STC_The Mole Lesson

The Mole

Basketball Planet imageImagine trying to complete an experiment that called for 10 molecules of water. There is no common device that could make that measurement, and it would be impossible to see the molecules to actually count them out. In chemistry, a quantity called the mole is used to measure molecules. A mole refers to the number of items that are being counted. Much like a dozen is just a quantity, 12, a mole is just a quantity, 6.02 x 1023. A mole of basketballs is 6.02 x 1023 basketballs. A mole of pencils is 6.02 x 1023 pencils. A mole of iron atoms is 6.02 x 1023 atoms. A mole of water molecules is 6.02 x 1023 molecules. A mole of chemical compounds or elements is a measurable amount. It is based on the exact number of atoms that are present in exactly 12 g of carbon-12. This number, Avogadro's number, is named in honor of Amadeo Avogadro, an Italian scientist that lived from 1776 - 1856. Although the true value of Avogadro's number is impossible to know, the experimental value is 6.0221367 x 1023. Because atoms are so small, a mole of carbon-12 has a mass of just 12 grams. But consider this:

  • A mole of dollar bills is enough to give each person on Earth over $3,000,000 per second, for 100 years.
  • A mole of sheets of paper would stack beyond our solar system.
  • A mole of basketballs would be larger than the size of planet Earth.
  • A mole of rice grains would cover every land mass on our planet a depth of 75 meters.

Depending on what type of substance we are talking about in chemistry, the unit of mole has different interpretations. If we are talking about an element, a mole counts atoms. If we are referring to a molecular compound, a mole counts the number of molecules. Last, if we are referring to an ionic compound, a mole counts the number of formula units.

CONVERSION 1 mol = 6.02 x 1023 atoms 1 mol = 6.02 x 1023 molecules 1 mol = 6.02 x 1023 formula units (1 mol = 6.02 x 1023 donuts!)

To do these problems we will use the dimensional analysis method we learned on the previous page. Watch this to see how.

When you do a math problem, you should always think about the "reasonableness" of your answer. In other words, does this answer make sense? Did I expect a big or small number?

For example, if you calculate the number of atoms or molecules in a sample, you would normally expect to get a very large number because atoms and molecules are so very small. So, always think about what kind of answer you expect before you do your calculation!

Mole Ratio

The mole can also be used to relate the amounts of chemicals that are in a compound or molecule.

Let's look at a water molecule as an example.

In H₂O, the subscripts indicate that there are 2 hydrogen atoms and 1 oxygen atom in each molecule of water. This can be written in fact form, like this: 1 molecule of H₂O = 1 atom H = 2 atoms O The 2:1 ratio of hydrogen to oxygen atoms can also be carried over to dozens, hundreds, or thousands. There are 2,000 H atoms to every 1,000 O atoms. We can also say that there are 2 moles of H atoms to every 1 mole of O atoms. This relationship between the elements is known as molar ratio. Or, each relationship can be written as a conversion factor, like this: 1 molecule H20 / 2 atom H 2 atom H / 1 atoms O

We will learn more about mole ratios when we learn how to balance equations.

The Mole and Mass

So, what if an experiment requires a certain amount of a chemical? How do you measure this? You can't count out individual atoms by hand. You can't even count by the mole. So, how do we measure chemicals? We often measure them using grams and then can convert between moles and grams using the factor label method.

The next question you should be asking yourself is, what is the relationship between moles and grams (mass)? We use the atomic mass found on the periodic table. One mole of any element (or compound or molecule) has a mass that is equal to the atomic mass in grams. This is called molar mass.

Example: Look on the periodic table find these molar masses. Molar mass of carbon is 12.011 grams. The molar mass of iron is ___ grams

As we did with Avogadro's Number, let's list this fact. You should keep your own list of facts and formulas for each unit.

MolarMass.png

We can use the periodic table to determine the molar mass of compounds and molecules as well. When we are talking about a compound, we can refer to this as formula mass. When referring to a molecule, we can call this molecular mass. It is also still appropriate to refer to these as molar mass. To find the molar mass of a compound, just add the atomic masses of each element, remembering to multiply if there is a subscript. If you have a polyatomic ion, you will need to distribute any of the subscripts outside of the parenthesis.

Molar mass is another conversion fact you will use to do dimensional analysis problems where you need to convert from grams to moles or from moles to grams. Watch this to see how.

Remember to work on the module practice problems as you complete each section of content.

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