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Moles

Moles, molar mass and molar concentrations are an extremely simply idea which cause a lot of confusion to the beginning chemistry student.

The idea of a mole is really no different than the idea of a dozen.

It is simply a number, usually a number of molecules. As it happens a very BIG number of molecules but still just a counting of molecules.

If you wanted a dozen dinner rolls you would simply count them out one at a time until you had counted out 12 single rolls.

If you wanted a mole of dinner rolls you would spend several life times counting them out one at a time, similarly if you wanted a mole of NaCl molecules you would never be able to count out a mole of them.

What we do instead is weigh the NaCl in grams and then divide by what we know to be the weight of one mole of NaCl molecules. This gives us a count of NaCl molecules expressed in moles, rather than in dozens.

But how do we know what a mole of NaCl molecules weigh?

Avagadro's number (i.e. a mole) was picked in a way that makes it true, that if a single atom or molecule weighes the equivalent of exactly "x" hydrogen atoms, then a mole of those same atoms or molecules, will weigh exactly "x" grams.

This way we can calculate an atomic or molecular weight using the periodic table and use the number we get that way, to also identify the weight in grams of one mole of those same atoms or molecules in bulk form.

In other words:
the concept of a mole gives us a way to establish a numerical equivalency between the weight of a single molecule expressed in multiples of the mass of a hydrogen atom, with a weight of a large but well defined number of those same molecules when expressed in grams.


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