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Calculate the pH at 25°C of a 0.31M solution of potassium acetate KCH3CO2 . Note that...

Calculate the pH at 25°C of a 0.31M solution of potassium acetate KCH3CO2 . Note that acetic acid HCH3CO2 is a weak acid with a pKa of 4.76 . Round your answer to 1 decimal place.

Solutions

Expert Solution

Ans :- pH = 9.24

Explanation :-

ICE table is :

...........................CH3CO2-(aq).............. +.......... H2O ----------> HCH3CO2 (aq)............. +............ OH- (aq)

Initial (I)................0.31 M................................................................0.0 M........................................0.0 M

CHange (C).............-y........................................................................+y..............................................+y

Equilibrium (E)......(0.31-y) M...............................................................y M..........................................y M

Pka = 4.76

So,

-logKa  = 4.76

Ka= 10-4.76  = 1.8 x 10-5  

and , kb= Kw/Ka = 1x10-14/1.8 x 10-5   = 5.6 x 10-10

Kb   = [HCH3CO2][OH-]/[CH3CO2-]

5.6 x 10-10 = y2 /(0.31 - y)

As y <<<0.31, So neglect y as compare to 0.31 M.

y2 = (5.6 x 10-10)(0.31)

y = 1.74 x 10-5

So, [OH-] = y = 1.74 x 10-5 M

pOH   = -log[OH-]

= - log 1.74 x 10-5 M

= 4.76

So,

pH = 14 - pOH

= 14 - 4.76

= 9.24

Hence, pH = 9.24


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