The+pH+scale

//The pH scale is defined as follows: // pH = -log10[H+ (aq)] //pure water, for instance, dissociates slightly: //  H 2 O (l) H +  (aq) + OH -  (aq) At 25 o C the concentration of H +  ions in pure water is 1.0 X 10 -7  mol dm -3 . So the pH of water is  pH = -log 10 (1.0 X 10 -7 ) = 7 Pure water is neutral. Any neutral solution has a pH of 7 at 25 o C

//<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">If the solution is acidic: //
 * <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ion concentration is greater than 1.0 X 10 -7 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> mol dm -3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">. The pH will be lower than 7.

//<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">If the solution is basic: //
 * <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ion concentration is less than 1.0 X 10 -4 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> mol dm -3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">. The pH will be greater than 7.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The pH scale runs from 0 to 14. As the pH scale is a logarithmic scale to base 10, a one unit change in pH is equivalent to a tenfold change in H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ion concentration.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Eg: 0.1 mol dm -3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> aqueous solution of NaOH has pH 13 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">While 0.01 mol dm -3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> aqueous solution of NaOH has pH 12

By the end of this lesson you should be able to:
 * Distinguish between acidic, neutral and alkaline solutions using the pH scale
 * Be aware that a 1 unit change in the pH implies a tenfold change in the hydrogen ion concentration

**<span style="color: #800080; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">NEXT UP- OXIDATION AND REDUCTION **
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