Question

The pH of a 0.00125 M solution of an aqueous solution of XOH (X = an unknown element) is 3.55. Is XOH a strong acid, weak acid, strong base, or weak base? How do you know? Is X a metal or non-metal? Explain

Answer 1

With the pH we can know the concentration of [H+] that we have in the solution.

pH= 3,55=-log [H+] ------ [H+] = 10 ^-3,55 = 2,8*10-4

So having a concentration of H+ in the order of 10-4 we most have an acid in the solution, because we know for the ionic product that having a concentration of [H+] higher than 1*10-7 we have an acid solution.

Now, we can write the reaction

XOH ============= X+ + OH-

In Equilibrium 0,00125M -  2,8*10-4   2,8*10-4    2,8*10-4

[XOH] =0,00125M -  2,8*10-4= 9,7*10-4M

%I=  2,8*10-4/9,7*10-4*100= 28%

The acid that we have in solution most be a week acid because the porcentage of ionization its 28%, wich indicates that in the solution we have an equilibrium between XOH and X+, and that is harder to take an H+ of the acid and stay protonated so it most be a week acid.

Now because we have in the equilibrium an OH- specie in an acidic solution, X+ most be a positive charged molecule that most be a non metal so it can take H+ of the medium and make the solution acidic.