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?

Solution

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

Known

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

Unknown

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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