pH/pOH
pH stands for power of hydrogen and pOH stands for power of hydroxide. pH and pOH indicate the acidity/basicity of a solution.
A few formulas...
- Use this tip: p always equals [-log], so therefore, pH = -log [H+]. pH is the negative log of the H+ concentration. If the H+ concentration is 1 x 10^-4, then pH = 4. Just take the 4 from the exponent and you have your pH. Write only the number of the exponent; do not use the negative sign.
- pOH = -log [OH-]. pOH is the negative log of the OH- concentration.
- pH + pOH = 14. Therefore, 14 - pH = pOH, and 14 - pOH = pH.
How to solve pH and pOH problems on graphing calculator
1. When given [H+] or [OH-] and asked to solve for pH or pOH:
a. hit the negative button on the bottom right
b. hit the log button on the left
c. now input the number given for [H+] or [OH-]. To input the power of ten, hit the "2nd" button on the upper left, followed by the comma button in the left center. When putting in the exponent after the 2nd comma, don't make it an exponent; simply put it in as an ordinary number. When putting in a negative exponent, make sure to hit the negative button at the bottom right, not the minus button. No need to close the parenthetical when putting the calculation into your calculator.
Note
When you are given the pH or pOH, you can find the H+/OH- without using a calculator:
[H+] = 10^-pH
[OH-] = 10^-pOH
And when given the pH or pOH, don't forget: pH + pOH = 14.
Here are some examples:
1. Calculate the pH of a 0.00001 M HCl solution.
0.00001= 1 x 10^-5. Since HCl is an acid, 1 x10^-5 is the H+ concentration. Write only the number of the exponent. Therefore, pH = 5.
2. Calculate the pOH of a solution whose [OH-] is 2 x 10^-5.
pOH = 4.7
3. Calculate the pH and pOH of a 0.0003 M solution of KOH.
Since KOH is a base, 3 x10 ^-4 is its OH- concentration. pOH = -log [OH-] = -log (3 x 10-4) = 3.5
pH = 14 - pOH = 10.5
4. Calculate the pH and pOH of 0.04 M HCl.
Since HCl is an acid, 4 x10^-2 is its H+ concentration. pH = -log [H+] = -log (4 x 10-2) = 1.4
pOH = 14 - pH = 12.6
5. What is the molarity if a substance with a pH of 3?
[H+] = 10^-pH
[H+] (molarity) = 1 x10^-3
6. Calculate the pH, pOH,OH- of a solution in which the H+ concentration is 7.94 x 10^-2.
pH = -log [H+] = 1.1
pOH = 14 - pH = 12.9
[OH-] = 1 x10 ^-12.9
For a quiz on pH/pOH calculations, go to: /phpoh-quiz.html
[H+] = 10^-pH
[OH-] = 10^-pOH
And when given the pH or pOH, don't forget: pH + pOH = 14.
Here are some examples:
1. Calculate the pH of a 0.00001 M HCl solution.
0.00001= 1 x 10^-5. Since HCl is an acid, 1 x10^-5 is the H+ concentration. Write only the number of the exponent. Therefore, pH = 5.
2. Calculate the pOH of a solution whose [OH-] is 2 x 10^-5.
pOH = 4.7
3. Calculate the pH and pOH of a 0.0003 M solution of KOH.
Since KOH is a base, 3 x10 ^-4 is its OH- concentration. pOH = -log [OH-] = -log (3 x 10-4) = 3.5
pH = 14 - pOH = 10.5
4. Calculate the pH and pOH of 0.04 M HCl.
Since HCl is an acid, 4 x10^-2 is its H+ concentration. pH = -log [H+] = -log (4 x 10-2) = 1.4
pOH = 14 - pH = 12.6
5. What is the molarity if a substance with a pH of 3?
[H+] = 10^-pH
[H+] (molarity) = 1 x10^-3
6. Calculate the pH, pOH,OH- of a solution in which the H+ concentration is 7.94 x 10^-2.
pH = -log [H+] = 1.1
pOH = 14 - pH = 12.9
[OH-] = 1 x10 ^-12.9
For a quiz on pH/pOH calculations, go to: /phpoh-quiz.html