pH Scale
Core Concept
Definition: The pH scale measures the concentration of hydrogen ions (H+H^+H+) or hydronium ions (H3O+H_3O^+H3O+) in a solution.
Range: Typically ranges from 0 to 14, but values can be below 0 or above 14 in extreme cases.
Acidic Solutions: pH<7\text{pH} < 7pH<7
Neutral Solutions: pH=7\text{pH} = 7pH=7
Basic (Alkaline) Solutions: pH>7\text{pH} > 7pH>7
Practice Tips
Understand the Logarithmic Nature: A change of 1 pH unit corresponds to a tenfold change in H+H^+H+ concentration.
Memorize Key Values:
pH = 7 is neutral ONLY at 25C.
Acids have pH<7; bases have pH>7.
Practice with Units: Always express [H+] and [OH−] in molarity (M).
Check Your Math: Double-check logs and powers of 10 during calculations.
Podcast Episode
The pH Formula
$\text{pH} = -\log[H^+]$
[$H^+$]: The molar concentration of hydrogen ions in the solution.
Related Formulas
pOH: $\text{pOH} = -\log[OH^-]$
Relationship Between pH and pOH: $\text{pH} + \text{pOH} = 14 \,\, (\text{at } 25^\circ \text{C})$
Hydrogen and Hydroxide Ion Product: $[H^+][OH^-] = K_w = 1.0 \times 10^{-14} \,\, (\text{at } 25^\circ \text{C})$
How to Interpret pH Values
Low pH (0−6): Indicates a high concentration of H^+ ions; the solution is acidic.
Example: Lemon juice (pH≈2).
Neutral pH (7): Indicates equal concentrations of H^+ and OH^− ions.
Example: Pure water (pH=7).
High pH (8−14): Indicates a high concentration of OH^− ions; the solution is basic.
Example: Soap solution (pH≈10).