Calculating pH
The pH of a substance is defined by:
pH = −log[H+]
where the quantity [H+] represents the concentration of hydrogen ions, measured in moles per liter, in the substance.
- A substance with a pH < 7 is considered an acid
- A substances with a pH > 7 is considered a base
Example #1:
Find the pH of milk whose concentration of hydrogen ions, [H+] = 4 x 10-7 moles/liter.
Solution #1:
Using pH = −log[H+]:
pH = −log(4 x 10-7)
pH ≈ −(−6.4)
pH ≈ 6.4
pH = −log(4 x 10-7)
pH ≈ −(−6.4)
pH ≈ 6.4
Example #2:
Find the concentration of hydrogen ions, [H+], for vinegar with a pH = 3.1.
Solution #2:
Using pH = −log[H+]:
3.1 = −log[H+]
Multiplying each side by −1:
−3.1 = log[H+]
Raising both sides to power of 10:
10−3.1 = [H+]
[H+] ≈ 7.94 x 10-4 moles/liter
3.1 = −log[H+]
Multiplying each side by −1:
−3.1 = log[H+]
Raising both sides to power of 10:
10−3.1 = [H+]
[H+] ≈ 7.94 x 10-4 moles/liter