Alkalinity and Acidity (SS1 Third Term Chemistry)

Introduction to Alkalinity and Acidity

Alkaliniy and Acidity are measured using a scale of numbers from 0 to 10 called the pH scale. A solution with pH value of 7 is neutral. A solution with a pH value less than 7 is acidic while a solution with pH value greater than 7 is alkaline. Acidity increases with decreasing pH while alkalinity increases with increasing pH.

Dissociation of water

Water is neutral and it ionizes very slightly to yield an equal number of hydrogen ions and hydroxide ions.

[H+] = [OH] = 1 x 10-7 mol dm-3 (at 25oC)

The product of the two ionic concentrations gives the ionic product of water.  It is represented by

Kw = [H+] = [OH] = 1 x 10-7  (mol dm-3)2 (at 25oC)
= 10-14 mol2 dm-6 (at 25oC)

Kw is constant under al circumstances at 25oC.

pH Scale

the pH of a solution is defined as the negative logarithm of the hydrogen ion concentration to the base 10. It can also be defined as the measure of the concentration of hydrogen ions in solution or the measure of the alkalinity or acidity of a solution. example, if the hydrogen ion concentration of a given aqueous medium is 10-5 mol dm-3 the acidity of the solution could be written in terms of its pH as follows

[H+] = 10-5

Log [H+] = Log 10-5

= -5

pH = -Log [H+] = – (-5) = 5.

pOH = 14 – pH = 14 – 5 = 9…

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