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

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