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# What causes one gas to be denser than another?

Question #120567. Asked by star_gazer. (Mar 04 11 12:40 AM)

looney_tunes

A number of factors need to be considered. If we are comparing two gasses that are stored at the same temperature and pressure, then the difference in density (which is mass per unit volume) is determined by the difference in molecular weights of the two gasses. Larger molecules have more mass, and the same number of molecules per unit volume, so their density is greater.

Density can also be increased by increasing concentration of the molecules. If you increase the pressure, the same number of molecules occupy a smaller volume, so the density increases. So a sample of (for example) oxygen at 2 atmospheres pressure has a greater density than one at the same temperature but 1 atmosphere pressure. Likewise, increasing temperature will cause the gas to expand and occupy a greater volume (if the pressure does not change), resulting in a lower density.

So the overall comparison depends on the molecular mass, temperature, and volume of the gas. It's not simple, especially if more than one of these variables differs.

http://www.grc.nasa.gov/WWW/K-12/airplane/fluden.html

 Mar 04 11, 2:16 AM
Watchkeeper

The Ideal Gas Equation is

pV = mRT/Mm

where p is the gas pressure, V is its volume, m its mass, T its temperature, Mm its molar mass and R the gas constant (8.314 J/K/mol).

Rearranging, we have

m/V = density = pMm/RT

Thus density increases as the pressure increases and decreases as the temperature increases. It also increases as the molar mass, Mm, increases. This is the mathematical basis for what looney_tunes has stated.

The molar mass is the relative molecular mass expressed in grams, and you find the relative molecular mass by looking up the relative atomic mass(es) in the Periodic Table. Consider the gases carbon monoxide, CO, and hydrogen chloride, HCl. The Periodic Table shows the relative atomic mass of carbon to be 12, of oxygen to be 16, of hydrogen to be 1 and of chlorine to be 35.5. Hence the relative molecular mass of CO is 12 + 16 = 28, and of HCl is 1 + 35.5 = 36.5. The molar masses are thus 28g/mol and 36.5g/mol respectively. This means, under the same physical conditions, HCl is 30% more dense than CO (36.5/28 = 1.304).

 Mar 06 11, 10:31 AM

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