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Sunday, May 3, 2015

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






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

Friday, May 1, 2015

Ideal Gas Law Problems, 2

Category: Chemical Engineering Math

"Published in Newark, California, USA"

A sample of pure zinc metal reacts completely with an excess of hydrochloric acid. The hydrogen gas produced is collected over water at 25°C and 1 atm. If the metal weighs 253.02 mg, how much hydrogen gas is collected?

Solution:

Consider the chemical reaction of pure zinc metal with hydrochloric acid to form hydrogen gas collected over water as follows


From the chemical reaction above, the number of moles of hydrogen gas collected over water is


  
The absolute temperature of oxygen gas is




The universal gas law constant for mL, mgmole, K, and atm is  .

Therefore, by Ideal Gas Law, the amount of  hydrogen gas collected over water