CHEMISTRY - BROWN 13E CH.3 - STOICHIOMETRY: CALCULATIONS WITH CHEMICAL FORMULAS & EQUATIONS

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Walter Wood
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2 ONEPT: EMPIRIAL FORMULA The empirical formula is also known as the. It represents the ratio of moles of each element in the compound. The molecular formula is also known as the. 6 H 12 O 6 = It represents the ratio of moles of each element in the compound. EXAMPLE: What is the empirical formula of dimethylhydrazine, 2H8N2, a colorless liquid used as a rocket fuel? EXAMPLE: Elemental analysis of a sample of an ionic compound showed 2.82 g of Na, 4.35 g of l, and 7.83 g of O. What is the empirical formula and name of the compound? EXAMPLE: After a workout session, lactic acid (M = g/mol) forms in muscle tissue and is responsible for muscle soreness. Elemental analysis shows that this compound contains 40%, 6.7% H and 53.3% O. Determine the molecular formula. Page 2

3 ONEPT: OMBUSTION ANALYSIS Under a combustion reaction a compound made of or reacts with. The products formed will be and. EXAMPLE: A g sample contains carbon, hydrogen and oxygen and undergoes complete combustion to produce g of O2 and g of H2O. What is the empirical formula of the compound? Page 3

ONEPT: HEMIAL OMPOSITION The mass per mole of an element is called its (M). The mass per mole of a compound is called its (M). They both have the units of. 1. Elements.

4 ONEPT: HEMIAL OMPOSITION The mass per mole of an element is called its (M). The mass per mole of a compound is called its (M). They both have the units of. 1. Elements. To find mass of an element just look up its atomic mass in the periodic table. EXAMPLE: What is the total mass of each of the following elements? a. Sodium b. Gold c. Mercury 2. ompounds. The mass of a compound is the sum of the individual masses of the elements in the chemical formula. EXAMPLE: What is the total mass of each of the following compounds? a. N2O5 b. 12H22O11 c. (NH4)3PO4 Page 4

5 ONEPT: MASS PERENT Mass or weight percent is the percentage of a given element in a compound. Mass Percent (%) = (Massomponent) 100 (TotalMass) EXAMPLE: What is the percentage of carbon in sodium hydrogen carbonate, NaHO3? EXAMPLE: A sample of toothpaste contains tin (II), SnF2. Analysis of a 5.25 g sample contains 8.77 x 10-3 g of F. What is the percentage of tin (II) fluoride in the sample? PRATIE: Hemoglobin contains 0.33% iron and has a molecular weight of 68 kg. How many iron atoms are in each molecule of hemoglobin? Page 5

6 ONEPT: BALANING HEMIAL REATIONS When balancing an equation always make sure the and of atoms on both sides of the arrow are equal. EXAMPLE: Write balanced equations for each of the following by inserting the correct coefficients in the blanks: a. Al (s) + l2 (g) All3 (s) b. Ba3(PO4)2 (s) + KOH (aq) K3PO4 (aq) + Ba(OH)2 (aq) c. 4H10 (aq) + O2 (g) O2 (g) + H2O (l) Page 6

7 ONEPT: STOIHIOMETRI REATIONS 2 H 2 (g) + 1 O 2 (g) 2 H 2 O (g) In the above equation the numbers that are in bold are called. They tell us the number of of each compound that reacts. This numerical relationship between compounds in a balanced equation is called. STOIHIOMETRI HART Before we get into solving stoichiometric reactions lets work out a plan of attack. Entities means, or. Entities of Given Entities of Unknown Moles of Given Moles of Unknown Grams of Given Grams of Unknown Use this chart when given a chemical equation with the quantity of a compound or element and asked to find the quantity of another compound or element. 2 H EXAMPLE: How many grams of H2O are produced when 12.3 g H2 reacts? 2 (g) + 1 O 2 (g) 2 H 2 O (g) Page 7

8 PRATIE: STOIHIOMETRI REATIONS EXAMPLE 1: The oxidation of chromium solid is represented by the following equation: 4 r (s) + 3 O2 (g) 2 r2o3 (s) a. How many moles of chromium (III) oxide are produced when g r reacts with excess oxygen gas? b. How many grams of O2 were needed to produce 4.28 x 10 3 molecules r2o3? EXAMPLE 2: If the density of ethanol, H3H2OH, is g/ml, how many milliliters of ethanol are needed to produce 4.8 g of H2O in the following reaction? H3H2OH (l) + 3 O2 2 O2 (g) + 3 H2O (l) PRATIE: Dinitrogen monoxide gas decomposes to form nitrogen gas and oxygen gas. How many molecules of oxygen are formed when 8.00 g of dinitrogen monoxide decomposes? Page 8

9 ONEPT: LIMITING REAGENT In a chemical reaction the reactant that is consumed when a reaction occurs and determines the maximum amount of product formed is called the. The amount of product it forms is called the yield. The reactant that remains after the completion of the chemical reaction is called the reactant. EXAMPLE: hromium (III) oxide reacts with hydrogen sulfide (H2S) gas to form chromium (III) sulfide and water: r2o3 (s) + 3 H2S (g) r2s3 (s) + 3 H2O (l) [Balanced] a. What is the mass of chromium (III) sulfide formed when g r2o3 reacts with g H2S? b. Identify the limiting reactant, excess reactant and theoretical yield. c. What mass of excess reactant remains? Page 9

10 ONEPT: PERENT YIELD The percent yield of a reaction is used to determine how effective the chemist was in creating their desired products. A high percent yield would signify that the reaction is. EXAMPLE 1: A scientist performs an experiment in the laboratory and obtains g r2s3. If his calculations on scratch paper give him a theoretical yield of g what is the percent yield? PRATIE: onsider the following balanced chemical reaction: 2 6H6 (l) + 15 O2 (g) 12 O2 (g) + 6 H2O (l) a. If a 2.6 g sample of 6H6 reacted with excess O2 to produce 1.25 g of water, what is the percent yield of water? b. If the above reaction only went to 75% completion, how many moles of O2 would be produced if 1.57 x 10-5 molecules of 6H6 were reacted with excess oxygen? Page 10

11 ONEPT: GROUP REATIVITY The central principle of Organic hemistry is based on the. The reactivity of an organic compound is all based on which type is present. H 3 OH H 3 H 3 O H 2 H 2 H 2 H 2 H 3 Tetrahydrocannabinol (TH) Alkane Alkene Alkyne Alcohol Amine Aldehyde Ketone arboxylic Acid Ester Page 11

12 ONEPT: GROUP REATIVITY (PRATIE) EXAMPLE: In each of the following molecules, identify the type(s) of functional groups present. a. b. O OH PRATIE: In each of the following molecules, identify the type(s) of functional groups present. a. b. O O Page 12

CHEMISTRY - MCMURRY 7E CH.3 - MASS RELATIONSHIPS IN CHEMICAL REACTIONS

CHEMISTRY - MCMURRY 7E CH.3 - MASS RELATIONSHIPS IN CHEMICAL REACTIONS !! www.clutchprep.com ONEPT: ALLOYS An alloy represents a mixture composed of two or more elements in which at least one of the elements is a metal. The predominant metal component, which is can be up