b) A weak acid HA, has a K, of 435 x10 at 20°C. A the dissociation of this acid

Question

b) A weak acid HA, has a K, of 435 x10 at 20°C. A the dissociation of this acid an exothermic or endothermicprocese By At 5°C, the K. for the same acid is 4.90x10 a) The following table shows six acids and their K, values. Formula HF HNO2 HAc HOCI HCN PhOH Acid 6.6 x 10 Hydroflouric acid Nitrous acid Acetic acid 4.0x 10 18 x 10 3.3 x 10 Hypochlorous acid 62x 10 Hydrocyanic acid 1.0 x 10 Phenol 3 marks (i) Which acid would be the best choice for preparing a pi-4.50 buffer? Explain your choice Supposed you are not given the conjugate base of the acid, briefly describe how th can be formed from the acid you choose. I ma (ii)

Explanation / Answer

(b) As the Ka value is lower at higher at lower temperature than that at higher temperature, the dissociation is uunfavourable at higher temperature or with more energy provided. Therefore, this is an endothermic reaction. Wherein, according to the Lechatellier's principle, for an exothermic reaction if temperature is increased for an equilibrium reaction, the reaction shifts to left handside and forms more reactants to reetsablish equilirbium state again.

(a) given the table

(i) the pH of the buffer should be closest to the pKa of the weak acid component of the buffer. Therefore for a buffer with pH 4.50, the best choice for the weak acid is acetic acid with a pKa of 4.74.

(ii) To prepare conjugate base of acetic acid, we would add NaOH to the acetic acid solution.

let x amount of NaOH is added.

then [NaOH] added = [Naacetate] formed

using hendersen-Hasselbalck equation,

pH = pKa + log(base/acid)

4.50 = 4.74 + log(sodium acetate/acetic acid)

(sodium acetate/acetic acid) = 0.575

so rato of conjugate base to acid taken is 0.575.

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