Metal ions in aqueous solutions are solvated, or hydrated by water molecules. Ty

Question

Metal ions in aqueous solutions are solvated, or hydrated by water molecules. Typically this primary hydration sphere is composed of six water molecules. The hydrated metal ion acts as a weak acid, undergoing a stepwise hydrolysis in which it donates an H^+ ion from its water ligands to the surrounding free water molecules. Shown below is the hydrolysis of AI(H_2O)_6^3+ (pK_a = 4.85) in water. Al(H_2O)_6^3+(aq) + H_2O irreversible H_3O^+(aq) + Al(H_2O)_5 OH^2+(aq) Calculate the pH of a 0.00239 M AICl_3 solution and determine what fraction of the aluminum is in the form Al(H_2O)_5OH^2+. pH = alpha_Al(H_2O)_5(OH)^2+ =

Explanation / Answer

For PH ,
PH = [PKa - log(0.00239) ] = [4.85 - log(0.00239)] = 7.47
PH = PKa + log10(0.00239 / [Al(H2O)5OH2+] )
substitute previous PH and PKa   
7.47 = 4.85 + log10 (0.00239 / [Al(H2O)5OH2+] )
2.62 = log10 (0.00239) - log10  [Al(H2O)5OH2+]
-5.241602 = log10 [Al(H2O)5OH2+]
[Al(H2O)5OH2+] = 5.73321*10-6M

, fraction of [Al(H2O)5OH2+] =   (5.73321*10-6 M / 0.00239 M ) = 2.39883*10-3

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