Gases: The Ideal Gas Law
Combination of the three empirical gas laws, (Boyle's, Charles', and Avogadro's) described in the preceding three sections leads to the ideal gas law which is usually written as:

PV = nRT
where P = pressure, V = volume, n = moles, T = kelvin temperature and R. 
The constant R in this equation is known as the universal gas constant. It arises from a combination of the proportionality constants in the three empirical gas laws. The universal gas constant has a value which depends only upon the units in which the pressure and volume are measured. The best available value of the universal gas constant is:
8.3143510  J/mol K   or   8.3143510  kPa dm3/mol K
Another value which is sometimes convenient is 0.08206 dm3 atm/mol K.
This equation is used to determine molecular mass from gas data.
Example:  A liquid can be decomposed by electricity into two gases.   In one experiment, one of the gases was collected. The sample had a mass of 1.090 g, a volume of 850 mL, a pressure of 746 torr, and a temperature of 25oC.  Calculate its molecular mass.
To calculate the molecular mass we need the number of grams and the number of moles.  We can get the number of grams directly from the information in the question. We can calculate the moles from the rest of the information and the ideal gas equation.
        V = 850 mL = 0.850 L = 0.850 dm3
        P = 746 torr/760 torr = 0.982 atm
        T = 25.0oC + 273.15 =  298.15 K

        PV = nRT
       (0.982 atm)(0.850 L) = (n)(0.0821 L atm mol-1 K-1)(298.15 K)
       n = 0.0341 mol

molecular mass = g/mol = 1.090 g/ 0.0341 mol = 31.96 g/mol.  The gas is oxygen.

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