Thursday, April 28, 2011

Stoichiometry Note #1: The Mole and the Avogadro Constant

Next: Converting Moles to Molecules

There are tons of different units that we use to measure quantity, and they all allow us to communicate more easily when talking about certain things. For example, a dozen eggs means twelve eggs, and it will be much easier to say "twelve dozen eggs" than to say "one hundred and forty-four eggs." In short, by using these units, we can often make numbers a lot more simple.

One unit that is often used in chemistry is the mole. It is very much like the quantities"dozen" and "pair," in the way that it also refers a specific number. However, it refers to a seemingly strange and unusually huge number— 6.0221479×1023 .

This number is called the Avogadro constant, and has a symbol of NA. 

So how is the Avogadro constant determined? It is very simple—it is the number of molecules in exactly twelve grams of carbon-12 and it is obtained by conducting experiments. This means that as time progresses and our technology improves, we may be able to measure this number more accurately and the accept value for the Avogadro constant may continue to change over the years. 

Also, to avoid confusion, even though the value for mole is obtained by counting the number of molecules in 12 grams of carbon-12, the mole is a unit or a quantity that can be used for all types of molecules. For example, one mole of H2O will contain 6.0221479×1023  molecules of H2O and one mole of silver will also contain  6.0221479×1023 silver atoms.

Why are the mole and the Avogadro constant important? They are important because they give a standard to measuring the amount of molecules. As mentioned in the very first paragraph, they make it easier for people to work with numbers. The mole is especially necessary because molecules are extremely small and when dealing with molecules, the number will almost always be very huge. Just imagine, would you want to keep saying numbers like 1.2245 × 1093 or 7.530000002 1062 when talking about the number of molecules? Probably not, as this can make calculations very complicated. Instead, we can just say one mole, two moles, or 5.12 moles, which are clearly a lot easier. 

Overall, the mole refers to a specific quantity of molecules, and this number is 6.0221479×1023. The mole is very useful in chemistry (and physics), and the different uses of the mole will be explained in future post(s). 


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