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pH Calculations
We can convert any hydrogen ion concentration into a ph value using the formula:
pH = -log[H+]
If you are given the concentration of an acid (in M), you simply have to plug that value into the equation as the
[H+]. Then, you can use a scientific calculator to determine the pH value. Here is an example:
What is the pH of a .0056 M perchloric acid (HClO4) solution?
Step 1: The equation
pH = -log[H+]
Step 2: Plug in the concentration!
pH = -log(.0056)
Step 3: Using your calculator, you will see that:
2.24 = -log(.0056)
Directions: Practice finding the pH values when you are given an acid concentration. If you do not have a
calculator on hand, write out the full equation like step 2 above.
1.) .000678 M hypochlorous acid (HClO) solution:
2.) .0014 M chloric acid (HClO3) solution:
3.) 6.8 x 10-5 M nitrous acid (HNO2) solution:
4.) 3.1 x 10-3 M perchloric acid (HClO4) solution :
5.) .01 M chlorous acid (HClO2) solution:
6.) 1.2 x 10-12 M chloric acid (HClO3) solution:
We can also calculate the hydrogen ion concentration if we know the pH. Suppose we have a solution whose pH is
6. Its hydrogen ion concentration can be calculated as follows:
Step 1: The Equation
pH = -log[H+]
Step 2: Plug it in
6 = -log[H+]
Step 3: Move the negative sign
-6 = log[H+]
Step 4: Eliminate log by putting both sides to the power of 10
10-6 = 10log[H+]
10-6 = [H+]
Directions: Practice finding the hydrogen ion concentration when you are given the pH of an acidic
solution. You shouldn’t need a calculator to get your answer!
1.) Hypochlorous acid (HClO) solution whose pH is 3:
2.) Chloric acid (HClO3) solution whose pH is 5:
3.) Nitrous acid (HNO2) solution whose pH is 2.1:
4.) Perchloric acid (HClO4) solution whose pH is 6.89:
5.) Chlorous acid (HClO2) solution whose pH is 1.2:
6.) Chloric acid (HClO3) solution whose pH is 4.123: