Chemistry Practice Problems Kinetic Molecular Theory Practice Problems Solution: Two identical flasks contain O2 and H2 gas, respec...

Solution: Two identical flasks contain O2 and H2 gas, respectively, at the same temperature and pressure. In comparing the two flasks, which of the following is not a true statement.(a) the average kinetic energy of H2 is greater than that of O2(b) the average speed of a H2 molecule is greater than that of an O2(c) the number of impacts/(unit area)/(unit time) are different for the two gases(d) the number of moles is the same(e) total mass of each gas is different

Problem

Two identical flasks contain O2 and H2 gas, respectively, at the same temperature and pressure. In comparing the two flasks, which of the following is not a true statement.

(a) the average kinetic energy of H2 is greater than that of O2

(b) the average speed of a H2 molecule is greater than that of an O2

(c) the number of impacts/(unit area)/(unit time) are different for the two gases

(d) the number of moles is the same

(e) total mass of each gas is different

Solution

We are asked which of the statements are NOT TRUE given: 

Two identical flasks contain O2 and H2 gas, respectively, at the same temperature and pressure.

Analyze each statement:

(a) the average kinetic energy of H2 is greater than that of O2

  • The temperature is the same so the average kinetic energy is the same.
  • H2 is lighter than O2 so this is FALSE.


(b) the average speed of a H2 molecule is greater than that of an O2

Recall that the molecular speed of a gas is given by:


mol. speed = 3RTM


where R = gas constant, T = temperature, and M = molar mass of the gas.


  • We can see that the molar mass is in the denominator. 
  • This means that the lower the molar mass the faster the molecule is. 
  • H2 is lighter than O2 so this is TRUE.


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