When we run a reaction to create a gas, we expect it come rise right into the air. Many students have actually done experiments where gases such as hydrogen room formed. The gas can be trapped in a test tube hosted upside-down end the reaction. Carbon dioxide, top top the other hand, sinks as soon as it is released. Carbon dioxide has actually a density greater than air, for this reason it will certainly not rise favor the hydrogen gas.

You are watching: How does the density of a gas depend on the molar mass of the gas?

Gas Density

As girlfriend know, thickness is identified as the mass per unit volume of a substance. Because gases every occupy the same volume on a per mole basis, the density of a certain gas is dependency on its molar mass. A gas v a small molar mass will have actually a lower thickness than a gas through a large molar mass. Gas densities are frequently reported in \(\textg/L\). Gas density can it is in calculated from molar mass and also molar volume.

Figure \(\PageIndex1\): Balloons filled with helium gas to rise in air because the thickness of helium is less than the density of air.

Example \(\PageIndex1\)

What is the thickness of nitrogen gas at STP?


Step 1: perform the recognized quantities and plan the problem.


\(\ceN_2 = 28.02 \: \textg/mol\) \(1 \: \textmol = 22.4 \: \textL\)


density \(= ? \: \textg/L\)

Molar mass divided by molar volume yields the gas density at STP.

Step 2: Calculate.

\<\frac28.02 \: \textg1 \: \textmol \times \frac1 \: \textmol22.4 \: \textL = 1.25 \: \textg/L\>

When set up with a counter factor, the \(\textmol\) unit cancels, leave \(\textg/L\) as the unit in the result.

Step 3: Think around your result.

The molar fixed of nitrogen is slightly larger than molar volume, for this reason the thickness is slightly better than \(1 \: \textg/L\).

See more: The Microbes Commonly Known As ________ Are Single-Celled Eukaryotes That Are Generally Motile.

Alternatively, the molar mass of a gas can be established if the thickness of the gas in ~ STP is known.

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