Dalton's Law of Partial Pressure, 2

Category: Chemical Engineering Math

"Published in Vacaville, California, USA"

Ammonia decomposes completely to hydrogen and nitrogen gas. If the total pressure of the reaction vessel at the end of the reaction is 250 mm Hg, what is the partial pressure of nitrogen gas?

Solution:

Consider the decomposition reaction of ammonia into hydrogen and nitrogen gas as follows

Basis: 2 moles NH₃ = 3 moles H₂ = 1 mole N₂

If the total pressure of the reaction vessel at the end of the reaction is 250 mm Hg, therefore, by Dalton's Law of Partial Pressure, the partial pressure of nitrogen gas is

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.