Quantities in Chemical Reactions

The Mole

Did you know?

• Count Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e di Cerreto (1776-1856) was an Italian chemist, most noted for his contributions to the theory of molarity and molecular weight. He is commonly referred to as simply Amedeo Avogadro.

• In 1811, he proposed what is now known as Avogadro's law and Avogadro's number. He made a critical contribution (recognized only in 1860 after his death) which helped greatly with the measurement of atomic weights.

Counting in Chemistry

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

• However, it is not possible, or practical, to work with individual atoms in the lab, since they are too small and you wouldn't be able to see the results.

• It is necessary to work with large enough samples for you to see and weigh on a balance using units of grams.

• A pile of atoms big enough for you to see contains billions of atoms. It can be difficult to keep track of such large number in calculations, so chemists made up a new unit for it.

Avogadro's Number

• In 1811, Amedeo Avogadro published a hypothesis on the volume of gases, stating that equal volumes of gases, at the same temperature and pressure, contain the same number of molecules regardless of their chemical nature and physical properties.

• This number is so important that it has a name. It was later called Avogadro's number and has the symbol N.

• The value of Avogadro’s number was precisely determined to be 6.022 x 1023.

This is a very large number (602 200 000 000 000 000 000 000) and has no unit.

The Mole • How many of something is in a “dozen”? What is a “gross”? A

pair? These are common groupings that correspond to specific values. The same thing occurs in chemistry. You can say there is an Avagadro’s number of atoms in a “mole” of atoms.

• Here is how the International Union of Pure and Applied Chemistry (IUPAC) defines the mole: – “The mole is the amount of substance of a system which contains as

many elementary entities as there are atoms in 0.012 kilogram of carbon-12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.”

• In other words, one mole contains as many entities as there are in 12 g of carbon-12 (or 0.012 kg). There are 6.022 x 1023 atoms in 12 g of carbon-12.

The Mole (con’t)

• The mole is abbreviated as “mol”• Mol is the SI base unit for measuring how much of a

substance is present.

• One mole of ANY specified entity contains 6.022 x 1023 of that entity:– One mole of donuts contains 6.022 x 1023 donuts – One mole of H2O contains 6.022 x 1023 molecules – One mole of nails contains 6.022 x 1023 nails – One mole of Fe contains 6.022 x 1023 atoms

Example 1

• How many atoms are in 0.5 mol of sodium?

1 mole of anything corresponds to 6.02 x 1023

0.5 mol of sodium corresponds to x atoms of sodium

• There are 3.01 x 1023 atoms in 0.5 mol of sodium.

– This solution uses ratio reasoning. A similar answer could be obtained by multiplying the 0.5 mol of sodium by the unit factor [6.02 x 1023 atoms/mol].

Example 2

Check Your Understanding

1. How many atoms are then in one molecule of Al(NO3)3?a) 3b) 5c) 11d) 13

2. How many atoms are there in 2 mol of iron?

3. How many moles of tungsten correspond to 5.05 x 1020 atoms?