# Daily Practice Review 2/28-29/08

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2 Daily Practice Review 2/28-29/08 1. Why is it not correct to balance an equation by changing the subscripts in one or more of the formulas? If you change the subscripts in a formula you change the chemical identity of the substance. Ex. You make it something else. 2. What does STP stand for? Standard Temperature and Pressure 0 o C and 101.3kPa 3. What is the molar volume of any gas at STP? How many particles does it contain? 22.4 L/mol; 6.02 x particles

3 9.1 The Arithmetic of Equations Objectives Calculate the amount of reactants required or product formed in a nonchemical process. Interpret balanced chemical equations in terms of interacting moles, representative particles, masses, and gas volume at STP.

4 Stoichiometry stoicheion: element metron: measure

5 Reaction Stoichiometry The study of the mass relationships between reactants and products in a chemical reaction.

6 Interpreting Chemical Equations What information is represented in the following equation? N 2 (g) + 3H 2 (g) 2NH 3 (g)

7 Interpreting Chemical Equations Particles Moles Mass Volume

8 Interpreting Chemical Equations Mass- Can be calculated from a balanced chemical equation. A balanced chemical equation must obey the law of conservation of mass. That is that mass can not be created or destroyed.

9 Interpreting Chemical Equations N 2 (g) + 3H 2 (g) 2NH 3 (g) What is the combined mass of reactants? (2x14) + 3x(2x1) = 34g What is the mass of the product? 2 x (14+3) = 34 g This equation is balanced because the law of conservation of mass is conserved.

10 Interpreting Chemical Equations N 2 (g) + 3H 2 (g) 2NH 3 (g) Mass can also be interpreted for just one product. Let see what the mass is for the Hydrogen used in the equation. 1 st find the molar mass of hydrogen 2x1=2 But we have 3 times this so 6g is the mass used in the equation.

11 Review Gram molecular mass (gmm) is used to express the mass of 1 mol of a molecular compound. H 2 O 2 = 34g Gram formula mass (gfm) is used to express the mass of one mole of an ionic compound. (NH 4 ) 2 CO 3 = 96g

12 Interpreting Chemical Equations Volume- If STP the equation will also tell you the volume of gasses. Remember 1 mol of gas at STP occupies 22.4L

13 Interpreting Chemical Equations N 2 (g) + 3H 2 (g) 2NH 3 (g) What is the volume of nitrogen? 22.4L What is the volume of hydrogen? 3 x 22.4= 67.2L

14 Practice: Balance, then tell me the number of particles, moles, mass, and volume of everything in the equation. C 5 H 12 + O 2 CO 2 + H 2 O

15 9.2 Chemical Calculations Objectives Construct mole ratios from balanced chemical equations and apply these ratios in mole-mole stoichiometric calculations. Calculate stoichiometric quantities from balanced chemical equations using units of moles, mass, representative particles, and volume of gases at STP.

16 Mg (s) + 2HCl (aq) MgCl 2(aq) + H 2(g) If you know the amount of one substance you can determine the amounts of all the other substances. How many moles of H 2 are obtained from 2 mol of HCl? How many moles of Mg will react with 2 mol of HCl? Double the number of moles of reactants, how many moles of product would we have? How would you convert the moles of these substances to masses?

17 To solve stoichiometry problems you must have: A balanced chemical equation to determine the correct mole ratios

18 Mole Ratio A conversion factor that relates the amounts in moles of any two substances involved in a chemical reaction.

19 2H 2 O 2H 2 + O 2 2 mol H 2 and 1 mol O 2 1 mol O 2 2 mol H 2 2 mol H 2 and 2 mol H 2 O 2 mol H 2 O 2 mol H 2 1 mol O 2 and 2 mol H 2 O 2 mol H 2 O 1 mol O 2

20 2 Al 2 O 3 (l) 4Al (s) + 3O 2 (g) What are the possible mole ratios?

21 Mole/Mole Problems 1. A balanced chemical equation 2. Determine the mole ratio 3. Set-Up problem.start with the given

22 Example 1: The elements lithium and oxygen react to form lithium oxide. How many mol of lithium oxide will form if 2 mol of lithium react?

23 Li + O 2 Li 2 O 1. Balance equation: 4 Li + O 2 2 Li 2 O 2. Mole ratio of lithium to lithium oxide: 4 mol lithium to 2 mol lithium oxide 2. Set-Up problem, start with given.

24 2 mol Li x 2 mol Li 2 O = 4 mol Li Answer: 1 mol Li 2 O

25 Problem 2 Hydrogen peroxide decomposes to form water and oxygen gas. How many moles of oxygen will result from the decomposition of 5.0 mol of H 2 O 2?

26 H 2 O 2 H 2 O + O 2 1. Balance equation: 2H 2 O 2 2H 2 O + O 2 2. Mole ration of oxygen & hydrogen peroxide: 2 mol H 2 O 2 to 1 mol O 2 3. Set-up problem starting with the given.

27 5.O mol H 2 O 2 x 1 mol O 2 = 2 mol H 2 O 2 Answer: 2.5 mol of O 2

28 Problem 3 Ammonia, NH 3 is widely used as a fertilizer and in many household cleaners. How many moles of ammonia are produced when 6 mol of hydrogen gas react with an excess of nitrogen gas?

29 N 2 + H 2 NH 3 1. Balance equation: N 2 + 3H 2 2NH 3 2. Determine mol ratio of hydrogen to ammonia: 3 mol hydrogen to 2 mol ammonia 3. Set-up problem starting with the given.

30 6 mol H 2 x 2 mol NH 3 = 3 mol H 2 Answer: 4 mol NH 3

31 Mass/Mole and Mole/Mass Problems Must have 3 pieces of information: 1. Balanced chemical equation 2. Mole ratio 3. Molar mass

32 Problem 1 What mass in grams of magnesium oxide is produced from 2.00 mol of magnesium? Mg + O 2 MgO 2 Mg + O 2 2 MgO Molar Mass of MgO g/mol 2.00 mol Mg x 2 mol MgO x 40.31g MgO = 2 mol Mg 1 mol MgO

33 Answer: g MgO = 80.6 g MgO

34 Problem 2 What mass in grams of oxygen combine with 2.00 mol of magnesium? 2 Mg + O 2 2MgO Molar mass of oxygen: g/mol 2.00 mol Mg x 1 mol O 2 x g O 2 = 2 mol Mg 1 mol O 2 Answer: 32.0 g O 2

35 Problem 3 How many mol of HgO are needed to produce 125 grams of oxygen? HgO Hg + O 2 2 HgO 2 Hg + O 2 Molar mass of oxygen: g/mol 125 g O 2 x 1 mol O 2 x 2 mol HgO = g O 2 1 mol O 2 Answer: 7.81 mol HgO

36 Mass/Mass Problems 1. Balanced equation 2. Mole ratio 3. Molar Masses

37 Problem 1 Tin (II) fluoride, SnF 2 is used in toothpaste. How many grams of tin (II) fluoride are produced from the reaction of 30.0 g of HF with Sn? (Hydrogen is also a product.) Sn + HF SnF 2 + H 2 Sn + 2 HF SnF 2 + H 2

38 Sn + 2 HF SnF 2 + H 2 How many grams of tin (II) fluoride are produced from the reaction of 30.0 g of HF with Sn? Mole ratio? 2 mole HF : 1 mole SnF 2 Molar masses of HF and SnF 2? HF: g/mol SnF 2 : g/mol

39 How many grams of SnF 2 are produced from the reaction of 30.0 g of HF with Sn? 30.0gHF x mol HF x 1 mol SnF 2 x gSnF gHF 2 mol HF 1 mol SnF 2 Answer: 117 g SnF 2

40 Problem 2 Nitrous oxide, N 2 O (nitrogen I oxide) a.k.a. laughing gas is used as an anesthetic in dentistry. How many grams of NH 4 NO 3 is required to form 33.0g N 2 O? (the other product of this decomposition reaction is water) NH 4 NO 3 N 2 O + H 2 O NH 4 NO 3 N 2 O + 2 H 2 O

41 NH 4 NO 3 N 2 O + 2 H 2 O Mole Ratio? 1 mol NH 4 NO 3 :1 mol N 2 O Molar Mass of NH 4 NO g/mol Molar Mass of N 2 O g/mol

42 NH 4 NO 3 N 2 O + 2 H 2 O 33.0g N 2 O x 1 mol N 2 O x 1 mol NH 4 NO gN 2 O 1 mol N 2 O x g NH 4 NO 3 = 1 mol NH 4 NO 3 Answer: 60.0 g NH 4 NO 3

43 9.3 Limiting Reagent and Percent Yield Objectives Identify and use the limiting reagent in a reaction to calculate the maximum amount of product(s) produced and the amount of excess reagent. Calculate theoretical yield, actual yield, or percent yield given appropriate information.

44 Limiting & Excess Reactants Limiting Reactant: the reactant is used up first in a chemical reaction. Excess Reactant: the reactant that is not totally used up in a chemical reaction.

45 Identify the limiting & excess reactants Suppose you have 10 slices of bread and four slices of cheese. How many sandwiches can you make? 1 slice of cheese per sandwich.

46

47 What is the limiting reactant? What is the excess reactant?

48 Musical Chairs In the game Musical Chairs, what is the limiting reagent and what is the excess reagent?

49 C (s) + O 2 (g) CO 2 (g) Suppose in this reaction we have 3 C atoms and 6 molecules of O 2 (g) C atoms + 6O 2 molecules 3CO 2 molecules + 3O 2 molecules

50 Limiting Reactant Problems 1. Balanced chemical equation. 2. Determine the number of moles of each reactant. 3. Divide each number of moles of each reactant by the coefficient in the balanced equation. The smallest number is the limiting reactant. 4. Proceed with solving problem.

51 Limiting Reactant Problem #1 SiO 2 + 4HF SiF 4 + 2H 2 O If 2.0 mol of HF are exposed to 4.5 mol of SiO 2, which is the limiting reactant?

52 SiO 2 + 4HF SiF 4 + 2H 2 O If 2.0 mol of HF are exposed to 4.5 mol of SiO 2, which is the limiting reactant? HF: 2.0 mol 4 = 0.5 SiO2: 4.5 mol 1 = 4.5 Limiting Reactant? HF

53 Limiting Reactant Problem #2 If 20.5 g of chlorine is reacted with 20.5 g of sodium, which is the limiting reactant? How much salt is formed? Cl 2 + Na NaCl Cl Na 2 NaCl

54 Cl Na 2 NaCl If 20.5 g of chlorine is reacted with 20.5 g of sodium, which is the limiting reactant? 20.5 g Cl 2 x 1 mol Cl 2 = mol 70.9 g 20.5 g Na x 1 mol Na = mol g Cl 2 : = Na : = 0.445

55 How much salt is formed? Limiting Reactant? Chlorine mol Cl 2 x 2 mol NaCl x g NaCl 1 mol Cl mol NaCl 33.8 g NaCl

56 Percent Yield Theoretical Yield: the maximum amount of product that can be produced from a given amount of reactant. Actual Yield: the measured amount of product obtained from a reaction.

57 Percent Yield = Actual yield x 100 Theoretical yield

58 Example 1 Chlorobenzene is used in the production of many important chemicals, such as aspirin, dyes, and disinfectants. One industrial method of preparing chlorobenzene is to react benzene, with chlorine, as represented in the following reaction: C 6 H 6 (l) + Cl 2 (g) C 6 H 5 Cl (s) + HCl (g)

59 When 36.8g of C 6 H 6 react with an excess of Cl 2, the actual yield of C 6 H 5 Cl is 38.8g. What is the percent yield of C 6 H 5 Cl? C 6 H 6 (l) + Cl 2 (g) C 6 H 5 Cl (s) + HCl (g)

60 Step 1: Do a mass-mass calculation to find the theoretical yield. Answer: 53.0 g

61 Step 2: Use percent yield equation: 38.8g x g Answer: 73.2%

62 Practice Problems 1.) If 75.0g of CO react to produce 68.4g of methanol, what is the percent yield of methanol? CO (g) + H 2 (g) CH 3 OH (l) (unbalanced) 2.) If 1.85g of Al react and the percent yield of Cu is 56.6%, what mass of Cu is produced? Al (s) + CuSO 4 (aq) Al 2 (SO 4 ) 3 (aq) + Cu (s) (unbalanced) MAKE SURE YOU SHOW ALL OF YOUR WORK

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