Molar Mass and Molecular Weight of Air

What is the molecular weight of air? The molecular weight of air (also called the molar mass of air) is 28.9647 g/mol. This is one of the most fundamental constants in thermodynamics, fluid mechanics, and atmospheric science.

Air is not a pure substance — it's a mixture of gases, primarily nitrogen (N₂), oxygen (O₂), argon (Ar), and trace amounts of carbon dioxide (CO₂). Because it's a mixture, the molar mass of air must be calculated as a weighted average based on the volumetric composition of dry air:

The terms "molar mass of air," "molecular weight of air," "molar weight of air," "molecular mass of air," "mol wt of air," "mw of air," "air atomic mass," and "atomic weight of air" all refer to this same value: 28.9647 g/mol. These are interchangeable terms in the context of air properties.

You'll often see the molar mass of air rounded to 28.97 g/mol or even 29 g/mol in textbooks and engineering handbooks. This rounding introduces negligible error for most engineering calculations (less than 0.2%), but when high precision is required — particularly in atmospheric modeling or combustion analysis — the full value of 28.9647 g/mol should be used.

The molar mass of air is used to convert between mass-based and mole-based quantities using the ideal gas law. For example, the specific gas constant of air is derived from the universal gas constant divided by the molar mass: R_air = 8314.46 J/(kmol·K) / 28.9647 kg/kmol = 287.058 J/(kg·K).

Specific Heat of Air — c_p, c_v, and γ Explained

The specific heat of air is one of its most important thermodynamic properties. There are two types of specific heat for gases: specific heat at constant pressure (c_p) and specific heat at constant volume (c_v).

Specific Heat at Constant Pressure (c_p of air)

The specific heat of air at constant pressure is c_p = 1005.7 J/(kg·K) at 25°C. In English units, this is 0.240 BTU/(lb·°F). This is the heat capacity of air when heated or cooled at constant pressure (the most common condition in open systems like atmospheric flow, HVAC systems, and turbines).

The cp value of air increases slightly with temperature due to increased molecular vibration at higher temperatures. At 0°C, c_p ≈ 1006 J/(kg·K), while at 1000°C, c_p ≈ 1141 J/(kg·K). For most room-temperature calculations, c_p = 1006 J/(kg·K) or 1005 J/(kg·K) is used as the standard reference value.

Specific Heat at Constant Volume (c_v of air)

The specific heat at constant volume is c_v = 718.6 J/(kg·K) at 25°C. This is the heat capacity when air is heated in a rigid container (constant volume). The relationship between c_p and c_v is given by Mayer's relation:

Ratio of Specific Heats (γ = gamma)

The ratio of specific heats is γ = c_p/c_v. For air at room temperature, γ = 1.400 exactly. This value is fundamental in compressible flow calculations, isentropic processes, and the speed of sound equation.

For an ideal diatomic gas (like N₂ and O₂, which make up 99% of air), γ = 1.4 is exact at moderate temperatures due to having 5 degrees of freedom (3 translational + 2 rotational). At very high temperatures (above ~1000 K), vibrational modes become active and γ decreases slightly.

The terms "cp of air," "cv of air," "specific heat value of air," "heat capacity of air," "sp heat of air," "cp value of air," "specific heat capacity of air kj kg k," and "heat constant of air" all refer to these same properties. In SI units, the specific heat of air is measured in J/(kg·K) or kJ/(kg·K), while in English units it's BTU/(lb·°F).

Dynamic and Kinematic Viscosity of Air

Air has two types of viscosity: dynamic viscosity (μ, also called absolute viscosity) and kinematic viscosity (ν, pronounced "nu").

Dynamic Viscosity of Air (μ)

The dynamic viscosity of air is a measure of its resistance to shear stress. At 20°C and atmospheric pressure, μ = 1.81×10⁻⁵ Pa·s (= 1.81×10⁻⁵ kg/(m·s)). In centipoise units, this is 0.0181 cP. At 25°C, the dynamic viscosity of air increases slightly to μ = 1.85×10⁻⁵ Pa·s.

Unlike liquids, the viscosity of gases increases with temperature. This is because gas molecules move faster at higher temperatures, increasing momentum transfer and thus viscosity. The relationship is governed by Sutherland's Law:

Sutherland's Law is one of the most commonly tested formulas in fluid mechanics courses because it accurately predicts gas viscosity over a wide temperature range (−50°C to 1000°C at atmospheric pressure).

Kinematic Viscosity of Air (ν)

The kinematic viscosity of air is the dynamic viscosity divided by density:

Kinematic viscosity is used in Reynolds number calculations (Re = VL/ν) to predict whether flow is laminar or turbulent. Because density decreases with temperature while dynamic viscosity increases, kinematic viscosity increases rapidly with temperature.

Common search terms like "dynamic viscosity of air at 20°C," "viscosity of air at 20°C," "kinematic viscosity of air at 25°C," "viscosity of air centipoise," "air viscosity," "mu of air," and "viscosity of air at room temperature" all refer to these values: μ ≈ 1.81×10⁻⁵ Pa·s and ν ≈ 1.52×10⁻⁵ m²/s at standard room temperature conditions.

Thermal Conductivity and Prandtl Number of Air

Thermal Conductivity of Air (k)

The thermal conductivity of air measures its ability to conduct heat. At 20°C, k = 0.02551 W/(m·K). At 25°C, k = 0.02625 W/(m·K). In English units, this is approximately 0.0148 BTU/(hr·ft·°F) at 20°C.

Like viscosity, the thermal conductivity of air increases with temperature because faster-moving molecules transfer kinetic energy (heat) more effectively. A polynomial fit gives:

Prandtl Number of Air (Pr)

The Prandtl number is a dimensionless number that relates momentum diffusivity (kinematic viscosity) to thermal diffusivity:

For air, Pr < 1, which means momentum diffuses more slowly than heat. This has important implications for convective heat transfer: the thermal boundary layer is thicker than the velocity boundary layer. The Prandtl number of air remains remarkably constant (0.71 ± 0.02) across a wide temperature range from −50°C to 500°C, making it a convenient constant in many heat transfer correlations.

Searches for "thermal conductivity of air," "heat conductivity of air," "conductivity of air," "pr number for air," and "heat transfer properties of air" all relate to these fundamental values: k ≈ 0.0255 W/(m·K) and Pr ≈ 0.71 at room temperature.

How Air Properties Change With Temperature

Air properties vary significantly with temperature, and understanding these trends is critical for HVAC design, combustion analysis, aerodynamics, and atmospheric modeling. Here are the key trends from −50°C to 1000°C at atmospheric pressure:

🔽 Density decreases with temperature

From the ideal gas law (ρ = P/(R·T)), density is inversely proportional to absolute temperature. At constant pressure, doubling the absolute temperature (e.g., 300 K to 600 K) halves the density. At −50°C, ρ = 1.582 kg/m³; at 1000°C, ρ = 0.277 kg/m³ — nearly a 6× decrease.

🔼 Dynamic viscosity increases with temperature

Sutherland's Law shows μ increases by approximately T^1.5 over moderate temperature ranges. At 20°C, μ = 1.81×10⁻⁵ Pa·s; at 1000°C, μ = 4.83×10⁻⁵ Pa·s — a 2.7× increase. This is opposite to liquids, where viscosity decreases with temperature.

🔼 Kinematic viscosity increases rapidly with temperature

Because ν = μ/ρ, and μ increases while ρ decreases, kinematic viscosity increases dramatically. At 20°C, ν = 1.51×10⁻⁵ m²/s; at 1000°C, ν = 17.4×10⁻⁵ m²/s — more than a 10× increase.

🔼 Specific heat (c_p) increases slightly with temperature

At room temperature, c_p ≈ 1006 J/(kg·K). At 500°C, c_p ≈ 1093 J/(kg·K). At 1000°C, c_p ≈ 1141 J/(kg·K). The increase is due to vibrational modes becoming accessible at higher temperatures. For many engineering calculations below 200°C, c_p can be treated as constant.

🔼 Thermal conductivity increases with temperature

At 20°C, k = 0.0255 W/(m·K); at 1000°C, k = 0.083 W/(m·K) — a 3.3× increase. This follows similar physics to viscosity: faster molecules transfer heat more effectively.

≈ Prandtl number stays nearly constant

Despite all the other properties changing dramatically, Pr remains between 0.68 and 0.73 from −50°C to 1000°C. This is because μ, c_p, and k all increase with temperature in ways that approximately cancel in the ratio Pr = μc_p/k.

These temperature dependencies are why engineers can't simply use "handbook values" — accurate calculations require temperature-corrected properties, especially for high-temperature applications like gas turbines, combustion chambers, and supersonic flight.

Air Properties in English/Imperial Units

Many engineering applications in the United States use English (Imperial) units rather than SI units. Here are the key air properties at standard conditions (68°F, 14.696 psia) in English units:

The most commonly searched English unit value is the cp of air in BTU/(lb·°F), which is 0.240 BTU/(lb·°F). This is sometimes rounded to 0.24 or written as 1.00 BTU/(lb·°R) in absolute temperature units.

Conversion factors between SI and English units:

• 1 J/(kg·K) = 0.000239006 BTU/(lb·°F)
• 1 W/(m·K) = 0.5779 BTU/(hr·ft·°F)
• 1 Pa·s = 0.020885 lb/(ft·s) = 1000 cP
• 1 kg/m³ = 0.062428 lb/ft³
• 1 m²/s = 10.7639 ft²/s

Moles of Air — Using the Molar Mass

The molar mass of air (28.9647 g/mol) is used to convert between mass and moles of air. This is essential for stoichiometric combustion calculations, ideal gas law applications, and atmospheric chemistry.

Worked Examples — Air Properties Calculations

Air Properties Reference Table — Complete Values from −50°C to 1000°C

The table below contains pre-calculated values for all ten thermophysical properties of dry air at atmospheric pressure (101.325 kPa) across the full temperature range from −50°C to 1000°C. All values are hardcoded for search engine indexing and can be used as a quick reference or printed as a cheat sheet.

Notes on the reference table:

• All SI values are at atmospheric pressure (101.325 kPa = 1 atm = 14.696 psia)
• Dynamic viscosity (μ) values are calculated using Sutherland's Law
• Kinematic viscosity (ν) = μ/ρ
• Specific heat (c_p) values from polynomial correlation, accurate to ±0.5%
• Thermal conductivity (k) from polynomial fit, accurate to ±1%
• Prandtl number (Pr) calculated from Pr = μc_p/k
• Speed of sound (c) from c = √(γRT), where γ = c_p/c_v

Frequently Asked Questions