Metal ions in aqueous solutions are solvated, or hydrated by water molecules. Ty
ID: 947826 • Letter: M
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 Al(H2O)63 (pKa = 4.85) in water.
Calculate the pH of a 0.00151 M AlCl3 solution and determine what fraction of the aluminum is in the form Al(H2O)5OH2 .
Explanation / Answer
[Al (H2O)6 ]+3 -------------------> [Al(H2O)5 (OH) ]+2 + H+
0.00151 0 0
0.00151-x x x
Ka = [[Al(H2O)5 (OH) ]+2 ][ H+]/[[Al (H2O)6 ]+3 ]
1.41 x 10^-5 = x^2 / 0.00151 -x
x^2 + 1.41 x 10^-5 x - 2.129 x 10^-8 = 0
x = 1.39 x 10^-4
[H+] = x = 1.39 x 10^-4 M
pH = -log [H+] = -log (1.39 x 10^-4 )
pH = 3.86
fraction of aluminum is in the form [Al(H2O)5OH ]+2 = x / C
= 1.39 x 10^-4 / 0.00151
= 0.092