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Derive Henderson - Hassel Balch equation. |
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Answer» Solution :(i) The concentration of hydronium ion in an ACIDIC buffer solution depends on the RATIO of the concentration of the weak acid to the concentration of its conjugate base present in the solution i.e, `[H_3O^+]=K_a(["acid"]_(aq))/(["base"]_(aq))` The weak acid is dissociated only to a small extent. Moreover, due to common ion effect the dissociation is further suppressed and hence the equilibrium concentration of the acid is nearly equal to the initial concentrationof the unionised acid. Similarly, the concentration of the conjugate base is nearly equal to the initial concentration of the added SALT. `[H_3O^+]=K_a(["acid"])/(["salt"])` (iii) Here [acid] and [salt] represent the initial concentration of the acid and salt, respectively used to prepare to buffer solution Taking logarithm on both sides of the equation `LOG[H_3O^+]=logK_a+log.(["acid"])/(["salt"])` reverse the sign on both sides `-log[H_3O^+]=-logK_a-log.(["acid"])/(["salt"])` We know that `pH=-log[H_3O^+] and pK_a=-log K_a` `rArr pH=pK_a-log.(["acid"])/(["salt"])` `rArr pH=pK_a+log.(["acid"])/(["salt"])` Similarly for a basic buffer, `pOH=pK_a+log.({"salt"])/(["base"])` |
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