+ (0.0050 g/mol), then

pH = -log (0.0050) = 2.30

Considerations: Note that pH depends on the temperature, thus the pH of the sam solutions will change if the temperature increases or decreases.