Calculate the pH of weak acid solutions using the acid dissociation constant (Ka) and concentration. Enter molarity and Ka value to determine solution pH.
pH
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pKa
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[H⁺] Concentration
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Dissociation Fraction (α)
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Strong acids dissociate completely in water (Ka >> 1), while weak acids only partially dissociate (Ka < 1). Weak acids require equilibrium calculations to determine pH, whereas strong acids can be calculated directly from concentration.
For weak acids, use the quadratic equation derived from Ka = [H⁺][A⁻]/[HA]. The exact solution is [H⁺] = (-Ka + √(Ka² + 4KaC))/2, where C is the initial concentration. Then pH = -log[H⁺].
Ka is the acid dissociation constant that measures acid strength. Common values: acetic acid (1.8×10⁻⁵), formic acid (1.8×10⁻⁴), benzoic acid (6.5×10⁻⁵). You can find Ka values in chemistry reference tables or textbooks.
Temperature affects the equilibrium constant Ka and the autoionization of water (Kw). Higher temperatures generally increase Ka values, making acids appear stronger. At 25°C, pH + pOH = 14.00, but this changes at other temperatures.
pKa = -log(Ka). It's often more convenient to use than Ka because it gives positive numbers. Lower pKa values indicate stronger acids. For example, acetic acid has pKa = 4.74.
The approximation pH ≈ 0.5(pKa - log C) works when the dissociation is less than 5% (α < 0.05). This occurs when C/Ka > 400. For more accurate results, always use the quadratic solution.
The dissociation fraction (α) is the percentage of acid molecules that dissociate: α = [H⁺]/C × 100%. It indicates how much the weak acid ionizes. Values typically range from 0.1% to 10% for common weak acids.