pH Scale

Core Concept

Definition: The pH scale measures the concentration of hydrogen ions ($H^+$) or hydronium ions ($H_3O^+$) in a solution.

Range: Typically ranges from 0 to 14, but values can be below 0 or above 14 in extreme cases.

Acidic Solutions: $\text{pH} < 7$
Neutral Solutions: $\text{pH} = 7$
Basic (Alkaline) Solutions: $\text{pH} > 7$

Key Tips

Understand the Logarithmic Nature: A change of 1 pH unit corresponds to a tenfold change in $H^+$ concentration.
Memorize Key Values:
- pH = 7 is neutral ONLY at 25°C.
- 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.

Test Yourself

Assorted Multiple Choice

A chemist dilutes a sample of hydrochloric acid, causing the hydronium ion concentration, $[H_3O^+]$, to decrease by a factor of 100. How does the pH of the solution change as a result of this dilution?

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Logarithmic Relationships and Scale Magnitude

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Relationship Between pH, pOH, and Kw

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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).

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