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Constants library

10 values, each with its units, its uncertainty, and where it came from.

Chemistry 10

Ionic Product of Water (Kw at 25 °C) measured

Kw=1×10−14K_{\mathrm{w}} = 1 \times 10^{-14}

dimensionlessThe autoionisation constant of pure water at 25 °C, [H⁺][OH⁻] = 1.0 × 10⁻¹⁴, the equilibrium behind the whole 0–14 pH scale.

pKw of Water at 25 °C measured

pKw=13.995\mathrm{p}K_{\mathrm{w}} = 13.995

dimensionlessThe negative logarithm of water's ionic product at 25 °C, pKw = 13.995, universally rounded to 14.00 for the pH + pOH identity.

Molar Mass of Calcium Carbonate measured

M(CaCO3)=0.100086 kg/molM(\mathrm{CaCO_3}) = 0.100086\ \text{kg/mol}

kg/molThe formula mass of calcium carbonate, 100.086 g/mol — the reference substance for reporting hardness and alkalinity as mg/L as CaCO₃.

Atomic Mass of Calcium measured

Ar(Ca)=0.040078 kg/molA_{\mathrm{r}}(\mathrm{Ca}) = 0.040078\ \text{kg/mol}

kg/molThe standard atomic weight of calcium, 40.078 g/mol — the ion that dominates water hardness and the scale it leaves behind.

First Dissociation Constant of Carbonic Acid measured

Ka1(H2CO3)=4.45×10−7K_{\mathrm{a1}}(\mathrm{H_2CO_3}) = 4.45 \times 10^{-7}

dimensionlessThe first acid dissociation constant of carbonic acid at 25 °C, Kₐ₁ = 4.45 × 10⁻⁷, governing the CO₂-bicarbonate equilibrium in natural water.

First pKₐ of Carbonic Acid measured

pKa1(H2CO3)=6.352\mathrm{p}K_{\mathrm{a1}}(\mathrm{H_2CO_3}) = 6.352

dimensionlessThe first pKₐ of carbonic acid at 25 °C, 6.352 — the pH at which dissolved CO₂ and bicarbonate are present in equal concentrations.

Second Dissociation Constant of Carbonic Acid measured

Ka2(H2CO3)=4.69×10−11K_{\mathrm{a2}}(\mathrm{H_2CO_3}) = 4.69 \times 10^{-11}

dimensionlessThe second acid dissociation constant of carbonic acid at 25 °C, Kₐ₂ = 4.69 × 10⁻¹¹, the bicarbonate-to-carbonate step that drives scaling.

Second pKₐ of Carbonic Acid measured

pKa2(H2CO3)=10.329\mathrm{p}K_{\mathrm{a2}}(\mathrm{H_2CO_3}) = 10.329

dimensionlessThe second pKₐ of carbonic acid at 25 °C, 10.329 — the pH at which bicarbonate and carbonate ions are present in equal concentrations.

Solubility Product of Calcium Carbonate measured

Ksp(CaCO3)=3.36×10−9K_{\mathrm{sp}}(\mathrm{CaCO_3}) = 3.36 \times 10^{-9}

dimensionlessThe solubility product of calcite at 25 °C, K_sp = 3.36 × 10⁻⁹ — the number that decides whether a water scales or corrodes.

Solubility Product of Calcium Sulfate measured

Ksp(CaSO4)=0.0000493K_{\mathrm{sp}}(\mathrm{CaSO_4}) = 0.0000493

dimensionlessThe solubility product of anhydrous calcium sulfate at 25 °C, K_sp = 4.93 × 10⁻⁵ — the gypsum scale that acid cleaning cannot remove.