Weak Acid pH Calculator

The Weak Acid pH Calculator finds the pH of a weak acid solution using its concentration and dissociation constant — two acids can have the same concentration but very different pH values depending on how much they dissociate. Enter your acid concentration (C) in molarity, the acid dissociation constant (Ka), and temperature to get the pH along with pKa, [H⁺] concentration, and the dissociation fraction (α).

M

Enter the molarity of the weak acid solution

Enter Ka value (e.g., 1.8e-5 for acetic acid)

°C

Temperature affects equilibrium calculations

Results

pH

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pKa

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[H⁺] Concentration

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Dissociation Fraction (α)

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Frequently Asked Questions

What is the difference between strong and weak acids?

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.

How do I calculate pH for a weak acid?

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⁺].

What is the Ka value and how do I find it?

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.

Why does temperature affect pH calculations?

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.

What is pKa and how does it relate to Ka?

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.

When can I use the simplified approximation for weak acid pH?

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.

What is the dissociation fraction and why is it important?

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.