🔬 pH Weak Base Calculator
Calculate pH from Kb and initial base concentration.
NH₃: 1.8×10⁻⁵
How to Use This Calculator
Enter the Kb (base dissociation constant) and the initial concentration of your weak base in mol/L. The calculator solves the equilibrium equation exactly using the quadratic formula, returning pH, pOH, pKb, and percent dissociation.
Find the Kb for your base. Ammonia (NH₃) has Kb = 1.8×10⁻⁵. Methylamine has Kb = 4.4×10⁻⁴. Pyridine has Kb = 1.7×10⁻⁹. Enter the value in scientific notation (e.g., 1.8e-5).
Enter the initial concentration in mol/L. For household ammonia at approximately 0.1 M, type 0.1.
Click Calculate pH. The tool uses the full quadratic rather than the 5% approximation, so results are accurate even at low concentrations or high Kb values.
Note the percent dissociation. If it is above 5%, the shortcut formula [OH⁻] ≈ √(Kb × C) would give a noticeably wrong answer. Always trust the quadratic result here.
Weak Base pH Formula
Weak bases partially accept protons from water via the equilibrium B + H₂O ⇌ BH⁺ + OH⁻. Kb equals [BH⁺][OH⁻] divided by [B]. Setting x as the amount reacted gives the quadratic equation shown above. Solving for x with the positive root gives the equilibrium [OH⁻], and converting via pOH and pH = 14 − pOH gives the final pH.
Worked Examples
Where This Calculation Comes Up
Weak base pH calculations come up whenever you work with bases that do not fully dissociate. In buffer chemistry, you often pair a weak base with its conjugate acid to maintain a stable pH. For an ammonia-ammonium chloride buffer, you first calculate the pH of pure ammonia solution, then adjust with the Henderson-Hasselbalch equation. This is a standard task in both general chemistry and biochemistry courses.
Ammonia solutions are used as cleaning agents and in fertilizer production, and calculating their pH matters for both safety and effectiveness. In wastewater treatment, ammonia levels and pH together determine how much ammonia exists as the toxic free form (NH₃) versus the safer ammonium ion (NH₄⁺). Pharmaceutical chemists also calculate the pH of amine-based drug solutions to predict whether the drug molecule will be in its ionised or neutral form, which directly affects how well it is absorbed by the body.
Frequently Asked Questions
What is Kb?
Kb is the base dissociation constant, measuring how strongly a base accepts a proton in water. Larger Kb = stronger base.
How do you calculate pH of a weak base?
B + H₂O ⇌ BH⁺ + OH⁻. Kb=[BH⁺][OH⁻]/[B]. Solve quadratic for [OH⁻], then pOH=−log[OH⁻], pH=14−pOH.
What is the Kb of ammonia?
Ammonia (NH₃) has Kb ≈ 1.8×10⁻⁵, the same Ka as acetic acid but as a base.
How are Ka and Kb related?
For a conjugate acid-base pair: Ka × Kb = Kw = 1×10⁻¹⁴ at 25°C.
What is percent dissociation for a weak base?
% dissociation = ([OH⁻] produced / initial concentration) × 100. Higher Kb and lower concentration both increase this value.