Chemistry · Reference table

Chemical reference conventions

A standard state defines a reference convention, but does not automatically fix a temperature. Pressure, concentration scale and temperature must match the source.

Values: 5
Edition / reference: IUPAC Gold Book, 5th edition, online version 5.0.0 (2025)

Reference values

Reviewed September 30, 2026

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Chemical reference conventions
Term	Reference	Note
standard pressure p°	10⁵ Pa = 1 bar	IUPAC has recommended this value since 1982; 1 atm is different.
standard atmosphere	101 325 Pa	The unit atm; do not confuse it with IUPAC standard pressure.
standard concentration c°	häufig 1 mol·dm⁻³	A chosen reference value; the choice must be stated.
STP	273.15 K and 10⁵ Pa	A defined abbreviation for standard temperature and pressure, not a universal laboratory temperature.
standard state	state and pressure convention	Different conventions apply to gases, pure phases and solutes.

Additional reference tables

Additional reference conventions and activities

Standard states are conventions at a temperature that must be specified. Activities are dimensionless. The chosen γc and γb symbols identify their scale.

Additional reference conventions and activities
Term	Reference/relation	Condition	Row source
Standard molality b°	usually 1 mol/kg	Reference mass is solvent, not solution.	IUPAC S05918
Gas standard state	p° = 1 bar	Hypothetical pure ideal gas at specified T; real gas uses fugacity.	IUPAC S05925
Pure condensed reference phase	a = 1	Pure phase at standard pressure and specified T; not a mixture or automatically another crystal phase.	IUPAC S05925
Concentration-based activity	a = γc·c/c°	Dimensionless; γc = 1 is an additional ideal approximation.	IUPAC A00115
Molality-based activity	a = γb·b/b°	Different scale from c; do not equate without appropriate conversion.	IUPAC S05918
Gas activity	a = f/p° ≈ p/p°	Fugacity f; p is absolute pressure; final form only for an ideal gas.	IUPAC S05925
Common thermochemical table temperature	298.15 K = 25 °C	Not implied by the standard symbol; always state T.	IUPAC S05925

References: IUPAC: standard state · IUPAC: standard molality · IUPAC: activity

Four explicitly specified ideal molar gas volumes

Vₘ = RT/p with R = Nₐkᵦ exact in SI. Not a universal measured value for real gases; extra digits support arithmetic checks.

Four explicitly specified ideal molar gas volumes
Temperature	Absolute pressure	Vₘ in the ideal model	Model
273.15 K	100000 Pa	22.710955 L/mol	Ideal gas, absolute pressure; calculated and rounded to 6 decimal places.
273.15 K	101325 Pa	22.413970 L/mol	Ideal gas, absolute pressure; calculated and rounded to 6 decimal places.
298.15 K	100000 Pa	24.789570 L/mol	Ideal gas, absolute pressure; calculated and rounded to 6 decimal places.
298.15 K	101325 Pa	24.465404 L/mol	Ideal gas, absolute pressure; calculated and rounded to 6 decimal places.

References: BIPM: SI defining constants · IUPAC: standard conditions for gases

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Understand and use this reference

A chemical reference is unambiguous only when pressure, temperature, phase and standard-state convention agree. This table separates defined reference states from actual laboratory conditions and shows how to calculate gas volumes and dimensionless activities consistently.

Which entries must be read together

For each property, identify temperature and phase first. Then check pressure reference and, for solutions, the concentration or molality scale. The superscript ° identifies a standard state; it does not specify temperature. Standard values at 298.15 K and 273.15 K may differ physically even though both carry the standard symbol.[2]

A gas volume also requires amount of substance and the gas model. A reaction property requires the complete reaction equation including phases. A heading saying only “standard conditions” is insufficient for reliable calculation. Mark missing conditions as unknown rather than silently adding room temperature or atmospheric pressure.

One bar is standard pressure; one atmosphere remains a different unit

p^\circ=10^5\,\mathrm{Pa}=1\,\mathrm{bar},\qquad1\,\mathrm{atm}=101325\,\mathrm{Pa}

These two defined pressure values are not identical.[1]

Thermodynamic equations use absolute pressure, referenced to vacuum. A gauge pressure of one bar above ambient is not the same absolute pressure as p° = 1 bar. Historical tables may use 1 atm as standard pressure; retain their convention when selecting data rather than merely relabelling it.

At equal temperature and amount, ideal volume is inversely proportional to pressure. The two reference pressures therefore give different molar volumes. The 1.325% pressure difference may be small for an estimate but matters in precise stoichiometry.

A standard state does not fix temperature

IUPAC standard conditions for gases specify 273.15 K and 100 kPa. The commonly used 298.15 K equals 25 °C and is a separate temperature specification for many thermochemical tables. It does not follow from the standard symbol. Interpret abbreviations on instruments or in older documents using their actual stated values.[3]

T/\mathrm K=t/{}^\circ\mathrm C+273.15,\qquad\Delta T/\mathrm K=\Delta t/{}^\circ\mathrm C

A temperature value needs the offset; temperature differences do not.[7]

273.15 K corresponds to 0 °C, not zero temperature in the gas law. Insert thermodynamic temperature in kelvin into pV = nRT. A Celsius ratio 25/0 therefore has no meaning as a ratio of thermodynamic temperatures.

Gas, pure phase and solute

The gas standard state is an idealized reference. For a real gas, thermodynamic behaviour differs from pressure alone; fugacity represents this departure. Pure condensed-phase references use the pure phase at standard pressure and the stated temperature.[2]

A solute standard state may use amount concentration or molality. Usual reference values are c° = 1 mol/dm³ and b° = 1 mol/kg. Molality uses solvent mass; concentration uses solution volume. They cannot simply be equated without density and composition.[4][5]

A hypothetical solute standard state does not mean that an actual 1 mol/L solution is ideal. Its reference is fixed by a thermodynamic convention. Activity coefficients and interactions still matter for real solutions.

Activity provides dimensionless reaction quotients

\mu=\mu^\circ+RT\ln a,\qquad a=\gamma_c\frac{c}{c^\circ}

The logarithm requires dimensionless activity; the concentration form uses the stated scale convention.[6]

a=\gamma_b\frac{b}{b^\circ},\qquad a_{\mathrm{gas}}=\frac{f}{p^\circ}\approx\frac{p}{p^\circ}

For gases, replacing fugacity by pressure is an ideal-gas approximation.

A pure phase in its chosen standard state has activity 1. Pure liquids or solids near standard pressure are often approximated this way; larger pressure departures may require corrections. A solution, mixture or different crystal phase cannot automatically be treated as the same pure reference.

The approximation γ ≈ 1 is an additional assumption, not a consequence of units mol/L or the word “dilute”. Electrolytes commonly use mean ionic activities; individual-ion activities need additional conventions. Activity definitions for all species must match the equilibrium-constant data.

Calculate molar gas volumes from specified conditions

V=\frac{nRT}{p},\qquad V_m=\frac{RT}{p},\qquad n=\frac{pV}{RT}

Ideal gas, absolute pressure, kelvin and consistent units.

R = N_akᵦ = 8.31446261815324 J/(mol·K) is exact in the present SI. Pressure in Pa produces volume in m³; multiply by 1000 for litres. Calculated ideal molar volume is not a universal measured value for every gas: real gases can depart through compressibility factor Z.[7]

V_m=Z\frac{RT}{p}

Z = 1 denotes the ideal model; real Z values need gas-specific state data.

Two calculations with deliberately specified references

Compare bar and atmosphere for 0.35 mol gas at 0 °C

Given: Ideal gas, n = 0.35 mol, T = 273.15 K; first 100000 Pa, then 101325 Pa absolute.
Find: Find both volumes and relative difference.

Calculate ideal molar volume at one bar.
$V_{m,\mathrm{bar}}=8.31446261815324\cdot273.15/100000=0.0227109546415\,\mathrm{m^3/mol}$
Multiply by amount.
$V_{\mathrm{bar}}\approx7.948834\,\mathrm L$
Insert the second absolute pressure.
$V_{\mathrm{atm}}=0.35RT/101325\approx7.844889\,\mathrm L$

Result: Model values: 7.948834 L at 1 bar and 7.844889 L at 1 atm; the first is 1.325% larger.

Check: Vbar/Vatm = 101325/100000 = 1.01325. Back-substitution pV/(RT) gives 0.35 mol in both cases. Extra digits check arithmetic rather than replace measurement uncertainty.

A concentration with nonideal activity

Given: c = 0.080 mol/L, c° = 1 mol/L, specified γc = 0.75; T = 298.15 K.
Find: Find a and μ − μ°.

Form the dimensionless ratio.
$a=0.75\cdot0.080/1=0.060$
Insert the natural logarithm.
$\mu-\mu^\circ=RT\ln0.060\approx-6974.324\,\mathrm{J/mol}$
Interpret the sign.

Result: Activity is 0.060; chemical potential is about 6.974 kJ/mol below μ°.

Check: exp[(μ − μ°)/(RT)] returns 0.060. γc is a problem assumption, not a claimed measured value for a particular substance; c = 0.080 alone would not determine a.

Apply references consistently yourself

What is ideal molar volume at 298.15 K and 1 bar?

Show hint

Use V_m = RT/p and convert m³ to L.

Show solution and calculation

V_m = 0.0247895703 m³/mol ≈ 24.78957 L/mol.
This belongs to 25 °C, not 0 °C.

V_m=RT/p\approx24.78957\,\mathrm{L/mol}

An ideal gas portion occupies 2.40 L at 1 bar and 300 K. Find its amount.

Show hint

V = 0.00240 m³ and p = 100000 Pa.

Show solution and calculation

n = 240/(8.31446261815324·300) ≈ 0.0962179 mol.
Back-substitution nRT gives 240 Pa·m³.

n=pV/(RT)\approx0.0962179\,\mathrm{mol}

For b = 0.20 mol/kg, b° = 1 mol/kg and γb = 0.85, find activity.

Show hint

Form b/b° first; the result has no unit.

Show solution and calculation

a = 0.85·0.20 = 0.17.
This does not determine amount concentration without further solution data.

a=\gamma_b b/b^\circ=0.17

Combine data sets and detect errors

Thermochemical calculations require matching reaction equation, phases, temperature and reference states for all values. A phase change is not a typographical detail: liquid and gaseous water have different molar properties. A zero standard formation quantity for an element in its reference state is a reference assignment, not a claim that it contains no energy.

For lookup data, record source edition and stated conditions. Transfer values between concentration and molality conventions only with the corresponding thermodynamic conversion. Ideal-gas behaviour, activity approximations and pure-phase assumptions are separate models whose limits require separate checks.

Sources for this explanation

Quanta · Updated October 1, 2026

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Term

Reference

Note

standard pressure p°

10⁵ Pa = 1 bar

IUPAC has recommended this value since 1982; 1 atm is different.

standard atmosphere

101 325 Pa

The unit atm; do not confuse it with IUPAC standard pressure.

standard concentration c°

häufig 1 mol·dm⁻³

A chosen reference value; the choice must be stated.

STP

273.15 K and 10⁵ Pa

A defined abbreviation for standard temperature and pressure, not a universal laboratory temperature.

standard state

state and pressure convention

Different conventions apply to gases, pure phases and solutes.

Additional reference tables

Additional reference conventions and activities

Standard states are conventions at a temperature that must be specified. Activities are dimensionless. The chosen γc and γb symbols identify their scale.

Additional reference conventions and activities
Term	Reference/relation	Condition	Row source
Standard molality b°	usually 1 mol/kg	Reference mass is solvent, not solution.	IUPAC S05918
Gas standard state	p° = 1 bar	Hypothetical pure ideal gas at specified T; real gas uses fugacity.	IUPAC S05925
Pure condensed reference phase	a = 1	Pure phase at standard pressure and specified T; not a mixture or automatically another crystal phase.	IUPAC S05925
Concentration-based activity	a = γc·c/c°	Dimensionless; γc = 1 is an additional ideal approximation.	IUPAC A00115
Molality-based activity	a = γb·b/b°	Different scale from c; do not equate without appropriate conversion.	IUPAC S05918
Gas activity	a = f/p° ≈ p/p°	Fugacity f; p is absolute pressure; final form only for an ideal gas.	IUPAC S05925
Common thermochemical table temperature	298.15 K = 25 °C	Not implied by the standard symbol; always state T.	IUPAC S05925

References: IUPAC: standard state · IUPAC: standard molality · IUPAC: activity

Four explicitly specified ideal molar gas volumes

Vₘ = RT/p with R = Nₐkᵦ exact in SI. Not a universal measured value for real gases; extra digits support arithmetic checks.

Four explicitly specified ideal molar gas volumes
Temperature	Absolute pressure	Vₘ in the ideal model	Model
273.15 K	100000 Pa	22.710955 L/mol	Ideal gas, absolute pressure; calculated and rounded to 6 decimal places.
273.15 K	101325 Pa	22.413970 L/mol	Ideal gas, absolute pressure; calculated and rounded to 6 decimal places.
298.15 K	100000 Pa	24.789570 L/mol	Ideal gas, absolute pressure; calculated and rounded to 6 decimal places.
298.15 K	101325 Pa	24.465404 L/mol	Ideal gas, absolute pressure; calculated and rounded to 6 decimal places.

References: BIPM: SI defining constants · IUPAC: standard conditions for gases

Which entries must be read together

One bar is standard pressure; one atmosphere remains a different unit

p^\circ=10^5\,\mathrm{Pa}=1\,\mathrm{bar},\qquad1\,\mathrm{atm}=101325\,\mathrm{Pa}

These two defined pressure values are not identical.[1]

A standard state does not fix temperature

T/\mathrm K=t/{}^\circ\mathrm C+273.15,\qquad\Delta T/\mathrm K=\Delta t/{}^\circ\mathrm C

A temperature value needs the offset; temperature differences do not.[7]

Gas, pure phase and solute

Activity provides dimensionless reaction quotients

\mu=\mu^\circ+RT\ln a,\qquad a=\gamma_c\frac{c}{c^\circ}

The logarithm requires dimensionless activity; the concentration form uses the stated scale convention.[6]

a=\gamma_b\frac{b}{b^\circ},\qquad a_{\mathrm{gas}}=\frac{f}{p^\circ}\approx\frac{p}{p^\circ}

For gases, replacing fugacity by pressure is an ideal-gas approximation.

Calculate molar gas volumes from specified conditions

V=\frac{nRT}{p},\qquad V_m=\frac{RT}{p},\qquad n=\frac{pV}{RT}

Ideal gas, absolute pressure, kelvin and consistent units.

V_m=Z\frac{RT}{p}

Z = 1 denotes the ideal model; real Z values need gas-specific state data.

Two calculations with deliberately specified references

Compare bar and atmosphere for 0.35 mol gas at 0 °C

Given: Ideal gas, n = 0.35 mol, T = 273.15 K; first 100000 Pa, then 101325 Pa absolute.
Find: Find both volumes and relative difference.

Calculate ideal molar volume at one bar.
$V_{m,\mathrm{bar}}=8.31446261815324\cdot273.15/100000=0.0227109546415\,\mathrm{m^3/mol}$
Multiply by amount.
$V_{\mathrm{bar}}\approx7.948834\,\mathrm L$
Insert the second absolute pressure.
$V_{\mathrm{atm}}=0.35RT/101325\approx7.844889\,\mathrm L$

Result: Model values: 7.948834 L at 1 bar and 7.844889 L at 1 atm; the first is 1.325% larger.

Check: Vbar/Vatm = 101325/100000 = 1.01325. Back-substitution pV/(RT) gives 0.35 mol in both cases. Extra digits check arithmetic rather than replace measurement uncertainty.

A concentration with nonideal activity

Given: c = 0.080 mol/L, c° = 1 mol/L, specified γc = 0.75; T = 298.15 K.
Find: Find a and μ − μ°.

Form the dimensionless ratio.
$a=0.75\cdot0.080/1=0.060$
Insert the natural logarithm.
$\mu-\mu^\circ=RT\ln0.060\approx-6974.324\,\mathrm{J/mol}$
Interpret the sign.

Result: Activity is 0.060; chemical potential is about 6.974 kJ/mol below μ°.

Check: exp[(μ − μ°)/(RT)] returns 0.060. γc is a problem assumption, not a claimed measured value for a particular substance; c = 0.080 alone would not determine a.

Apply references consistently yourself

What is ideal molar volume at 298.15 K and 1 bar?

Show hint

Use V_m = RT/p and convert m³ to L.

Show solution and calculation

V_m = 0.0247895703 m³/mol ≈ 24.78957 L/mol.
This belongs to 25 °C, not 0 °C.

V_m=RT/p\approx24.78957\,\mathrm{L/mol}

An ideal gas portion occupies 2.40 L at 1 bar and 300 K. Find its amount.

Show hint

V = 0.00240 m³ and p = 100000 Pa.

Show solution and calculation

n = 240/(8.31446261815324·300) ≈ 0.0962179 mol.
Back-substitution nRT gives 240 Pa·m³.

n=pV/(RT)\approx0.0962179\,\mathrm{mol}

For b = 0.20 mol/kg, b° = 1 mol/kg and γb = 0.85, find activity.

Show hint

Form b/b° first; the result has no unit.

Show solution and calculation

a = 0.85·0.20 = 0.17.
This does not determine amount concentration without further solution data.

a=\gamma_b b/b^\circ=0.17

Combine data sets and detect errors

Sources for this explanation

Quanta · Updated October 1, 2026

Editorial responsibility: Quanta Study editorial team

Review scope

An internal human subject review is documented for an earlier release of this page. That confirmation does not apply automatically to the revised explanation and new exercises.

An independent, named human subject review has not yet been documented. Our exercise numbers are worked examples, rather than measurements published by the cited sources.

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Cite this reference page

Quanta. Chemical reference conventions. Updated October 1, 2026. https://quanta-study.de/en/formula-sheet/tables/chemische-referenzzustaende

When citing a specific scientific statement, also cite the specialist source linked to that statement.

Suggest a correction with a reference