Part 6 – Moles and Counting Atoms

Measuring Matter When we talk about “how much” of something we have we use three different ways of measuring: –Counting- the number of students in a class, or the number of bowling pins struck down, etc. –Weighing (or massing)- purchase potatoes by the pound –Volume- a gallon of gasoline, or 3L bottle of soda.

Chemists also measure substances in these three areas and convert between each type.

COUNTING IN CHEMISTRY Chemists are very interested in counting atoms, formula units and molecules of substances that participate in chemical reactions.

Representative particles In most cases a representative particle is “one” of something. “Representative particle”refers to a substance whether it commonly exists as atoms, ions, or molecules. It’s a “catch all” phrase.

The Mole This is how chemists “count” the number of representative particles in a certain amount of a substance. A mole is a grouping of representative elements.

How many of something is in a “dozen”? What about a “gross”? These are common everyday groupings… Same thing occurs in chemistry. There are an Avagadro’s number of atoms in a “mole” of atoms.

Avagadro’s Number Amedeo Avagadro would publish a hypothesis on the volume of gases, stating that equal volumes had equal numbers of atoms A German scientist would determine the number.

1 mole of any substance contains 6.02 X representative particles. This is known as Avagadro’s Number.

Using Avagadro's Number If we want the number of atoms of an element, or ion, within a compound, we use Avagadro’s number and the compounds formula.

Example Problem How many Na atoms are in 0.5 moles of sodium? How many moles of magnesium is 3.01 X 1022 atoms of magnesium?

Your try these… How many Fl atoms are in 2.0 mol Fe? How many moles of Fe is 5.05 X atoms of iron?