Themes > Science > Chemistry > Inorganic Chemistry > More Information About Gas Laws > Gas Index > Molecular weight of a gas

You can use the ideal gas law to compute the molecular weight of a gas. Since n, the number of moles, is just equal to the mass m divided by the molecular weight M, we can substitute in the m/M for n in the ideal gas law and solve for M.
PV = nRT
PV = mRT / M
M = mRT / PV
This is how we know that molecules like oxygen and nitrogen are diatomic gases: we measure P,V m and T for a given amount of gas and compute M

Example: A sample of oxygen gas is studied. If a 1.00 gram sample of oxygen gas at 25.0oC and 1.00 atm pressure has a volume of 0.765 L, what is the molecular weight of oxygen gas

Solution: Simply use the above equation to get the molecular weight

M = mRT/PV
M = (1.00 g* 0.0821 (L*atm/mol*K) * 298 K) / (1.00 atm*0.765 L)
M = 32.0 g/mole


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