Use this professional calculator to determine the Vapor Density (relative to Hydrogen) and Specific Gravity (relative to Air) of any gas or vapor. Essential for chemical engineering, safety assessments, and HVAC design.
Enter the molecular weight of the gas/vapor (e.g., CO₂ is approx 44.01).
Please enter a valid positive molecular weight.
Hydrogen (Standard Vapor Density)
Air (Specific Gravity / Relative Density)
"Vapor Density" strictly refers to Hydrogen (H₂), while "Specific Gravity" compares to Air.
Vapor Density (Relative to H₂)
0.00
Formula: MW / 2.016
Specific Gravity (vs Air)0.00
Density @ STP (g/L)0.00
Buoyancy in Air–
Figure 1: Comparison of Molecular Weights (Your Gas vs. Air vs. Hydrogen)
Table 1: Calculated Gas Properties based on inputs.
Property
Value
Unit / Scale
Molecular Weight
–
g/mol
Vapor Density (H₂ ref)
–
Dimensionless
Specific Gravity (Air ref)
–
Dimensionless
What is the Calculation of Vapor Density from Molecular Weight?
When professionals need to calculate vapor density from molecular weight, they are determining how heavy a gas or vapor is compared to a reference standard—usually hydrogen gas or air. This calculation is a fundamental concept in physical chemistry, industrial hygiene, and fire safety engineering.
Vapor density is a dimensionless quantity. It tells you whether a gas will rise and dissipate or sink and accumulate in low-lying areas. Understanding this property is critical for managing hazardous materials, designing ventilation systems, and ensuring workplace safety in environments where volatile chemicals are present.
While often confused, "Vapor Density" strictly compares a gas to Hydrogen (H₂), whereas "Specific Gravity" (or Relative Vapor Density) compares it to Air. This guide and calculator allow you to derive both figures directly from the substance's molecular weight.
Vapor Density Formula and Mathematical Explanation
To calculate vapor density from molecular weight, we rely on Avogadro's Law, which states that equal volumes of gases at the same temperature and pressure contain the same number of molecules. Because of this, the ratio of the densities of two gases is equal to the ratio of their molecular weights.
1. Formula Relative to Hydrogen (Classical Vapor Density)
Hydrogen (H₂) is the lightest gas with a molecular weight of approximately 2.016 g/mol (often rounded to 2). The formula is:
Vapor Density = Molecular Weight of Gas / Molecular Weight of Hydrogen
or simply: VD = MW / 2
2. Formula Relative to Air (Specific Gravity)
Dry air is a mixture of gases (mostly Nitrogen and Oxygen) with an average molecular weight of approximately 28.97 g/mol (often rounded to 29). The formula is:
Specific Gravity (Air) = Molecular Weight of Gas / 28.97
Variable Definitions
Variable
Meaning
Unit
Typical Range
MW
Molecular Weight (Molar Mass)
g/mol
2 (H₂) to 200+ (Heavy Organics)
VD
Vapor Density (H₂ scale)
None (Ratio)
1.0 to 100+
SG
Specific Gravity (Air scale)
None (Ratio)
0.07 to 10+
28.97
Avg Molecular Weight of Air
g/mol
Constant
Practical Examples of Vapor Density Calculations
Here are two real-world scenarios showing how to calculate vapor density from molecular weight to assess risk.
Example 1: Carbon Dioxide (CO₂) Leaks
Scenario: A safety engineer needs to determine where CO₂ sensors should be placed in a brewery fermentation room.
Calculation (VD): 44.01 / 2.016 ≈ 21.83 (Relative to Hydrogen).
Calculation (SG): 44.01 / 28.97 ≈ 1.52 (Relative to Air).
Interpretation: Since the Specific Gravity (1.52) is greater than 1.0, CO₂ is heavier than air. It will sink to the floor, creating an asphyxiation hazard in low-lying areas. Sensors should be placed near the ground.
Example 2: Methane (CH₄) Leak Detection
Scenario: Determining the flow path of a natural gas leak in a residential utility room.
Interpretation: With a Specific Gravity of 0.55 (less than 1.0), Methane is lighter than air. It will rise and pool near the ceiling. Ventilation and detectors should be installed at the highest points of the room.
How to Use This Vapor Density Calculator
Our tool is designed to simplify the math so you can focus on the safety implications. Follow these steps:
Find the Molecular Weight: Enter the known molecular weight of your substance in the "Molecular Weight" field. You can find this on an MSDS (Material Safety Data Sheet) or periodic table.
Select Reference Standard: Choose "Hydrogen" for classical vapor density (common in academic chemistry) or "Air" for specific gravity (common in fire safety and HVAC).
Review Intermediate Metrics: Look at the "Density @ STP" and "Buoyancy" indicators to understand the physical behavior of the gas.
Analyze the Chart: The dynamic bar chart visually compares your gas against Air and Hydrogen to give you an immediate sense of its relative weight.
Copy for Reports: Use the "Copy Results" button to paste the data directly into your safety reports or lab notebooks.
Key Factors That Affect Vapor Density Results
While the calculation from molecular weight is straightforward, several real-world factors can influence the effective behavior of a vapor.
Temperature: Gases expand when heated. A heavy gas heated significantly might temporarily become lighter than the surrounding cooler air, affecting its flow path.
Pressure: Variations in atmospheric pressure affect absolute density (g/L), though the ratio (vapor density) remains relatively constant if both gas and air are at the same pressure.
Humidity: Moist air is actually lighter than dry air because water vapor (MW ~18) is lighter than nitrogen/oxygen (MW ~29). This can slightly skew the specific gravity comparison in highly humid environments.
Gas Mixtures: Industrial vapors are rarely pure. If you have a mixture, you must calculate the weighted average molecular weight of all components to get an accurate vapor density.
Ventilation Currents: Even a gas that is heavy (high vapor density) can be dispersed by strong air currents or HVAC systems, overriding the effects of gravity.
Stratification: In still air, gases with densities very close to air (1.0) may stratify or mix slowly rather than distinctly rising or sinking immediately.
Frequently Asked Questions (FAQ)
Why is the molecular weight of air considered 29?
Air is a mixture roughly composed of 78% Nitrogen (N₂, MW=28) and 21% Oxygen (O₂, MW=32), plus trace gases like Argon. The weighted average comes out to approximately 28.97 g/mol, which is rounded to 29 for most engineering calculations involving vapor density.
What is the difference between Vapor Density and Specific Gravity?
In strict chemical terms, Vapor Density is the ratio of the weight of a volume of gas to the weight of an equal volume of Hydrogen. Specific Gravity (Gas) is the ratio of the gas to Air. However, in many industrial safety contexts, "Vapor Density" is used colloquially to mean density relative to air.
Does this calculator apply to liquids?
No. This calculator is specifically for gases and vapors. Liquids use a specific gravity relative to water (where water = 1.0). The physics of gas expansion and buoyancy differ from liquid hydrostatics.
How do I calculate vapor density if I don't know the molecular weight?
If the molecular weight is unknown, you can determine vapor density experimentally using the Dumas method or Victor Meyer method, which involve weighing a known volume of the vapor at a specific temperature and pressure.
Is a gas with a vapor density of 1.0 safe?
Not necessarily. A vapor density of 1.0 means it has the same weight as air. Carbon Monoxide (CO), for example, has a specific gravity of roughly 0.97, making it very close to air. It mixes easily and moves with air currents, making it highly dangerous as it doesn't settle or rise predictably.
Can I use this for ideal gases only?
The formula `VD = MW / 2` assumes Ideal Gas behavior. At very high pressures or very low temperatures (near liquefaction), real gases deviate from this behavior, and compressibility factors must be considered.
What is the unit for Vapor Density?
Vapor Density is a dimensionless unit because it is a ratio of two densities (or two molecular weights). It has no physical units like grams or pounds.
Why is knowing vapor density important for firefighters?
Firefighters must know if hazardous vapors from a spill will pool in basements (heavier than air) or rise to upper floors (lighter than air) to plan evacuation routes and ventilation strategies safely.
Related Tools and Internal Resources
Molar Mass Calculator – Determine the molecular weight of complex compounds before using this tool.