pH and Hydrogen Ion Concentration Calculator
Convert between pH and hydrogen ion concentration, calculate pOH and hydroxide concentration, and classify solutions as acidic, neutral, or alkaline.
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pH and Acid-Base Chemistry Guide
What's the key thing to understand about The pH Scale?
pH = −log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L. The scale runs 0–14 at 25°C: pH < 7 is acidic, pH = 7 is neutral (pure water at 25°C), pH > 7 is alkaline. Each unit change represents a 10-fold change in [H⁺]: pH 3 has 10× more H⁺ than pH 4, and 100× more than pH 5. Lemon juice ≈ pH 2. Black coffee ≈ pH 5. Pure water pH 7. Baking soda pH 9. Bleach pH 12. Human blood is maintained at pH 7.35–7.45 — outside this range is life-threatening.
What's the key thing to understand about Converting pH and [H⁺]?
From pH to [H⁺]: [H⁺] = 10^(−pH). pH 3 → [H⁺] = 10⁻³ = 0.001 mol/L. pH 7 → [H⁺] = 10⁻⁷ mol/L. From [H⁺] to pH: pH = −log₁₀[H⁺]. [H⁺] = 0.01 → pH = −log(0.01) = −(−2) = 2. The ionic product of water: Kw = [H⁺][OH⁻] = 1×10⁻¹⁴ at 25°C. This means pH + pOH = 14. If pH = 4, then pOH = 10, [OH⁻] = 10⁻¹⁰ mol/L. Strong acids fully dissociate: 0.01 mol/L HCl → [H⁺] = 0.01 mol/L → pH = 2.
What's the key thing to understand about Buffer Solutions?
A buffer resists pH change when small amounts of acid or alkali are added. Composed of a weak acid and its conjugate base (or weak base and conjugate acid). Example: ethanoic acid/sodium ethanoate buffer. Biological importance: blood is buffered at pH 7.4 by the carbonic acid/bicarbonate system. CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. The Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]) — used to calculate buffer pH and design buffers for specific pH values in lab work.
What should I know about Indicators and pH Measurement?
Indicators are weak acids that change colour when they donate or accept a proton — the colour depends on the ratio of acid to conjugate base form. Litmus: red in acid (pH < 7), blue in alkali. Universal indicator: continuous colour spectrum from red (pH 1) through green (pH 7) to purple (pH 14). Phenolphthalein: colourless below pH 8.3, pink above — used in alkaline endpoint titrations. Methyl orange: red below pH 3.1, yellow above pH 4.4 — used in acid endpoint titrations. For precision measurement beyond what an indicator's colour change can offer, a calibrated pH meter with a glass electrode gives a direct numerical reading accurate to about ±0.01 pH units, and is the standard method wherever precise pH values (rather than just crossing a known threshold) are needed.