Molar Mass of a Gas

QuickNotes

Molar Mass of a Gas: M = (dRT)/P

• M = molar mass of a gas (grams)
• Don't get this confused with the symbol M for molarity of a solution!
• d = density (grams/Liter)
• R = universal gas constant
• T = Kelvin temperature
• P = pressure (atm)

Example:

The density of a gas is measured at 1.853 g / L at 745.5 mmHg and 23.8 °C. What is its molar mass?

• Convert mmHg to atm and °C to K. Use 1.000 L. 

745.5 mmHg/760 = .981 atm

• Plug into PV = nRT and solve for n. 

(.981 atm) (1.000 L) = (n) (0.08206) (296.8 K)

n = 0.0402753 mol  

• Then, divide 1.853 g by n, the number of moles.

1.853 g / 0.0402753 mol = 46.01 g/mol

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Molar Mass Quiz