Question

What is the hydroxide concentration of a solution with a pH of 12.80?

Answer 1

Step1 : To convert from pH to ion concentrations,first apply equation 17-1 to calculate [H3O+]. Then make use of water equilibrium to calculate [OH-]
Step 2 : We must rearrange equation  pH = -log [H+]  , in order to solve for concentration
Step 3 : log [H+] = -pH and [H+] = 10 -pH   
Step 4 ; Now substitute and evaluate:
[H+] = 10-3.05 = 8.9 x 10-4 M
Step 5: To determine the hydroxide ion concentration,we need to rearrange the water equilibrium expression:
Kw = 1.0 x 10 -14 = [H3O+]eq [OH-]    and           [OH-]eq = 1.0 x 10- 14 / [H3O+]
now susbtitute and evaluate:
       = 1.0 x 10- 14 / 8.9 x 10-4  = 1.1 x 10-11 M   
 
Sample protblem 2 : What is the  OH- ion concetraion of a solution of NaOH with pH 13.40 ?
Solution:
Step 1 : First find pOH from the given pH:
     pH + pOH = 14 
      pOH = 14 - 13.40 = 0.60
Step 2 : As you have now the value of pOH you can find the OH- ion concentration using formula
pOH = -log [OH-]
[OH- ]  =  10 - pOH =  10 - 0.60 = 0.25 M