Italy Case

Determination of pH in the solution. need help. thanks?

Determine the pH of the following solutions.
(a) a 0.42 M CH3COOH solution

(b) a solution that is 0.42 M CH3COOH and 0.24 M CH3COONa

Lancenigo di Villorba (TV), Italy

Case A)
You get an aqueous solution of Acetic Acid (e.g. CH3COOH).

CH3COOH(aq) ---> H+(aq) + CH3COO-(aq)

OBEYING TO CHEMICAL EQUILIBRIUM : mathematically speaking, it exists an Equilibrium Constant which governs the concentration of chemical species involved in equilibrium.
This constant is Ka which results equal to

Ka = 1.8E-5 = |H+| * |CH3COO-| / |CH3COOH|

A very common assumpt holds that Hydrogen Ion and Acetate Ion (e.g. H+ and CH3COO- respectively) comes only for that equilibrium : this isn't strictly true but THIS ASSUMPT SIMPLIFIES YOUR CALCULATIONs (no-rigorous calculation)

|H+| = |CH3COO-|
|CH3COOH| = Ca - |H+| = 0.42 - |H+|

so I have to solve

Ka = 1.8E-5 = |H+|^2 / (Ca - |H+|)

which gives off

|H+| = 2.7E-3 M
pH = -LOG(|H+|) = -LOG(2.7E-3) = 2.6

Case B)
The HENDERSON Formula states about pH index of Chemical Buffers based on Weak and Monovalent Electrolytes.
In the case of Acetic Acid's Buffers, you retrieve

pH = pKa + LOG[|CH3COO-| / |CH3COOH|]

as you get starting from the Chemical Equilibrium.
So, by this formula

pH = 4.74 + LOG[0.24 / 0.42] = 4.5

I hope this helps you.

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