500 ml of 0.2M BOH (a weak base) is mixed with 500 ml of 0.1 M HCl and pH of the resulting solution is 9 . What is the pH of a 0.1 M BCl solution

you must have taken out the Kb.

the thing that might have confussed you would have been,“What is the pH of a 0.1 M BCl solution”. Now this is just salt hydrolysis i.e. reaction of a weak base and strong acid, so just apply the formula, to get the [H+] =\sqrt{K_{w}.C/K_{b}}. Then take the -ve log of it to get the pH.

 

Hope it helps