S TOICHIOMETRY Chemistry 11 Ms. McGrath

S TOICHIOMETRY The study of the quantities of reactants and products in a chemical reactions. ex. NaHCO 3 (s) + CH 3 COOH(aq)  NaCH 3 COO(aq) + H 2 O(l) + CO 2 (g) According to this reaction, one formula unit of baking soda reacts with one formula unit of acetic acid to form a salt, water and carbon dioxide. If we wanted to carry out this reaction, how would we know the amount of baking soda and vinegar to use? The particles are too small and numerous to be counted.

S TOICHIOMETRY L ET ’ S RECALL A FEW IMPORTANT CONCEPTS Isotopes: An element’s atom have different number of neutrons. ex. Carbon can exist with 6, 7, and 8 neutrons, therefore they have varying atomic masses (12, 13 and 14). These carbon atoms are called Carbon-12, Carbon-13 and Carbon-14.

S TOICHIOMETRY L ET ’ S RECALL A FEW IMPORTANT CONCEPTS Average Atomic Mass (u): is the average of all the masses of all the element’s isotopes. We have to take into account: the abundance of each isotope the mass of each isotope This is the mass that is seen on the periodic table.

S TOICHIOMETRY L ET ’ S RECALL A FEW IMPORTANT CONCEPTS Calculating Average Atomic Mass (u) ex. Carbon Carbon % abundance Carbon % abundance Carbon-14 1 x abundance We can determine that the average atomic mass is very close to 12.

A VERAGE A TOMIC M ASS ( U ) P RACTICE Page 45 #1, 2, 3, and 4

S ECTION R EVIEW Page 46 #1, 2, 3 and 4

T HE A VOGADRO C ONSTANT AND T HE M OLE We will learn how chemists group large numbers of atoms into amounts that are easily measured. Certain items, because of their size, are often handled in bulk. Some common quantities: ItemQuantityAmount glovespair2 soft drinkssix-pack6 eggsdozen12 pensgross (12 dozen)144 paperream500

T HE A VOGADRO C ONSTANT AND T HE M OLE Easily measurable amounts of elements contain huge numbers of atoms. Chemists use a quantity that is much larger than a dozen or ream to group atoms or molecules together. This quantity is the mole (symbol mol ). A mole = 6.02 x (Avogadro’s number) It has been determined that there are 6.02 x carbon atoms in a g sample of carbon-12.

T HE A VOGADRO C ONSTANT AND T HE M OLE

Consider the following balanced chemical reaction: Fe( s ) + S( s )  FeS( s ) The above reactants combine in a 1 : 1 ratio. One 56 u atom of Iron reacts with one 32 u atom of Sulfur to yield one 88 u formula unit of Iron Sulfide.

T HE A VOGADRO C ONSTANT AND T HE M OLE How big is a mole?