Unit 3. Stoichiometry Upon successful completion of this unit, the students should be able to: 3.1 Define atomic mass and solve related problems. 1. Gallium has two naturally occurring isotopes, and gallium-70 is not one of them. Suggest what those two isotopes could reasonably be AND their relative abundances. Explain. 2. Define atomic mass. 3. How is the mass number for an atom different from the atomic mass of the element? 3.2 Solve calculations involving isotopic masses and abundances. 1. Calculate the atomic mass of lead given the following data. Show all work. isotope mass (amu) relative abundance (%) lead-204 203.97302 1.4 lead-206 205.97444 24.1 lead-207 206.97587 22.1 lead-208 207.97663 52.4 2. Complete the following table of isotopic information for the element neon. isotope mass (amu) relative abundance (%) neon-20 19.99? neon-21 20.99 0.257 neon-22 21.99? 3.3 Solve problems which demonstrate an understanding of the mole concept. 1. Complete the following statement. A dozen is to 12 as a mole is to. 2. Give the value for Avogadro s number. 3. How many moles of each element are there in 1.00 mol of Ca(NO 3 ) 2? 4. a. How many oranges are there in a mole of oranges? b. How many pennies are there in a mole of pennies? c. How many water molecules (H 2 O) are there in a mole of water? d. How many hydrogen atoms are there in a mole of water? 3.4 Calculate the formula mass of a compound. 1. Calculate the formula mass of the following compounds: a. NH 4 HCO 3 b. Al 2 (CO 3 ) 3 c. BCl 3 d. CuSO 4 5H 2 O
3.5 Explain and apply the relationship between atomic mass, formula mass, molecular mass, and molar mass. 1. If the atomic mass of cadmium is 112.411 amu, what is the molar mass of cadmium (include units)? 2. If the formula mass of Na 2 SO 4 is 142.04 amu, what is the molar mass of Na 2 SO 4 (include units)? 3. Give the molecular mass and molar mass (including units for both) of C 3 H 8. 4. Calculate the molar mass of the following: a. C 6 H 12 O 6 b. K 3 PO 4 c. CH 3 CH 2 OH d. Cu(NH 3 ) 4 8H 2 O 5. Match the term from the right column with the formula it might be associated with in the left column. You can use choices more than once. SrBr 2 SCl 2 C 5 H 12 O Fe Mg(NO 3 ) 2 molecular mass molar mass formula mass atomic mass 6. What is wrong with the following statements, if anything? If the statement is wrong, reword it to make it correct. a. The molecular mass of BaSO 4 is 233.39 amu/molecule. b. The molar mass of BaSO 4 is 233.39 g/mol. 3.6 Interconvert between the following as it applies to a particular substance: mass, moles, molecules, formula units, atoms, and ions. 1. A sample of glucose, C 6 H 12 O 6, contains 4.0 x 10 22 atoms of carbon. a. How many moles of glucose does it contain? b. How many grams of glucose does it contain? 2. You have 30.7 grams of water, how many molecules would this be? 3. A sample of DDT, C 14 H 9 Cl 5, weighs 61.9 grams. How many atoms of chlorine are in the sample? 4. What is the mass in grams of 3.67 mol of Mg(OH) 2? 5. How many moles of manganese (IV) oxide, MnO 2, are there in a 47.50 g sample? 6. Bauxite, the principle ore used in the production of aluminum cans, has a formula of Al 2 O 3 2H 2 O. a. Determine the molar mass of bauxite. b. How many grams of Al are there in 0.48 moles of bauxite? c. How many atoms of Al are there in 0.48 moles of bauxite? d. What is the mass in grams of 3.4 x 10 25 formula units of bauxite?
3.7 Calculate the percent composition of a compound from its formula. 1. Calculate the mass percent chlorine in each of the following: a. HClO 2 b. Ca(ClO 4 ) 2 2. Calculate the mass percent of each element in barium sulfite, BaSO 3. 3.8 Define empirical and molecular formulas, write and recognize examples of each, and calculate the empirical and molecular formula of a compound from quantitative analytical data. 1. Ferrocene, a substance proposed for use as a gasoline additive, has the percent composition 5.42% H, 64.56% C, and 30.02% Fe. What is the empirical formula of ferrocene? 2. What is the molecular formula of a compound that has an empirical formula of CH 2 Cl and a molar mass of 98.96 g/mol? 3. A mystery compound contains 43.64 % P and 56.36 % O. If the molar mass of this mystery compound is 283.88 g / mol, what is the empirical and molecular formula for this compound? 4. A compound containing only silicon and bromine was analyzed, and found to contain 91.9% bromine by mass and have a molar mass of 347.69 g/mol. Calculate the empirical and molecular formulas of the compound. 5. Which of the following is not an empirical formula? Write its empirical formula. a. CH 2 O b. KMnO 4 c. C 2 H 6 O 2 3.9 Differentiate between reactants and products of a chemical equation and utilize symbols for various states of matter (s, l, g, aq). 1. Identify the products of the following reaction which are dissolved in water. C 6 H 8 ONCl(s) + NaOH(aq) C 6 H 7 ON(s) + H 2 O(l) + NaCl(aq) 3.10 Balance chemical equations by inspection and solve related problems. 1. Balance the following equations. a. Au 2 S 3 (s) + H 2 (g) Au(s) + H 2 S(g) b. B 2 O 3 (s) + HF(l) BF 3 (g) + H 2 O(l) 2. Balance the following equations. a. Na 2 SiO 3 (s) + HF(aq) H 2 SiF 6 (aq) + NaF(aq) + H 2 O(l) b. H 2 + O 2 H 2 O c. Li 2 CO 3 (aq) + AgNO 3 (aq) Ag 2 CO 3 (s) + LiNO 3 (aq)
3. Write a balanced chemical equation (including the phases) based on the descriptions of the chemical reactions given below. a) Aqueous ammonium carbonate reacts with aqueous calcium chloride to form solid calcium carbonate and aqueous ammonium chloride. b) Nitrogen dioxide gas reacts with water to form aqueous nitric acid and nitrogen monoxide gas. 3.11 Solve standard stoichiometric calculations. 1. For the reaction C + 2S CS 2, how many mol of C will you need for complete reaction if you begin with 65.2 g S? 2. Kerosene, a mixture of hydrocarbons, is used in domestic heating and as a jet fuel. Assume that kerosene can be represented as C 14 H 30 and that it has a density of 0.763 g/ml. How many grams of water are produced by the combustion (unbalanced equation below) of 1.00 gal (3.785 L) of kerosene? C 14 H 30 + O 2 CO 2 + H 2 O 3. How many grams of potassium iodide are necessary to completely react with 20.61 g of mercury (II) chloride? HgCl 2 (aq) + 2KI(aq) HgI 2 (s) + 2KCl(aq) 4. The reaction between potassium chlorate and red phosphorus is highly exothermic and takes place when you strike a match with a matchbox. If you were to react 52.9 g of potassium chlorate with red phosphorus, how many grams of tetraphosphorus decaoxide would be produced? KClO 3 (s) + P 4 (s) P 4 O 10 (s) + KCl(s) (unbalanced) 3.12 Identify the limiting reactant of a chemical reaction and calculate theoretical yields based upon this information. 1. Consider the following reaction: CH 4 (g) + 4Cl 2 (g) CCl 4 (g) + 4HCl(g) If 1.20 mols of methane (CH 4 ) react with 1.60 mols of chlorine (Cl 2 ): a. What is the limiting reactant? b. How many grams of CCl 4 can be produced? c. How many grams of excess reactant will be left over? 2. Consider the following reaction in which 6.00 g of CS 2 gas react with 10.0 g of Cl 2 gas: CS 2 (g) + 3Cl 2 (g) CCl 4 (l) + S 2 Cl 2 (l) a. What is the theoretical yield of CCl 4 in grams? b. How many grams of excess reactant are left over?
3. When 10.0 g of octane (C 8 H 18 ) is burned in the presence of 25.0 g of oxygen, 7.36 g of H 2 O is produced (carbon dioxide is the other product). a) What is the excess reactant and how much is left? b) What is the percent yield of this reaction? 3.13 Solve problems involving the percentage yield of a reaction. 1. If the percent yield is 75% and 45 g of NO 2 are used in the following reaction, how many grams of nitric acid are produced? 3NO 2 (g) + H 2 O(l) 2HNO 3 (aq) + NO(g) Additional Unit 3 Sample Questions: 1. Aqueous strontium chloride reacts with aqueous lithium phosphate to form solid strontium phosphate and aqueous lithium chloride. Using this equation, answer the following: a. If 94.2 grams of strontium chloride reacts with 61.7 grams of lithium phosphate, how many grams of strontium phosphate could be formed? b. What is the limiting reactant in this chemical reaction? c. When you conduct the above chemical reaction, 83.5 grams of strontium phosphate is actually produced. What would be the percent yield for this reaction?