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Chemistry constants

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

Universal & Atomic 2

Avogadro constant exact

NA=6.02214076×1023 mol−1N_{\mathrm{A}} = 6.02214076 \times 10^{23}\ \text{mol}^{-1}

mol⁻¹Exactly 6.022 140 76 × 10²³ entities per mole — the fixed number that has defined the mole since the 2019 SI revision.

Atomic mass constant (unified atomic mass unit) measured

mu=1.66053906892×10−27 kgm_{\mathrm{u}} = 1.66053906892 \times 10^{-27}\ \text{kg}

kgOne twelfth of the mass of a free carbon-12 atom at rest, 1.660 539 069 × 10⁻²⁷ kg — the dalton used in every mass spectrum.

Electromagnetic 2

Faraday constant exact

F=96485.33212331001 C/molF = 96485.33212331001\ \text{C/mol}

C/molThe charge carried by one mole of electrons, N_A×e ≈ 96485 coulombs — the bridge between the coulombs you meter and the moles you plate.

Relative permittivity of water measured

εr,H2O=80.1 —\varepsilon_{r,\mathrm{H_2O}} = 80.1\ \text{—}

—Typical static dielectric constant of liquid water at 20 °C — an outlier among common liquids and the reason water dissolves salts so well.

Thermodynamic 10

Molar gas constant exact

R=8.31446261815324 J/(mol⋅K)R = 8.31446261815324\ \text{J/(mol}{\cdot}\text{K)}

J/(mol·K)The universal gas constant, R = k·N_A = 8.314462618 J/(mol·K), exact since 2019 and the R in PV = nRT and in every entropy table.

Molar volume of an ideal gas at STP (0 °C, 100 kPa) exact

Vm STP=22.71095464 L/molV_{\mathrm{m}}^{\,\mathrm{STP}} = 22.71095464\ \text{L/mol}

L/mol22.71095464 L/mol at IUPAC standard temperature and pressure, 273.15 K and 100 kPa exactly — not the older 22.4 L/mol.

Molar volume of an ideal gas at 0 °C and 1 atm exact

Vm atm=22.41396954 L/molV_{\mathrm{m}}^{\,\mathrm{atm}} = 22.41396954\ \text{L/mol}

L/molThe textbook 22.414 L/mol: one mole of ideal gas at 273.15 K and 101.325 kPa, the pre-1982 definition of standard conditions.

Molar volume of an ideal gas at 25 °C and 1 atm exact

Vm NTP=24.4654037 L/molV_{\mathrm{m}}^{\,\mathrm{NTP}} = 24.4654037\ \text{L/mol}

L/mol24.4654 L/mol at 298.15 K and 101.325 kPa, the ambient reference used for gas concentrations in ppm-to-mg/m³ conversions.

Loschmidt constant exact

n0=2.686780111×1025 m−3n_0 = 2.686780111 \times 10^{25}\ \text{m}^{-3}

m⁻³Number density of an ideal gas at 273.15 K and 101.325 kPa: 2.6867801e25 molecules per cubic metre, or 2.69e19 per cubic centimetre.

Sackur-Tetrode constant measured

S0/R=−1.15170754 —S_0/R = -1.15170754\ \text{—}

—Reduced absolute entropy of an ideal monatomic gas at 1 K and 100 kPa, -1.1517075, the constant that puts Planck's h inside a classical gas.

Molar mass constant measured

Mu=0.00100000000105 kg/molM_{\mathrm{u}} = 0.00100000000105\ \text{kg/mol}

kg/molM_u = 1.00000000105e-3 kg/mol, the factor turning a relative atomic mass into a molar mass — no longer exactly 1 g/mol since 2019.

Standard state pressure exact

p∘=100,000 Pap^{\circ} = 100,000\ \text{Pa}

PaThe thermodynamic standard state pressure, exactly 1 bar = 100 kPa, the p° in every tabulated ΔG°, ΔH° and equilibrium constant.

Heat capacity ratio of argon

γAr=1.667 —\gamma_{\mathrm{Ar}} = 1.667\ \text{—}

—γ = 1.667 for argon and the other monatomic gases, the theoretical maximum 5/3 predicted by kinetic theory for point-like atoms.

Calorie (thermochemical) exact

cal=4.184 J\mathrm{cal} = 4.184\ \text{J}

JThe thermochemical calorie is exactly 4.184 J; the food Calorie is a kilocalorie, 4184 J, a factor of a thousand larger.

Chemistry 45

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 Water measured

M(H2O)=0.018015 kg/molM(\mathrm{H_2O}) = 0.018015\ \text{kg/mol}

kg/molThe molar mass of water, 18.015 g/mol — one mole of H₂O is 18.015 g and occupies almost exactly 18.07 mL of liquid at 25 °C.

Molar Mass of Carbon Dioxide measured

M(CO2)=0.044009 kg/molM(\mathrm{CO_2}) = 0.044009\ \text{kg/mol}

kg/molThe molar mass of carbon dioxide, 44.009 g/mol — the conversion that turns tonnes of burnt carbon into tonnes of CO₂ emitted.

Molar Mass of Dioxygen measured

M(O2)=0.031998 kg/molM(\mathrm{O_2}) = 0.031998\ \text{kg/mol}

kg/molThe molar mass of molecular oxygen, 31.998 g/mol — the basis of every stoichiometric air requirement in combustion calculations.

Molar Mass of Dinitrogen measured

M(N2)=0.028014 kg/molM(\mathrm{N_2}) = 0.028014\ \text{kg/mol}

kg/molThe molar mass of molecular nitrogen, 28.014 g/mol — the dominant term in the 28.96 g/mol average molar mass of dry air.

Molar Mass of Methane measured

M(CH4)=0.016043 kg/molM(\mathrm{CH_4}) = 0.016043\ \text{kg/mol}

kg/molThe molar mass of methane, 16.043 g/mol — the working figure for natural gas, converting between cubic metres, kilograms and moles.

Molar Mass of Sodium Chloride measured

M(NaCl)=0.05844 kg/molM(\mathrm{NaCl}) = 0.05844\ \text{kg/mol}

kg/molThe formula mass of sodium chloride, 58.44 g/mol — the number behind saline, brine strength and softener regeneration dosing.

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.

Molar Mass of Sulfuric Acid measured

M(H2SO4)=0.098072 kg/molM(\mathrm{H_2SO_4}) = 0.098072\ \text{kg/mol}

kg/molThe molar mass of sulfuric acid, 98.072 g/mol — the basis for converting between percent strength, molarity and normality in acid feed.

Molar Mass of Sodium Hydroxide measured

M(NaOH)=0.039997 kg/molM(\mathrm{NaOH}) = 0.039997\ \text{kg/mol}

kg/molThe formula mass of sodium hydroxide, 39.997 g/mol — the reason a 1 M caustic solution is made from almost exactly 40 g per litre.

Molar Mass of Ammonia measured

M(NH3)=0.017031 kg/molM(\mathrm{NH_3}) = 0.017031\ \text{kg/mol}

kg/molThe molar mass of ammonia, 17.031 g/mol — the conversion between mg/L as N and mg/L as NH₃ in every wastewater report.

Molar Mass of Glucose measured

M(C6H12O6)=0.180156 kg/molM(\mathrm{C_6H_{12}O_6}) = 0.180156\ \text{kg/mol}

kg/molThe molar mass of glucose, 180.156 g/mol — the conversion between blood sugar in mg/dL and mmol/L, and the unit of cellular energy accounting.

Atomic Mass of Hydrogen measured

Ar(H)=0.001008 kg/molA_{\mathrm{r}}(\mathrm{H}) = 0.001008\ \text{kg/mol}

kg/molThe standard atomic weight of hydrogen, 1.0080 g/mol — the lightest entry on the periodic table and the anchor of the original mass scale.

Atomic Mass of Carbon measured

Ar(C)=0.012011 kg/molA_{\mathrm{r}}(\mathrm{C}) = 0.012011\ \text{kg/mol}

kg/molThe standard atomic weight of carbon, 12.011 g/mol — the element that anchored the atomic mass scale from 1961 until the 2019 redefinition.

Atomic Mass of Nitrogen measured

Ar(N)=0.014007 kg/molA_{\mathrm{r}}(\mathrm{N}) = 0.014007\ \text{kg/mol}

kg/molThe standard atomic weight of nitrogen, 14.007 g/mol — the divisor behind every fertiliser grade and every mg/L as N in a water report.

Atomic Mass of Oxygen measured

Ar(O)=0.015999 kg/molA_{\mathrm{r}}(\mathrm{O}) = 0.015999\ \text{kg/mol}

kg/molThe standard atomic weight of oxygen, 15.999 g/mol — the reference element for chemical atomic masses from Berzelius until 1961.

Atomic Mass of Sodium measured

Ar(Na)=0.02298976928 kg/molA_{\mathrm{r}}(\mathrm{Na}) = 0.02298976928\ \text{kg/mol}

kg/molThe standard atomic weight of sodium, 22.98976928 g/mol — a mononuclidic element, so the value is known to eleven significant figures.

Atomic Mass of Chlorine measured

Ar(Cl)=0.03545 kg/molA_{\mathrm{r}}(\mathrm{Cl}) = 0.03545\ \text{kg/mol}

kg/molThe standard atomic weight of chlorine, 35.45 g/mol — the textbook example of a fractional atomic weight produced by isotope mixing.

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.

Atomic Mass of Iron measured

Ar(Fe)=0.055845 kg/molA_{\mathrm{r}}(\mathrm{Fe}) = 0.055845\ \text{kg/mol}

kg/molThe standard atomic weight of iron, 55.845 g/mol — the element with the highest binding energy per nucleon, where fusion stops paying.

Atomic Mass of Sulfur measured

Ar(S)=0.03206 kg/molA_{\mathrm{r}}(\mathrm{S}) = 0.03206\ \text{kg/mol}

kg/molThe standard atomic weight of sulfur, 32.06 g/mol — the divisor for sulfate, sulfuric acid and every fuel sulfur specification.

Acid Dissociation Constant of Acetic Acid measured

Ka(CH3COOH)=0.0000175K_{\mathrm{a}}(\mathrm{CH_3COOH}) = 0.0000175

dimensionlessThe acid dissociation constant of acetic acid at 25 °C, Kₐ = 1.75 × 10⁻⁵, the reference weak acid of every textbook and every buffer.

pKₐ of Acetic Acid measured

pKa(CH3COOH)=4.756\mathrm{p}K_{\mathrm{a}}(\mathrm{CH_3COOH}) = 4.756

dimensionlessThe pKₐ of acetic acid at 25 °C, 4.756 — the pH at which acetic acid and acetate are present in exactly equal amounts.

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.

First Dissociation Constant of Phosphoric Acid measured

Ka1(H3PO4)=0.00711K_{\mathrm{a1}}(\mathrm{H_3PO_4}) = 0.00711

dimensionlessThe first acid dissociation constant of phosphoric acid at 25 °C, Kₐ₁ = 7.11 × 10⁻³, a moderately strong first proton on a triprotic acid.

First pKₐ of Phosphoric Acid measured

pKa1(H3PO4)=2.148\mathrm{p}K_{\mathrm{a1}}(\mathrm{H_3PO_4}) = 2.148

dimensionlessThe first pKₐ of phosphoric acid at 25 °C, 2.148 — the centre of the low-pH buffering region used in HPLC mobile phases.

Second Dissociation Constant of Phosphoric Acid measured

Ka2(H3PO4)=6.32×10−8K_{\mathrm{a2}}(\mathrm{H_3PO_4}) = 6.32 \times 10^{-8}

dimensionlessThe second acid dissociation constant of phosphoric acid at 25 °C, Kₐ₂ = 6.32 × 10⁻⁸, the step that buffers cells and biological media.

Second pKₐ of Phosphoric Acid measured

pKa2(H3PO4)=7.199\mathrm{p}K_{\mathrm{a2}}(\mathrm{H_3PO_4}) = 7.199

dimensionlessThe second pKₐ of phosphoric acid at 25 °C, 7.199 — almost exactly physiological pH, which is why phosphate buffers biology.

Third Dissociation Constant of Phosphoric Acid measured

Ka3(H3PO4)=4.5×10−13K_{\mathrm{a3}}(\mathrm{H_3PO_4}) = 4.5 \times 10^{-13}

dimensionlessThe third acid dissociation constant of phosphoric acid at 25 °C, Kₐ₃ = 4.5 × 10⁻¹³, a proton so tightly held it needs strong alkali to remove.

Third pKₐ of Phosphoric Acid measured

pKa3(H3PO4)=12.35\mathrm{p}K_{\mathrm{a3}}(\mathrm{H_3PO_4}) = 12.35

dimensionlessThe third pKₐ of phosphoric acid at 25 °C, 12.35 — the pH above which free orthophosphate finally becomes the dominant species.

Base Dissociation Constant of Ammonia measured

Kb(NH3)=0.0000177K_{\mathrm{b}}(\mathrm{NH_3}) = 0.0000177

dimensionlessThe base dissociation constant of ammonia at 25 °C, K_b = 1.77 × 10⁻⁵, making it the textbook weak base and the mirror of acetic acid.

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.

Standard Hydrogen Electrode Potential exact

E∘(H+/H2)=0 VE^{\circ}(\mathrm{H^+}/\mathrm{H_2}) = 0\ \text{V}

VThe standard potential of the hydrogen electrode, defined as exactly 0 V at 25 °C — the zero point of the entire electrochemical series.

Standard Potential of the Zinc Half-Cell measured

E∘(Zn2+/Zn)=−0.7618 VE^{\circ}(\mathrm{Zn^{2+}}/\mathrm{Zn}) = -0.7618\ \text{V}

VThe standard reduction potential of Zn²⁺ + 2e⁻ → Zn at 25 °C, −0.7618 V — the anode of the Daniell cell and of every sacrificial anode.

Standard Potential of the Copper Half-Cell measured

E∘(Cu2+/Cu)=0.3419 VE^{\circ}(\mathrm{Cu^{2+}}/\mathrm{Cu}) = 0.3419\ \text{V}

VThe standard reduction potential of Cu²⁺ + 2e⁻ → Cu at 25 °C, +0.3419 V — the cathode half of the Daniell cell and of copper electroplating.

Standard Potential of the Ferric–Ferrous Couple measured

E∘(Fe3+/Fe2+)=0.771 VE^{\circ}(\mathrm{Fe^{3+}}/\mathrm{Fe^{2+}}) = 0.771\ \text{V}

VThe standard reduction potential of Fe³⁺ + e⁻ → Fe²⁺ at 25 °C, +0.771 V — the redox couple that sets the character of natural water.

Standard Potential of the Silver Half-Cell measured

E∘(Ag+/Ag)=0.7996 VE^{\circ}(\mathrm{Ag^+}/\mathrm{Ag}) = 0.7996\ \text{V}

VThe standard reduction potential of Ag⁺ + e⁻ → Ag at 25 °C, +0.7996 V — the basis of the silver-silver chloride reference electrode.

Standard Potential of the Oxygen–Water Couple measured

E∘(O2/H2O)=1.229 VE^{\circ}(\mathrm{O_2}/\mathrm{H_2O}) = 1.229\ \text{V}

VThe standard reduction potential of O₂ + 4H⁺ + 4e⁻ → 2H₂O at 25 °C, +1.229 V — the couple that drives corrosion and limits water electrolysis.

Cryoscopic Constant of Water measured

Kf=1.86 K⋅kg/molK_{\mathrm{f}} = 1.86\ \text{K}{\cdot}\text{kg/mol}

K·kg/molThe freezing-point depression constant of water, 1.86 K·kg/mol — one molal of dissolved particles lowers the freezing point by 1.86 °C.

Ebullioscopic Constant of Water measured

Kb=0.512 K⋅kg/molK_{\mathrm{b}} = 0.512\ \text{K}{\cdot}\text{kg/mol}

K·kg/molThe boiling-point elevation constant of water, 0.512 K·kg/mol — one molal of dissolved particles raises the boiling point by 0.512 °C.