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Saturday, May 2, 2015

Ideal Gas Law Problems, 3

Category: Chemical Engineering Math

"Published in Newark, California, USA"

A certain gas weighs 2.478 grams and occupies a volume of 712.5 mL at 25°C and 1.2 atm. What is the molecular formula of the gas?

Solution:

In the given word problem, the weight of a gas is given. We need to solve for the number of moles of a gas in order to get its molecular weight as well as its molecular formula. 

The absolute temperature of a gas is 



The universal gas law constant for L, gmole, K, and atm is 
.
  
Hence, by Ideal Gas Law, the number of moles of a gas is


  

The molecular weight of a gas is
 
 
 

Next, we need to get the Periodic Table of Elements and check the atomic mass of the gas elements (except the noble gasses). The molecular weight of a gas is equal to twice the atomic mass of a gas element. The molecular weight of the gasses are:
 
 
 
 
 
 

By comparing the calculated molecular weight of a gas with the molecular weight of the gasses above, chlorine gas is the closest value.

Therefore, the molecular formula of a certain gas is Cl2