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Derive an expression for the hydrolysis constant and degree of hydrolysis of salt of strong acid and weak base. |
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Answer» Solution :LET US consider the reactions between a strong acid, HCl and a weak BASE, `NH_4OH` to produce a salt, `NH_4Cl` and water `HCl(aq)+NH_4OH(aq) leftrightarrowNH_4Cl(aq)+H_2O(l)` `NH_4Cl(aq)toNH_4^+ +Cl^(-) (aq)` `NH_4^+` is a strong conjugate acid of the weak base `NH_4OH` and it has a tendency to react with `OH^(-)` from water to produce unionised `NH_4OH` shown below. `NH_4^(+) (aq)+H_2O(l) leftrightarrow NH_4OH(aq)+H^+ (aq)` There is no such tendency shown by `Cl^-` and therefore `[H^+] gt [OH^-]`, the solution is acidic and the pH is less than 7. As discussed in the salt hydrolysis is strong base and weak acid . In this case also,we can establish a relationship between the `K_h and K_b` as `K_h.K_b=K_w` Let us calculate the `K_h` value in TERM of degree of hydrolysis (h) and the concentration of salt `K_h=h^2Cand[H^+]=sqrt(k_h.C)` `[H^+]=sqrt(K_w/K_b.C)` `pH=-LOG[H^+]=-log(K_w/K_b.C)^(1/2)` `=-1/2logK_w-1/2logC+1/2logK_b` `pH=7-1/2 pK_b-1/2 log C` |
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