Mole – EP – 1

A mole is a unit in chemistry used to count collections of extremely small particles, such as atoms, molecules, ions, or electrons. One mole contains exactly 6.02 x 10^23 particles, a quantity known as Avogadro's number.

Everyday units compared to a mole

• A pair refers to a collection of two items, like slippers.
• A dozen means a group of twelve items, like eggs.
• A ream is a bundle of five hundred sheets of paper.
• A mole is a collection of 6.02 x 10^23 particles, taking its name from the Latin word for a heap of soil.

How Avogadro's number was found

Avogadro discovered that equal volumes of any gas held at the same temperature and pressure always contain an identical number of atoms or molecules. The scientist Perrin later calculated this count and named it Avogadro's number to honor him. Later scientists refined the value to 6.02 x 10^23, a quantity so large that stacking that many paper sheets would stretch between the Earth and the Moon eighty million times.

Counting particles using mass

Just as the number of bricks in a load can be calculated by dividing total weight by the mass of one brick, atoms are counted using their mass. Scientists agreed to take the atomic mass value from the periodic table and measure that amount in grams. When 12 grams of carbon-12 is divided by the mass of a single carbon-12 atom, the result is 6.02 x 10^23 atoms.

Understanding molar mass

• Molar mass is the mass in grams equal to the atomic or molecular mass shown on the periodic table.
• Taking four grams of helium, its atomic mass, yields 6.02 x 10^23 helium atoms.
• For molecular oxygen, taking two times 15.99 grams yields 6.02 x 10^23 oxygen molecules.
• This rule works for every element and compound, including water and sodium chloride, allowing chemists to convert between mass and particle count.