Question

Calculate the pH of a solution obtained by mixing equal volumes of a strong acid solution of pH 3.0 and a strong base solution of pH 12.0

Answer 1

H+ + OH- ===== H2O

pH= -log [H+]= 3

H+]= 10^-3=0.001 So this is the concentration of H+ in the astrong acid solution.

Now we can calculate the amount of OH- in the solution.

pH + pOH = 14

pOH= 14-pH= 14-12= 2

pOH= -log[OH-]= 10^-2= 0.01

According to the reaction at the begining 1 mol of H+ reacts with 1 mol of OH- to form water, so if we have only 0.001 mol of H+ and 0,1mol of OH-, the OH- is the one that is in excess and will remain in the solution giving the alkalinity of the solution.

0.01 mol-0.001 mol=0.009 mol OH-

pOH= -log ( 0.009 ) = 2.045

pH= 14-pOH

=14-2.045

=11.955