pH and pOH are logarithmic scales used to describe how acidic or basic an aqueous solution is. They matter because small changes in hydrogen ion concentration can strongly affect reactions, living systems, water quality, and laboratory measurements. A low pH means a higher concentration of H3O+ or H+ ions, while a low pOH means a higher concentration of OH- ions.
At 25 °C, the pH and pOH scales are linked by the constant behavior of water.
Key Facts
- pH = -log[H3O+] or pH = -log[H+]
- pOH = -log[OH-]
- Kw = [H3O+][OH-]
- At 25 °C, Kw = 1.0 x 10^-14
- At 25 °C, pH + pOH = 14.00
- If [H3O+] = 1.0 x 10^-3 M, then pH = 3.00 and the solution is acidic
Vocabulary
- pH
- pH is a logarithmic measure of hydrogen ion or hydronium ion concentration in an aqueous solution.
- pOH
- pOH is a logarithmic measure of hydroxide ion concentration in an aqueous solution.
- Kw
- Kw is the ion product constant of water, equal to [H3O+][OH-] for aqueous solutions.
- Hydronium ion
- A hydronium ion, H3O+, forms when a water molecule accepts a hydrogen ion.
- Neutral solution
- A neutral solution has equal concentrations of H3O+ and OH-, giving pH 7.00 at 25 °C.
Common Mistakes to Avoid
- Treating pH as directly proportional to [H3O+] is wrong because pH is logarithmic, so a pH change of 1 means a tenfold change in ion concentration.
- Forgetting the negative sign in pH = -log[H3O+] gives an incorrect sign and can make acidic solutions appear to have negative or unrealistic values.
- Using pH + pOH = 14 at every temperature is wrong because 14.00 comes from Kw = 1.0 x 10^-14, which applies specifically at 25 °C.
- Confusing [H3O+] with [OH-] leads to reversed acid and base conclusions because acids have higher [H3O+] and bases have higher [OH-].
Practice Questions
- 1 A solution has [H3O+] = 2.5 x 10^-4 M at 25 °C. Calculate the pH, then decide whether the solution is acidic, basic, or neutral.
- 2 A solution has pOH = 3.20 at 25 °C. Calculate the pH and [OH-].
- 3 Two solutions differ by 2 pH units. Explain which solution has the greater [H3O+] if one has pH 4 and the other has pH 6, and state how many times greater it is.