CC 102: QUALITATIVE AND QUANTITATIVE CHEMISTRY College of Liberal Arts, Sciences and Education Prepared by: ERWIN R. ABRENCILLO, PhD DOHN ARTURO M. TABERNILLA MODULE 6 STOICHIOMETRY PART II Brief Introduction or Description This module discusses the continuation of the previous module. This includes the mole ration concept, mass to mass stoichiometry, and mole to mass stoichiometry. Learning Outcomes: By the end of the module, you should be able to: 1. Calculate the mass of a product involve in chemical reaction from a given mass of reactants. 2. Determine the mole of a product involve in a chemical reaction from a given move of reactants. 3. Solve the mass of a product involve in a chemical reaction from a given mole and vice versa. Lesson 1 – Mole Ratios Stoichiometry problems can be characterized by two things: (1) the information given in the problem, and (2) the information that is to be solved for, referred to as the unknown. The given and the unknown may both be reactants, both be products, or one may be a reactant while the other is a product. The amounts of the substances can be expressed in moles. However, in a laboratory situation, it is common to determine the amount of a substance by finding its mass in grams. The amount of a gaseous substance may be expressed by its volume. Figure 1. Mole ratio relationship. Page 1 of 8 CC 102: QUALITATIVE AND QUANTITATIVE CHEMISTRY College of Liberal Arts, Sciences and Education Prepared by: ERWIN R. ABRENCILLO, PhD DOHN ARTURO M. TABERNILLA Chemical equations express the amounts of reactants and products in a reaction. The coefficients of a balanced equation can represent either the number of molecules or the number of moles of each substance. The production of ammonia (NH3) from nitrogen and hydrogen gases is an important industrial reaction called the Haber process, after German chemist Fritz Haber. N2(g)+3H2(g)→2NH3 The balanced equation can be analyzed in several ways, as shown in the figure below. Figure 2. This representation of the production of ammonia from nitrogen and hydrogen shows several ways to interpret the quantitative information of a chemical reaction. We see that 1 molecule of nitrogen reacts with 3 molecules of hydrogen to form 2 molecules of ammonia. This is the smallest possible relative amount of the reactants and products. To consider larger relative amounts, each coefficient can be multiplied by the same number. For example, 10 molecules of nitrogen would react with 30 molecules of hydrogen to produce 20 molecules of ammonia. The most useful quantity for counting particles is the mole. So if each coefficient is multiplied by a mole, the balanced chemical equation tells us that 1 mole of nitrogen reacts with 3 moles of hydrogen to produce 2 moles of ammonia. This is the conventional way to interpret any balanced chemical equation. Finally, if each mole quantity is converted to grams by using the molar mass, we can see that the law of conservation of mass is followed. 1mol1mol of nitrogen has a mass of 28.02 g while 3 mol of hydrogen has a mass of 6.06 g, and 2 mol of ammonia has a mass of 34.08 g. 28.02 g N2 + 6.06 g H2→ 34.08 g NH3 Mass and the number of atoms must be conserved in any chemical reaction. The number of molecules is not necessarily conserved. Page 2 of 8 CC 102: QUALITATIVE AND QUANTITATIVE CHEMISTRY College of Liberal Arts, Sciences and Education Prepared by: ERWIN R. ABRENCILLO, PhD DOHN ARTURO M. TABERNILLA A mole ratio is a conversion factor that relates the amounts in moles of any two substances in a chemical reaction. The numbers in a conversion factor come from the coefficients of the balanced chemical equation. The following six mole ratios can be written for the ammonia forming reaction above. In a mole ratio problem, the given substance, expressed in moles, is written first. The appropriate conversion factor is chosen in order to convert from moles of the given substance to moles of the unknown. Example No. 1 Page 3 of 8 CC 102: QUALITATIVE AND QUANTITATIVE CHEMISTRY College of Liberal Arts, Sciences and Education Prepared by: ERWIN R. ABRENCILLO, PhD DOHN ARTURO M. TABERNILLA Lesson 2: Mass-Mass Stoichiometry Mass-mass calculations are the most practical of all mass-based stoichiometry problems. Moles cannot be measured directly, while the mass of any substance can generally be easily measured in the lab. This type of problem is three steps, and is a combination of the two previous types. mass of given→ moles of given→ moles of unknown→ mass of unknown The mass of the given substance is converted into moles by use of the molar mass of that substance from the periodic table. Then, the moles of the given substance are converted into moles of the unknown by using the mole ratio from the balanced chemical equation. Finally, the moles of the unknown are converted to mass by use of its molar mass. Example No. 2 Page 4 of 8 CC 102: QUALITATIVE AND QUANTITATIVE CHEMISTRY College of Liberal Arts, Sciences and Education Prepared by: ERWIN R. ABRENCILLO, PhD DOHN ARTURO M. TABERNILLA Lesson 3: Mass-Mole and Mole-Mass Stoichiometry Mass to Moles Problems In this type of problem, the mass of one substance is given, usually in grams. From this, you are to determine the amount in moles of another substance that will either react with or be produced from the given substance. mass of given→ moles of given→ moles of unknown The mass of the given substance is converted into moles by use of the molar mass of that substance from the periodic table. Then, the moles of the given substance are converted into moles of the unknown by using the mole ratio from the balanced chemical equation. Page 5 of 8 CC 102: QUALITATIVE AND QUANTITATIVE CHEMISTRY College of Liberal Arts, Sciences and Education Prepared by: ERWIN R. ABRENCILLO, PhD DOHN ARTURO M. TABERNILLA Page 6 of 8 CC 102: QUALITATIVE AND QUANTITATIVE CHEMISTRY College of Liberal Arts, Sciences and Education Prepared by: ERWIN R. ABRENCILLO, PhD DOHN ARTURO M. TABERNILLA Moles to Mass Problems In this type of problem, the amount of one substance is given in moles. From this, you are to determine the mass of another substance that will either react with or be produced from the given substance. moles of given→ moles of unknown→ mass of unknown The moles of the given substance are first converted into moles of the unknown by using the mole ratio from the balanced chemical equation. Then, the moles of the unknown are converted into mass in grams by use of the molar mass of that substance from the periodic table. Page 7 of 8 CC 102: QUALITATIVE AND QUANTITATIVE CHEMISTRY College of Liberal Arts, Sciences and Education Prepared by: ERWIN R. ABRENCILLO, PhD DOHN ARTURO M. TABERNILLA Learning Tasks: Answer the following problems: Mole-Mole Stoichiometry: 1) a. b. c. N2 + H2 NH3 Write the balance chemical equation. How many moles of hydrogen are needed to react with seven moles of Nitrogen? How many moles of NH3 are produced from 9 moles of hydrogen? 2) a. b. c. KClO3 KCl + O2 Write the balance chemical equation. How many moles of oxygen are produced by the decomposition of six moles of KClO 3? How many moles of oxygen are produced when 0.5 moles of KCl are produced? Mass-Mass Stoichiometry: 1) LiOH + HBr LiBr + H2O a. Write the balance chemical equation. b. If you start with 10.0 grams of lithium hydroxide, how many grams of lithium bromide will be produced? 2) C2H4 + O2 CO2 + H2O a. Write the balance chemical equation. b. If you start with 45 grams of ethylene, how many grams of carbon dioxide will be produced? 3) Mg + NaF MgF2 + Na a. Write the balance chemical equation. b. If you start with 5.5 grams of sodium fluoride, how many grams of magnesium fluoride will be produced? Mole-Mass Stoichiometry: 1) Mg + HCl MgCl2 + H2 a. Write the balance chemical equation. b. How many grams of HCl are consumed by the reaction of 2.50 moles of magnesium? c. What is the mass in grams of H2 gas when 4.0 moles of HCl is added to the reaction? 2) CaC2 + H2O C2H2 + Ca(OH)2 a. Write the balance chemical equation. b. If 3.20 moles of CaC2 are consumed in this reaction, how many grams of H2O are needed? c. How many grams of Ca(OH)2 would be formed with 3.20 moles of CaC2? References: Harvey, D V. (2010). Modern analytical chemistry. De Pauw University West, D.M. (2015) Fundamentals of analytical chemistry 9th Edition Page 8 of 8
