Unit II – Quantities in Chemical Reactions

Question #1 What is a ‘dozen’ ?

Question #2 How do you know?

Question #3 What is a mole?

Is it this? Or this? Or this?

The Mole (for chemists) The mole is a unit of measurement used by chemists. It provides a method to ‘level the playing field’ and treat all matter equally One mole of a substance (an element, a molecule, an ion or anything else) is made from Avogadro’s number of particles

Rules Utilized With The Mole I. The chemical formula represents a mole of that substance. ( HCl would mean 1 mole of HCl) II. The formula mass, expressed in grams, represents the mass of one mole of that substance. III. One mole of any substance contains 6.02 x 10 23 particles. IV. One mole of any gas, at STP conditions, occupies 22.4 liters of volume.

History of the Mole The number of objects in one mole, that is, 6.02 x 1023 , is commonly referred to as Avogadro's number. Amadeo Avogadro was an Italian physics professor who proposed in 1811 that equal volumes of different gases at the same temperature contain equal numbers of molecules. About fifty years later, an Italian scientist named Stanislao Cannizzaro used Avogadro's hypothesis to develop a set of atomic weights for the known elements by comparing the masses of equal volumes of gas. Building on this work, an Austrian high school teacher named Josef Loschmidt calculated the size of a molecule of air in 1865, and thus developed an estimate for the number of molecules in a given volume of air. While these early estimates have since been refined, they led to the concept of the mole - that is, the theory that in a defined mass of an element (its atomic weight) there is a precise number of atoms: Avogadro's number.

How do we use the Mole? Here ‘s a problem that we’ll work on solving. When you add 10.0 grams of hydrogen gas to 10.0 grams of oxygen gas, how many grams of water will be made? The answer: 11.25 grams