How do determine the solubility product constant for Aluminum hydroxide?

Question

One mole of Al₂O₃ is added to water to make exactly 1.00 liters of solution.

(a) Write the expression for the solubility product constant.

(b) Using the given above and the expression written in (a) determine the K_sp for Al₂O₃.

Answer 1

Hi James M.,

I'm guessing that with enough coffee (in you) you might persuade Al2O3 == 2 Al(OH)3 to dissolve into 2[Al(OH)2]+1 + 2 (OH)- . If you jiggle that 7.40*10^-6 into products, each species has 14.8*10^-6M . So K(sp) = (14.8*10^-6)^4 is the calculation.

Now, you would expect to depress that solubility by adding NaHCO3, and also by adding Na2CO3. But not necessarily by adding NaOH. At what molarity of NaOH would the solubility of Al2O3 be unchanged vs. that in water?

--Cheers, --Mr. d.