Changing from pH or pOH to concentrations

MINI-TUTORIAL              Std 5

 

This little insight may be helpful to you when you convert pH or pOH to their respective ion concentrations .   Think of a big box.  The rule is, you can only convert vertically or horizontally.  You cannot convert diagonally.   Here are the conversions and the box:

 

  

               pH <---------------------> pOH

            

   [H+] <---------------------> [OH-]

 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Horizontal Conversions

 

         Going from [H+]   <-------------->  [OH-]   

 

              1 x  10-14                                  1 x  10-14

            -----------  =  [OH-]       or      ------------  =  [H+]

                [H+]                                        [OH-]

 

         Going from pH ------>  pOH      (remember pH + pOH = 14)

 

            14  -  pH   =   pOH

            14 -  pOH =  pH

 


 

Vertical Conversions

 

         Going from  [H+]  ------->  pH    or    [OH-]  -------->  pOH

 

            - log  [H+]  =  pH

            - log  [OH-] = pOH

 

         Going from pH ---------->  [H+]    or  pOH ---------> [OH-]

 

            Anti-log pH   =  [H+]

            Anti-log pOH  =  [OH-]

 

 

This is a great little box to have on your help sheet !!!

 

Another HOT item for your help sheet is “estimating the pH” below:

 

To figure out the pH from the [H+], you normally must figure out the –log [H+].  With this chart you can make a fairly accurate guess on the pH value to be sure you ran your calculator properly.  Also, in many multiple choice questions you can find the right answer without even using your calculator.

 

[H+]

Value of pH

  [OH-]

Value of pOH

1 x 10x

  (x)

1 x 10x

  (x)

2 x 10x

  (x-1) .70

2 x 10x

  (x-1) .70

3 x 10x

  (x-1) .52

3 x 10x

  (x-1) .52

4 x 10x

  (x-1) .40

4 x 10x

  (x-1) .40

5 x 10x

  (x-1) .30

5 x 10x

  (x-1) .30

6 x 10x

  (x-1) .22

6 x 10x

  (x-1) .22

7 x 10x

  (x-1) .15

7 x 10x

  (x-1) .15

8 x 10x

  (x-1) .10

8 x 10x

  (x-1) .10

9 x 10x

  (x-1) .05

9 x 10x

  (x-1) .05

10 x 10x

  (x-1)

10 x 10x

  (x-1)

 

* Note that pH and pOH are the same values as numbers but that they mean very different things!