Molarity. How do you define it?

[gregcropper]

what is the definition of molarity in lay-mans language? What is, say a mole of NaCl?

[Jumbuck]

Is this ok http://www.infoplease.com/ce6/sci/A0833619.html ?

[theprof]

You cannot define molarity in terms of lay-mans language; you either express it in scientific terms or not at all.

Molarity may be defined as the concentration of a solution expressed as the number of moles of solute per litre of solution.

In simple terms, one mole of a substance may be defined as equal to the atomic mass of an element in grams. If you check the Periodic Table, you will see that the atomic mass of sodium (Na) is 22.99 grams. Likewise, the atomic mass of chlorine (Cl) is 35.45 grams. Therefore, the atomic mass of sodium chloride (NaCl) is 22.99 + 35.45 = 58.44 grams.

[theprof]

One mole of sodium chloride is therefore 58.44 grams of the compound.

[jake-the-peg]

Perhaps the best way to understand it is to consider why we need it.

You might want to react Sodium with Chlorine to make sodium chloride or salt.

Now you know that one atom of Sodium will react with one atom of Chlorine but that doesn't mean that one gram of each will react with nothing left over. There are more atoms in one gram of something than of something else.

The relative atomic mass of an element gives you a measure of the relative number of atoms so if we take 22.99 grams of sodium and 35.45 grams of chlorine we know there are the same number of atoms in each.

This is hugely usefull in Chemistry so we give it a special name - we say that the relative atomic mass ( which is kind of just a measure of how dense an atom is compared to another ) of something expressed in grams is a mole.

The concentration of a solution is usually the number of grams in a fixed volume but often we're more interested in the relative number of atoms so we talk about the number of moles

If you're interested in how many atoms are in a mole it's 6.0221415×10²³ (Avagadro's number)

(A mole can also apply to molecules you just add up the combined mass)