The Mole Concept: Molecules and Atoms

Moles represent or stand in for other quantities. One such quantity is mass.  The atomic mass (for for an element) and the formula weight (for a compound) are equal to a mole. 

Related Byte: Moles to grams conversions

But moles can also represent the number of:

  • Atoms (for elements)
  • Molecules (for compounds)

Moles-to-Molecules

NOTE: atoms and molecules are different and moles will therefore represent different quantities depending on if you have a compound or element.

  • One mole is equivalent to “x” value of atoms of an element
  • One mole is equivalent to “x” value of molecules of a compound

What is “x?” The number of atoms or molecules equal to one mole is called Avogadro’s Number,  and its value is 6.022 x 1023.

  • 1 mole of an ELEMENT = 6.022 x 1023 ATOMS
  • 1 mole of COMPOUND = 6.022 x 1023 MOLECULES

Do these look familiar? They should—they are conversion factors!

You can write them as follows:

Writing-Moles

Starting with Moles

Use the proper Avogadro’s number conversion factor and dimensional analysis to solve in one simple step.

Steps:

Moles-to-Molecules-Graphic

Example: How many molecules are in 3.5 mols of CO2?

Solve using dimensional analysis:

Recall the set up of dimensional analysis such that given units are positioned to cancel out!

Related Byte: Dimensional Analysis

Dimensional-Analysis-Moles

Starting with Grams

So what if you have to find the number of molecules beginning with a mass rather than moles? Simple, just one more step.

Grams-Moles-Molecules

There are __________ molecules in 130 grams of C6H12O6?

Grams-Moles-Molecules-2

Calculating Atoms from Molecules for a Compound

You can find the number of atoms of a particular element or all elements in a molecule of a compound. For example, in CO3, there is 1 atom of C and 3 atoms of O or a total of 4 atoms altogether.

In C6H12O6 , there are 6 atoms of C, 12 atoms of H, and 6 atoms of O or a total of 24 atoms in the entire molecule, etc.

These are also conversion factors. Let’s try an example, using conversion factors and dimensional analysis.

Steps:

Grams-Moles-Molecules-Atoms

How many hydrogen atoms are there in 130 grams of C6H12O6?

Hint

All at once:

Hydrogen

Step by step:

Hydrogen-Steps

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