BackThe pH Scale and the Chemical Context of Life
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The pH Scale and Aqueous Solutions
pH Scale and Ion Concentrations
The pH scale is a measure of the hydrogen ion concentration in an aqueous solution, which is crucial for understanding chemical reactions in biological systems. At 25°C, the product of the concentrations of hydrogen ions (H+) and hydroxide ions (OH-) in water is a constant value.
Ion Product Constant for Water (Kw): The product of the molar concentrations of H+ and OH- is always at 25°C.
Mathematically:
Neutral Solutions
In a neutral solution at 25°C, the concentrations of H+ and OH- are equal.
M
M
This balance defines a neutral pH (pH 7).
Effect of Adding Acid or Base
Adding acids or bases to a solution alters the concentrations of H+ and OH-, but their product remains constant at M2 (at 25°C).
Adding Acid: If enough acid is added to increase to M, then must decrease to M to maintain the constant ().
Adding Base: If a base is added to raise to M, then must decrease to M ().
This relationship expresses the behavior of acids and bases in aqueous solutions and is fundamental to understanding biological chemistry.
Key Equations
Ion Product Constant:
pH Definition:
pOH Definition:
Relationship: (at 25°C)
Example Table: Effect of Acid and Base Addition
Condition | [H+] (M) | [OH-] (M) | pH |
|---|---|---|---|
Neutral | 1 × 10-7 | 1 × 10-7 | 7 |
After Acid Addition | 1 × 10-5 | 1 × 10-9 | 5 |
After Base Addition | 1 × 10-10 | 1 × 10-4 | 10 |
Additional info: The pH scale is logarithmic, so each unit change represents a tenfold change in H+ concentration. This concept is foundational for understanding acid-base balance in biological systems, enzyme activity, and cellular processes.
