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S TOICHIOMETRY Chemistry 11 Ms. McGrath

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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.

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S TOICHIOMETRY L ET S RECALL A FEW IMPORTANT CONCEPTS Mass number: the number of protons plus neutrons in the nucleus of the atom. Atomic number: the unique number of protons in the nucleus of the atom of a particular element. Average Atomic Mass: the average of all the masses of all an elements isotopes based on their abundance. This is the mass seen on the periodic table. Molar mass: the mass of one mole of a substance, numerically equivalent to the elements average atomic mass; expressed in g/mol.

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S TOICHIOMETRY L ET S RECALL A FEW IMPORTANT CONCEPTS Isotopes: Atoms of the same element can 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, respectively). These carbon atoms are called Carbon-12, Carbon-13 and Carbon-14.

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S TOICHIOMETRY L ET S RECALL A FEW IMPORTANT CONCEPTS Calculating Average Atomic Mass (u) ex. Carbon Carbon-1298.9% abundance Carbon-13 1.1% abundance Carbon-14 1 x 10 -10 abundance We can determine that the average atomic mass is very close to 12.

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THE AVOGADRO CONSTANT AND THE MOLE 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

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THE AVOGADRO CONSTANT AND THE MOLE Mole (mol): the amount of substance that contains as many particles atoms, molecules or formula units as exactly 12 g of carbon-12. One mole (1 mol) of a substance contains 6.022 141 99 x 10 23 particles of the substance. The constant 6.022 141 99 x 10 23 mol -1 is called the Avogardo constant. Its symbol is N A.

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T HE A VOGADRO C ONSTANT AND T HE M OLE

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Converting Moles to Number of Particles The mole is used to help us count atoms and molecules. The relationship between moles, number of particles and the Avogadro constant is: N = n x N A where: N = number of particles n amount (mol) N A = Avogadro constant (mol -1 )

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You try: Page 51 and 52, numbers 4 - 11