The Concept of Moles By Sho Morimoto, Class of 2000

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The Concept of Moles
By Sho Morimoto, Class of 2000

At first glance, moles sound like a complicated chemical concept. A quick read through the textbook gives you a bunch of numbers, exponents, and chemical formulas. In reality, though, the concept of moles is very easily understood when taken step-by-step.

First off, a mole is simply a grouping unit and a way to make counting and measuring small objects like atoms and ions A LOT easier. We already use this system in our daily lives. For example…

dozen eggs=12 eggs

score people=20 people

A mole is the same as a dozen or a score and represents 6.02 x 10²³ objects (which can be atoms, ions, molecules, or formula units). So, a mole of atoms contains 6.02x10²³ atoms just like a score of people means 20 people. This number is called the Avogadro constant and is considered a SI standard just like the kilogram, the meter, the second, etc. It was a convenient constant developed by Amadeo Avogadro so that a number of atoms (or other particles) would have a mass in grams equal to the mass of one atom (or another particle) in atomic mass units. This constant applies to all elements as equal numbers of different atoms always have the same mass ratio.

Now, one mole of particles has a mass in grams (g) equal to one particle in atomic mass units (u). So if one mole of a particle has a mass of 4.02 grams, then a single particle has a mass of 4.02 u. The mass of one mole of an object is called the molar mass of that object.

E.g. One mole of iron=55.847 g

One atom of iron=55.847 u

The last step in understanding moles is converting them. Keep in mind during the following examples that atoms/molecules/ions/formula units are interchangeable. So in the first example, atom can be replaced with molecule/ion/formula unit depending on the question or situation.

Atoms-Moles Conversions

If you had 48 eggs, how would you find how many dozens of eggs there were?

1. A dozen=12 eggs
2. There are 48 eggs
3. 48¸ 12=4
4. 48 eggs=4 dozens

The same concept can be applied to moles. If one had 6.02x10²⁴ atoms, how many moles of that atom are there?

1. A mole=6.02x10²³ atoms
2. There are 6.02x10²⁴ atoms
3. 6.02x10²⁴¸ 6.02x10²³=10 moles
4. 6.02x10²⁴ atoms=10 moles

On the other hand, if you wanted to convert the number of particles from mole, then do the opposite. Multiply 6.02x10²³ to the number of moles to get the number of particles.

Grams-Moles Conversions

If a mass of one egg was one gram, how many grams would a dozen eggs mass?

1. An egg=1 gram
2. You must find the mass of a dozen eggs
3. A dozen=12 eggs
4. 1 gram x 12 eggs=12 grams
5. A dozen=12 grams

Now if the mass of one molecule was 10 grams, how many grams would a mole of molecules mass?

1. A molecule=10 grams
2. You must find the mass of one mole of molecules
3. A mole=6.02x10²³ molecules
4. 10 grams x 6.02x10²³=6.02x10²⁴
5. A mole=6.02x10²⁴ grams

Volume of Gas-Moles Conversions

Plain and simple. Divide the volume of the gas at STP by 22.4dm³, the volume of one mole any gas at STP. When converting moles to volume of the gas at STP, simply multiply the number of moles by 22.4dm³.

Eg. What is the volume of 0.5 moles of carbon dioxide at STP?

0.5 x 22.4=11.2

The volume of carbon dioxide at STP is 11.2dm³or 10dm³ with appropriate usage of significant digits.

In actual mole conversion problems, you must find out the mass of a particle yourself. To do so, find the atomic mass of the element in question. For example, if you needed to convert Carbon, you would look for the atomic mass of C or 12.011. For ease of calculation, limit the mass number to 3 significant digits. So for all intents and purposes an atom of Carbon equals 12.0 grams. For the mass of compounds, simply add all the atomic masses of the atoms involved. So, water (H₂O) will have the following mass:

H=1.01 grams x2 (since there are 2 H atoms)=2.02 grams

O=16.0 grams

2.02+16.0=18.02 grams=18.0 grams

H₂O=18.0 grams

If you need practice, try these problems…

Mole Conversion Practice Problems