WATER and pH

WATER and pH


A water molecule is an irregular and irregular slightly skewed tetrahedron with an oxygen atom at its center. Water is a solvent for a wide range of organic molecules. Water molecules can dissociate into hydroxide ions(OH-) and protons (H+).

Structure of water

Water is a dipole molecule. Water molecules have inter and intra linked hydrogen bonds. Normally water is in liquid form at room temperature. The boiling point of water is 100°C and the freezing point is 0°C.
Water molecules are V-shaped with an angle of 104.5°. water is polar because of the high electronegativity of oxygen. A molecule of water consists of two hydrogen atoms and one oxygen atom that is attached by two sigma bonds and has two lone pairs of electrons. The geometry of water molecules is stated as bend or angular with sp3 hybridization.

Oxygen is highly electronegative in comparison to Hydrogen so the sharing of electron between oxygen and hydrogen is unequal. The electrons are more often in the vicinity of the oxygen rather than hydrogen. This results in developing a partial positive charge in each Hydrogen. This is due to the presence of Hydrogen bonds. Hydrogen bonds are relatively weak, and the energy required to break the bond(Bond dissociation energy) of about 23KJ/mol.

Water dissociation constant

  • Water molecules have a tendency to undergo reversible ionization.

  • The position of equilibrium of any chemical reaction is given by its equilibrium constant (Keq).

  • The equilibrium constant can be defined in terms of the conc. of the reactants and products.

  • In pure water the concentration of water at 25°C is 55.5M [1,000 g/L)/(18 g/mol)].

  • Keq value determined by the electrical conductivity measurement of pure water and that is 1.8 X 10-16.

The product of [H+][OH-] at 25°C in the aq. The solution is always 1.0 X 10-14 M2. . there is exactly equal concentrations of H+ and OH- in pure water, so the solution is at neutral pH.

What is pH?

pH is the negative log of hydrogen ions. The term pH was introduced by Sorensen in 1909. Based on pH different solutions are categorized as Acid and base.

Henderson Hassel-Balch equation

This equation is important for understanding buffer action and acid-base balance Henderson equation can be derived as follow:


  • Lehninger, A. L., Nelson, D. L., & Cox, M. M. (2005). Lehninger principles of biochemistry. Macmillan.


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