Molecular Weight of Common Fluids

Molar Mass Reference Table


fluid_name chemical_formula molar_mass
fluid_name chemical_formula molar_mass

Information and Definitions

What is Molecular Weight?

Molecular weight (also called molecular mass) is the sum of the atomic weights of all atoms in a molecule. It is expressed in kilograms per kilomole (kg/kmol) or grams per mole (g/mol). Accurate values are essential for mass balance, stoichiometry, and gas property calculations.

Units and Conversions

  • 1 g/mol = 1 kg/kmol
  • For mixtures, use molecular weight in kg/kmol when applying the universal gas constant R = 8.314 kJ/(kmol·K).

Common Engineering Uses

  • Ideal gas calculations: p·V = n·R·T with n = m/MW.
  • Mass and energy balances: Converting between mass and molar flow rates.
  • Compressibility and density: ρ = (p·MW)/(Z·R·T) for gases.
  • Diffusion and mixing: Lighter molecules diffuse faster; MW influences binary diffusion coefficients.

Mixtures and Apparent Molecular Weight

For gas mixtures, compute apparent molecular weight by molar fraction:

MW_mix = Σ (yi · MW_i)

where yi is the molar fraction of component i. Use MW_mix for density and flow calculations of blended streams (e.g., natural gas, air).

Data Quality Notes

Values in the table are rounded to typical reference precision. When high accuracy is needed, use standardized sources (e.g., NIST, API) and keep consistency with the property database used in your simulation or design tools.

Molecular Weight Data References

1 CRC Handbook of Chemistry and Physics (106th Edition, 2025-2026) — Molecular weights and elemental data.

2 Perry's Chemical Engineers' Handbook (10th Edition) — Physical and chemical properties tables.

3 NIST Chemistry WebBook (https://webbook.nist.gov/) — Molecular weight and thermophysical properties for common compounds.

4 DIPPR Project 801 Thermophysical Database (AIChE) — Evaluated molecular weight and property data for industrial chemicals.

5 Yaws, C. L. (2015) Transport Properties of Chemicals and Hydrocarbons, 2nd Edition — Molecular weight and related properties.

6 Lange's Handbook of Chemistry (16th Edition) — Reference molecular weights and constants.

7 IUPAC (2021) Standard Atomic Weights — Authoritative atomic weights for elemental composition calculations.

8 API Technical Data Book – Petroleum Refining (9th Edition) — Molecular weights for hydrocarbon families and petroleum fractions.

9 Engineering ToolBox – Hydrocarbon Gases and Liquids Molecular Weight (https://www.engineeringtoolbox.com/) — Quick reference values for process use.

10 DIPPR 801 Companion Guide (2020) — Guidance on data quality and usage for molecular weight–based calculations.

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Frequently Asked Questions

Q1 What is molecular weight and why does it matter?

A1 Molecular weight (molar mass) is the sum of atomic masses in a molecule, expressed in g/mol. It is crucial to convert between mass and molar flow, estimate density via equations of state, and balance reactions in process calculations.

Q2 How do I use this table in flow calculations?

A2 Take the molecular weight of the gas, use it to convert between volumetric, mass, and molar flow, and pair it with an equation of state (ideal, Peng-Robinson, etc.) to estimate density and compressibility. This is essential for sizing orifice plates, restriction devices, and other flow meters.

Q3 Why do molecular weights vary in real mixtures?

A3 Real streams (e.g., natural gas) are mixtures with changing composition. Molecular weight is the mole-fraction-weighted average. Composition shifts change density, sonic velocity, and pressure drop.

Q4 How does molecular weight affect Mach number and choked flow?

A4 It influences speed of sound; higher molecular weight generally lowers sonic velocity. That alters the critical (choked) flow condition in nozzles and orifices and must be considered when sizing restriction elements.

Q5 What is the link to specific heat ratio (k or γ)?

A5 Different mixtures with different molecular weights often have different k values. For compressible flow, both matter: molecular weight for density, k for pressure-temperature relationships.

Q6 How does it connect to viscosity and pressure drop?

A6 Molecular weight affects density, and together with viscosity sets Reynolds number, friction factor, and pressure drop. Use consistent molecular weight and viscosity data at the same temperature and pressure.

Q7 What accuracy is needed for mass and energy balances?

A7 Accurate molecular weights per component are needed for stoichiometry, emissions reporting, and ppm-to-mg/Nm³ conversions. Using the correct mixture average avoids errors in excess air or fuel consumption calculations.

Q8 Where can I get reliable data?

A8 Use trusted sources: API and GPA data for natural gas, NIST Webbook, or vendor/GC composition for specific streams. Then calculate the molar-average molecular weight from the actual composition.