PH (exp:1) - assignment - Q-A

(1) Q: Discuss the environmental significance of “pH”.
Ans: A controlled value of pH is desired in water supplies, sewage treatment and chemical process plants. In water supply pH is important for coagulation, disinfection, softening and corrosion control. In biological treatment of waste pH is the most significant. Organisms involved in treatment plants are operative within certain pH range.
According to Bangladesh Environment Conservation Rule (1997), drinking water standard for pH is 6.5 to 8.5.
(2) Q: Define pH in terms of hydrogen-ion (H+) concentration and hydroxyl-ion
(OH-) concentration. A decrease in pH of one unit represents how much increase in hydrogen-ion concentration?
Ans: pH is a measure of the acid or alkaline condition of water. It is way of expressing the hydrogen ion concentration, or more preciously, the hydrogen ion activity. pH is define as follows:
pH = -log{H+}
Where, {H+} is the concentration (or activity) of hydrogen ion (or proton) in moles per liter (M)
Water dissociates to form hydrogen ion ( H+) and hydroxyl ion (OH-) according to following equation:
H2O = H+ + OH-
At equilibrium, it can be written,
Kw = {H+} {OH-} / {H2O}
But , since concentration of water is extremely large (approximately 55.5 mol/L) and is diminished very little by the slight degree of ionization, it may be considered as a constant and its activity is taken as 1.0. Thus
Kw = {H+} {OH-}
Where Kw = Equilibrium Constant
For pure water at 250C, Kw = 10-7×10-7 = 10-14 . This known as the ion product of water or ionization constant for water. In other words, water (de-ionized or distilled water) at 250C dissociates to yield 10-7 mol/L of hydrogen ion (H+) and 10-7 mol/L f hydroxyl ion (OH-). Hence, according to equation pH = -log {H+}
, pH of deionized water is equal to 7.0.
(3) Q: At 250C, hydroxyl-ion concentration of a solution is 10-5.7. Determine the pH of the solution.
Ans: For pure water at 250C, Kw = 10-7×10-7 = 10-14 and given {OH-} = 10-5.7
{H+} = Kw / {OH-}
= 10-14 / 10-5.7
= 10-8.3
pH = -log{H+}
= -log 10-8.3 = 8.3

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