Question

The graph shows the average molecular speed of four gases a 27 °C. 

What, if any, relationship is observed between the average molecular speed and the molar mass of the gas? 

There is no relationship between the average molecular speed and the molar mass. 

The average molecular speed decreases as the molar mass of the gas increases. 

The average molecular speed increases as the molar mass of the gas increases. 

The average molecular speed decreases as the molar mass of the gas decreases. 

 Estimate the average molecular speed of helium at 27°C. 

average molecular speed of helium:

Answer 1

Correct option:

Molar mass of Xe (131.2 g/mol) > Molar mass of Ar (40 g/mol) > Molar mass of Ne (20.1 g/mol) > Molar mass of He (4 g/mol).

From the graph it is clear that the maximum of the curves shifts to the lower average molecular speed with the increase in the molar mass of the gas.

----

Average molecular speed,

where,

R = gas constant = 8.314 J/mol.K

T = temperature of the gas = 27 ^oC = (27 + 273) K = 300 K

M = molar mass of helium = 4 g/mol = 0.004 kg/mol       ( 1 g = 0.001 kg)

Thus,

         = 1260 m/s

Hence, the average molecular speed of helium = 1260 m/s