Question

What is the pH of an aqueous solution at 25.0 degrees Celsius in which [OH^-] is 0.00055 M? a. +3.26b. -3.26c. +11.4 d. +12.4 e. +10.7

Answer 1

Step 1 - Which equation should we use?

Note the exercise is asking us the pH, but gave instead the concentration of OH- ions. That's not a problem, since pH and pOH are related through the following equation:

`pH+pOH=14`

Therefore, if we discover the pOH, we can also find the pH of a solution.

Step 2 - Calculating the pOH of the solution

The definition of pOH is as follows:

`pOH=-log[OH^-\rbrack`

The concentration of OH- in the given solution is 5.5*10^-4 mol/L, so:

`pOH=-log5.5*10^(-4)=4-log5.5=4-0.74=3.26`

Step 3 - Finding the pH of the solution

Now we can find the pH:

`pH=14-pOH=14-3.26=10.74`

Answer: the pH is equal to 10.7