 
Avogadro’s number is the number of particles in one mole of any substance. It's equal to 6.022 × 1023 particles per mole. The mole is a unit for measuring large quantities of very small things, such as atoms, molecules, or other particles. One mole of a substance contains Avogadro’s number of particles.Number of atoms = Avogadro's number × number of moles Avogadro's number links microscopic and macroscopic worlds, so we can see how one affects the other. It let's us convert between a substance's mass and its number of atoms. Avogadro's Number 6.022 × 1023 is an incredibly large number. It's just a tenth in size of Earth's mass of Here's an example of how Avogadro's number is used. To calculate a mass of a single atom: 1. Determine the atomic mass of the element in grams per mole. This is the atomic mass for that element as given in the periodic table 2. Divide the mass by Avogadro's number The result is the mass of an atom in grams. Example: "Calculate the mass in grams of a single carbon atom" Solution: To calculate the mass of a single atom, first look up the atomic mass of carbon from the periodic table. This number, 12.01, is the mass in grams of one mole of carbon. One mole of carbon is 6.022 x 1023 atoms of carbon. This relation is then used to convert a carbon atom to grams by the ratio: mass of 1 atom = mass of a mole of atoms 6.022 x 1023 atoms/mole Plug in the atomic mass of Carbon to solve for the mass of 1 atom: Mass of 1 Carbon atom = 12.01 g = 1.994 x 10-23 g 6.022 x 1023 atoms The mass of a single carbon atom is 1.994 x 10-23 grams. The mass of a single atom is an extremely small number! This is why chemists use Avogadro's number. It makes working with atoms easier because we work with moles rather than individual atoms. For example, if you have one mole of water molecules, it contains approximately 6.022 x 1023 water molecules. More examples: "How many molecules are in 6 moles of methane?" Solution: To calculate this, multiply 6 moles by Avogadro's number: 6 × 6.022 × 10²³ = 3.61 × 10² molecules. Avogadro's Law: Equal volumes of any ideal gas contain the same number of moles, if measured at the same temperature and pressure. To find the number of molecules in a given mass of a substance, convert the mass to moles, and then use Avogadro's number to convert from moles to molecules. Example: "How many H2O molecules are in a snowflake weighing 1 mg?" Solution: First determine the mass of 1 mole of H2O: Snowflakes are made of water, or H2O. To obtain the mass of 1 mole of water, look up the atomic masses for hydrogen and oxygen from the Periodic Table. There are two hydrogen atoms and one oxygen for every H2O molecule, so the mass of one mole of H2O is: 2 x (mass of H) + mass of O = 2 (1.01 g) + 16.00 g = 2.02 g + 16.00 = 18.02 g So the mass of 1 mole of H2O molecules is 18.02 grams. Since 1 mole contains 6.022 x 1023 molecules, 1 gram must contain 6.022 x 1023 ÷ 18.02 = 3.34 x 1022 molecules. So a 1 mg snowflake would contain 3.34 x 1022 ÷ 1000 = 3.34 x 1019 molecules. Amedeo Avogadro was an Italian count and scientist, born in 1776. He received an education in law but decided to begin studying physics and math. When he was 35, he hypothesized that gases with equal volume, temperature, and pressure, would have the same number of molecules. This important revelation would form part of the ideal gas law equation, a crucial formula in thermodynamics. Avogadro's constant was named after him for his contributions to molecular theory. |