Weak Acid Weak Base pH Tool

Compute the pH of aqueous weak acid or weak base solutions given concentration and equilibrium constant. Supports scientific notation (e.g., 1e-5), M/mM/µM units, and runs entirely offline. For students, researchers, and lab technicians.

Educational only. Follow your lab SOPs and SDS.

Input Parameters

Solution Type

Concentration (C)

Enter total formal concentration

Equilibrium Constant (K)

K_a for acids, K_b for bases

Results

Calculated pH

[H⁺]

-- M

[OH⁻]

-- M

Degree of Ionization

-- %

pOH

Quick Converter

K ↔ pK

Concentration Unit

Understanding Weak Acid/Base pH Calculations

This tool calculates the pH of a monoprotic weak acid or weak base solution using the common approximation [H⁺] = √(K_a·C) for acids (similarly for bases). This is valid when K·C > 10⁻¹² and C/K > 100. For borderline cases, the exact quadratic equation is solved automatically.

Step-by-Step Usage

Select whether your compound is a weak acid or weak base.
Enter the total analytical concentration. Use the unit buttons (M, mM, µM) to match your measurement.
Enter the equilibrium constant (K_a, K_b, pK_a, or pK_b). Use scientific notation (e.g., 1.8e-5).
Click "Calculate pH". Review the results, intermediate values, and any validation warnings.

Realistic Examples

Scenario	Type	Conc	K (or pK)	Expected pH
Acetic acid	Weak Acid	0.1 M	K_a = 1.8e-5	≈ 2.87
Ammonia solution	Weak Base	0.05 M	K_b = 1.8e-5	≈ 10.97
Very dilute acid	Weak Acid	1e-4 M	pK_a = 4.75	≈ 4.38 (quadratic)

Common Mistakes Checklist

Using K_b for an acid (or vice versa).
Forgetting to convert mM or µM to M before calculation.
Using concentration of the conjugate species instead of the weak acid/base.
Applying the approximation outside its valid range (C/K < 100).
Ignoring temperature effects on K_w (assumed 1e-14 at 25°C).

Frequently Asked Questions

When does the approximation fail?

When C/K < 100 (i.e., the degree of ionization is large) or when the solution is extremely dilute (C very small relative to [H⁺] from water). The tool automatically switches to the exact quadratic solution in these cases.

What about polyprotic acids or amphiprotic species?

This tool is for monoprotic weak acids/bases only. For polyprotic systems (e.g., H₂CO₃), successive K values matter and the calculation is more complex.

How accurate is the pH result?

It assumes ideal dilute solution behavior, no ionic strength corrections (activity coefficients = 1), and K_w = 1e-14 at 25°C. For precise work, measure pH experimentally.

What safety notes apply?

Always consult SDS for the specific acid/base you are handling. Use appropriate PPE (gloves, goggles). Never add water to concentrated acid; always acid to water. Have spill kits and neutralizers ready.

Formulas & Assumptions

Equations Used

Weak acid: [H⁺] = √(K_a·C) when approximation valid, else solve [H⁺]² + K_a[H⁺] - K_aC = 0
Weak base: [OH⁻] = √(K_b·C) (or quadratic), then [H⁺] = K_w/[OH⁻]
pH = -log₁₀([H⁺])
Degree of ionization α = [H⁺]/C for acids, [OH⁻]/C for bases
K_w = 1×10⁻¹⁴ (at 25°C)

Assumptions & Constants

Monoprotic acid/base only.
Temperature 25°C (K_w fixed).
Ideal dilute solution (activity ≈ concentration).
No competing equilibria or side reactions.
Rounding: pH displayed to 0.01, concentrations in scientific notation with 2 significant figures.

For informational and educational purposes only. Estimates depend on your inputs and simplifying assumptions. This tool does not replace professional judgment, lab SOPs, or safety procedures. Always follow SDS guidance and institutional policies.

Sources & References

Common chemistry constants: R, N_A, F; Standard unit conversions and lab measurement best practices; General analytical chemistry and uncertainty concepts; Intro physical chemistry relationships (ideal gas, Arrhenius, equilibrium); Laboratory safety basics and SDS principles.

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Weak Acid Weak Base pH Tool

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