Gas Laws and Number of Molecules

 General Chemistry Index

Where are we going with this? Getting to the current models of atomic theory didn't happen over night. This page will support the ability to use the kinetic molecular theory with the combined and ideal gas laws to explain changes in volume, pressure, moles and temperature of a gas and apply the ideal gas equation (PV = nRT) to calculate the change in one variable when another variable is changed and the others are held constant.

Gas Laws and Number of Molecules
Yes, there will be math. Eventually. 

The number of molecules being observed… Well, this is rather intuitive: the number of molecules is… the number of molecules in the container.

Counting molecules is not easy. They are… small and do not take up much room. Any sample would have a bazillion molecules in it!

bazillion |bəˈzilyən|

 

cardinal number informal, chiefly N. Amer.

a very large exaggerated number.

Numbering molecules is usually done by saying how many moles are present.

The mole is the unit of measurement in the International System of Units (SI) for amount of substance. 

It is defined as the amount of a chemical substance that contains as many elementary entities, e.g., atoms, molecules, ions, electrons, or photons, as there are atoms in 12 grams of carbon-12 (12C), the isotope of carbon with relative atomic mass 12 by definition. This number is expressed by the Avogadro constant, which has a value of 6.022140857(74)×1023/mol. The mole is one of the base units of the SI, and has the unit symbol mol.

Avogadro's Number: 6.0221409e+23 = 6.0221409 X 1023= 602,214,090,000,000,000,000,000 

For reference and further understanding, one mole of a gas at standard temperature and pressure (273.15 K and 1 ATM) occupies a volume of 22.4 L.

How Does The Number of Molecules Affect Gases?

When the number of molecules goes up…

• more molecules in the container will result in more frequent collisions with the container.
  
  
• increasing the number of molecules will increase the pressure if the temperature and volume are kept constant. 
  
  
• increasing the number of molecules will increase the volume if the pressure and temperature are kept constant.

The most obvious example is adding molecules of air to a balloon. Blowing into a balloon obviously changes its volume. It is easy to see that one affect adding molecules can have is to increase volume. The opposite is also true.

If the volume cannot change, such as in a basketball or volleyball that "isn't flat" something else must happen. Pumping more air into a basketball or volleyball does not change the size (unless it was flat to start with). It makes the ball harder. Why? The more molecules inside the set, fixed volume, the more collisions with the inside wall of the ball, and therefore, the higher the pressure. So a second affect adding molecules can have is to increase pressure.

Understanding the affect adding molecules has on temperature is not as intuitive. To examine this, we have to imagine a situation where molecules are added but that neither the volume, nor pressure changes. In this case we have more molecules in the same space, which means more collisions with the unmovable wall. For there to be more collisions but to NOT have a pressure change must mean that the molecules are moving slower. Therefore, if molecules are added and if pressure and volume do not change THEN temperature must go down (it must be lowered on purpose).

The math… Universal Gas Law