Stoichiometry. A. The Meaning of Coefficients in a Reaction Equation 1. Consider the following reaction: 200 H O H 2 O or

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1 Stoichiometry A. The Meaning of Coefficients in a Reaction Equation 1. Consider the following reaction: 2H 2 + O 2 2H 2 O The coefficients in the equation tell us that two hydrogen molecules react with one oxygen molecule to produce two molecules of water. As long as the 2:1:2 ratio is maintained, the reaction is balanced, so this reaction is also balanced when written as 200 H O H 2 O or 2 x (6.02 x ) H x (6.02 x ) O 2 2 x (6.02 x ) H 2 O Which is the same as 2 mol H 2 + O 2 2 mol H 2 O Thus the coefficients in a balanced equation simply gives us the ratio of reactants and products in terms of atoms and molecule or in terms of moles of atoms and molecules. In other words, the balanced equation provides us with a MOLE RATIO that relates reactants and products. STOICHIOMETRY The relationship between the amounts of reactants used in a chemical reaction and the amounts of products produced by the reaction. VERSION: November 21, 2000

2 CHEMISTRY 11 UNIT VII STOICHIOMETRY 2 EXAMPLE VII.1 RATIOS IN TERMS OF MOLECULES Problem Consider the reaction equation: N 2(g) + 3H 2(g) 2NH 3(g) How many molecules of N 2 are required to react with 15 molecules of H 2? Solution According to the balanced equation, 1 molecule of N 2 reacts with 3 molecules of H 2. # N 2 = 15 molecules H 2 x ecule N 2 3 molecules H 2 = 5 molecules N 2 EXAMPLE VII.2 RATIOS IN TERMS OF MOLES Problem Consider the reaction equation: N 2(g) + 3H 2(g) 2NH 3(g) How many moles of NH 3 are produced when 18 mol of H 2 are reacted? Solution According to the balanced equation, 2 moles of NH 3 are produced when 3 moles of H 2 react. # NH 3 = 18 moles H 2 x 2 moles NH 3 3 moles H 2 = 12 moles NH 3

3 CHEMISTRY 11 UNIT VII STOICHIOMETRY 3 SAMPLE PROBLEM VII.1 RATIOS IN TERMS OF MOLES Problem Consider the reaction equation: N 2(g) + 3H 2(g) 2NH 3(g) (a) How many molecules of NH 3 are formed from the reaction of 54 H 2? (b) How many moles of N 2 are required to react with 27 moles of H 2? Solution (a) Assign 1-5 (b)

4 CHEMISTRY 11 UNIT VII STOICHIOMETRY 4 B. Stoichiometry Calculations Involving Moles, Mass, Gas Volume, and Molecules 1. Stoichiometry calculations allow us to find out how much of chemical #1 is involved in a chemical reaction based on the amount of chemical #2 involved. A typical problem might be How many grams of chemical #1 must be reacted to produce 25.0 g of chemical #2? or What volume of chemical #1 at STP will be produced when 15.0 g of chemical #2 is reacted? In most cases, the quantities of the chemicals will be given in terms of molecules, mass, or volume of a gas at STP. It is important to remember that the balanced equation provides with a MOLE RATIO that relates reactants and products. As such, before we can use the ratio, the quantities that are given must be converted to moles first. MASS of CHEMICAL #1 MASS of CHEMICAL #2 MOLECULES of CHEMICAL #1 MOLES of CHEMICAL #1 MOLE RATIO MOLES of CHEMICAL #2 MOLECULES of CHEMICAL #2 VOLUME of CHEMICAL #1 VOLUME of CHEMICAL #2

5 CHEMISTRY 11 UNIT VII STOICHIOMETRY 5 EXAMPLE VII.3 CALCULATIONS INVOLVING MASS Problem Consider the balanced equation: C 3 H 8(g) + 5O 2(g) 3CO 2(g) + 4H 2 O (l) What mass of C 3 H 8 is required to produce g of H 2 O? Solution mass H 2 O mol H 2 O mol C 3 H 8 mass C 3 H 8 mass of C 3 H 8 = g H 2 O x 18.0 g x 1 C 3 H 8 4 H 2 O x 44.0 g = 61.1 g C 3 H 8 EXAMPLE VII.4 CALCULATIONS INVOLVING VOLUME OF A GAS Problem Consider the balanced equation: C 3 H 8(g) + 5O 2(g) 3CO 2(g) + 4H 2 O (l) If a sample of propane is burned what mass of H 2 O (l) is produced if the reaction also produces 50.0 L of CO 2(g) at STP? Solution volume CO 2 mol CO 2 mol H 2 O mass H 2 O mass H 2 O = 50.0 L CO 2 x 22.4 L x 4H 2 O 3CO 2 x 18.0 g 1mol = 53.6 g H 2 O

6 CHEMISTRY 11 UNIT VII STOICHIOMETRY 6 EXAMPLE VII.5 CALCULATIONS INVOLVING MOLECULES Problem Consider the balanced equation: C 3 H 8(g) + 5O 2(g) 3CO 2(g) + 4H 2 O (l) A sample of porous, gas-bearing rock is crushed and 1.35 x 10-6 g of C 3 H 8(g) is extracted from the powdered rock. How many molecules of CO 2 are produced if the gas sample is burned in the presence of an excess of O 2(g)? Solution mass C 3 H 8 mol C 3 H 8 mol CO 2 molecules CO 2 # CO 2 = 1.35 x 10-6 g C 3 H 8 x 44.0 g x 3CO 2 1 C 3 H 8 x 6.02 x = 5.54 x molecules CO 2 SAMPLE PROBLEM VII.2 CALCULATIONS INVOLVING MASS, VOLUME, AND MOLECULES Problem Consider the reaction equation: Solution Assign 6-16 ace N 2(g) + 3H 2(g) 2NH 3(g) (a) What mass of NH 3 is formed from the reaction of 25.0 g of H 2? (b) What volume of N 2 at STP is required to react with 30.0 g of H 2? (c) How many molecules of NH 3 are produced from the reaction of 15.0 g of N 2? (a) (b) (c)

7 CHEMISTRY 11 UNIT VII STOICHIOMETRY 7 C. Stoichiometry Involving Molar Concentrations 1. Recall that molar concentration calculations involve the use of the equation: c = n V Since stoichiometry involves finding relationships between moles of one chemical to another, the equation can be re-arranged to give moles n = c V If a volume is mentioned in a question, and the problem involves molarity, DO NOT assume that 22.4 L should be used. The relationship 22.4 L = e only applies to gases at STP. EXAMPLE VII.6 CALCULATIONS INVOLVING VOLUME OF A SOLUTION Problem A tablet of Tums has a mass of g. What volume of stomach acid having [HCl] = M is neutralized by a g portion of CaCO 3? CaCO 3 + 2HCl CaCl 2 + CO 2 + H 2 O Solution moles of CaCO 3 = g CaCO 3 x g = moles moles of HCl = moles CaCO 3 x 2 HCl 1 CaCO 3 = moles Volume of HCl = = mol mol/l = 15 L Vol of HCl = g CaCO 3 x or g x 2 HCl 1 L 1 CaCO x mol 3 = 15 L

8 CHEMISTRY 11 UNIT VII STOICHIOMETRY 8 EXAMPLE VII.7 CALCULATIONS INVOLVING VOLUME OF A GAS Problem What volume of CO 2(g) at STP is produced if 1.25 L of M HCl reacts with an excess of CaCO 3? CaCO 3 + 2HCl CaCl 2 + CO 2 + H 2 O Solution mole of HCl = mol L x 1.25 L = mol moles of CO 2 = moles HCl x 1 CO 2 2 HCl = mol CO 2 volume of CO 2 = mol x 22.4 L = L or volume of CO 2 = x 1.25 L x 1 CO 2 2 HCl x 22.4 L = L 2. A special process called TITRATION is used to find the unknown concentration of a chemical in solution. TITRATION is a process in which a measured amount of a solution is reacted with a known volume of another substance (of unknown concentration) until a desired equivalence point is reached. The EQUIVALENCE POINT is the point in a titration where the ratio of the moles of each species involved

9 CHEMISTRY 11 UNIT VII STOICHIOMETRY 9 exactly equals the ratio of the coefficients of the species in the balanced equation. shfiles/stoichiometry/acid_base.html EXAMPLE VII.8 TITRATION CALCULATION Problem If 19.8 ml of H 3 PO 4 with an unknown molarity reacts with 25.0 m L of M KOH according to the following reaction. What is the molarity of the H 3 PO 4? H 3 PO 4 + 2KOH K 2 HPO 4 + 2H 2 O Solution mol moles of KOH = n = c V = L mol x L = moles of H 3 PO 4 = mol KOH x mol 1 H 3 PO 4 2 KOH = [H 3 PO 4 ] = n V = mol L or = M [H 3 PO 4 ] = mol KOH L x L x 1 H 3 PO 4 2 KOH x L = M

10 CHEMISTRY 11 UNIT VII STOICHIOMETRY 10 SAMPLE PROBLEM VII.3 TITRATION CALCULATION Problem Calculate the [H 3 PO 4 ] if ml of M KOH is required to titrate ml of H 3 PO 4 according to the reaction: Solution H 3 PO 4 + 3KOH K 3 PO 4 + 3H 2 O Assign 17-25odd

11 CHEMISTRY 11 UNIT VII STOICHIOMETRY 11 D. Stoichiometry of Excess Quantities 1. In all of the previous stoichiometric calculations we assumed that a given reactant was completely used up during a reaction. Reactions are frequently carried out in such a way that one or more of the reactants are present in EXCESS amounts. Some reasons for having an excess amount include: (i) (ii) adding an excess of one reactant to make sure all of a second reactant is completely used (the second reactant may be too expensive to waste or harmful to the environment). unavoidably having a reactant in excess because a limited amount of another reactant is available. 2. In these types of calculations, the reactant that is present in lesser amount is called the LIMITING REACTANT. Since the limiting reactant gets completely used up first, it sets the limit on the amount of product that can be formed and the amount of the excess reactant used in the reaction. LIMITING REACTANT is the reactant that sets a limit on the amount of product that can be formed (completely used up in a chemical reaction). EXCESS REACTANT is the reactant that is not completely used up in a chemical reaction. EXAMPLE VII.9 LIMITING REACTANTS AND EXCESS

12 CHEMISTRY 11 UNIT VII STOICHIOMETRY 12 Problem If 20.0 g of H 2(g) reacts with g of O 2(g) according to the reaction: 2H 2(g) + O 2(g) 2H 2 O (g) which reactant is in excess and by how much? Solution To determine the reactant in excess, calculate the mass of some arbitrarily-selected product. Find how much H 2 O can be formed. mass of H 2 O (using H 2 ) = 20.0 g H 2 x 2.0 g x 2 H 2 O 2H 2 x 18.0 g = 180 g mass of H 2 O (using O 2 ) = g O 2 x 32.0 g x 2 H 2 O 1 O 2 x 18.0 g = 113 g Although there is enough H 2 to make g of H 2 O, there is only enough O 2 to make g of H 2 O therefore, O 2 is called the LIMITING REACTANT and H 2 is the EXCESS REACTANT. Use the limiting reactant to calculate how much H 2 is reacted. mass of H 2 (reacted) = g O 2 x 32.0 g x 2 H 2 1 O 2 x 2.0 g = 13g mass of H 2 in excess = 20.0 g 13 g = 7 g

13 CHEMISTRY 11 UNIT VII STOICHIOMETRY 13 EXAMPLE VII.10 LIMITING REACTANTS AND EXCESS Problem If 56.8 g of FeCl 2, 14.0 g of KNO 3, and 40.0 g of HCl are mixed and allowed to react according to the reaction: 3FeCl 2 + KNO 3 + 4HCl 3FeCl 3 + NO + 2H 2 O + KCl (a) which reactant is the limiting reactant? (b) how many grams of each excess reactant remains?

14 CHEMISTRY 11 UNIT VII STOICHIOMETRY 14 Solution (a) Arbitrarily find the mass of NO which can be produced mass NO (FeCl 2 ) = 56.8 g FeCl 2 x g 1 NO x 3 FeCl x 30.0 g 2 = 4.48 g mass NO (KNO 3 ) = 14.0 g KNO 3 x g 1 NO x 1 KNO x 30.0 g 3 = 4.15 g mass NO (HCl) = 40.0 g HCl x 36.5 g x 1 NO 4 HCl = 8.22 g x 30.0 g Since KNO 3 produces the least amount of NO, KNO 3 is the limiting reactant. (b) Use the limiting reactant, KNO 3, to calculate the mass of FeCl 2 and HCl reacted. mass FeCl 2 (reacted) = 14.0 g KNO 3 x g x 3 FeCl 2 1 KNO x g 3 = 52.7 g

15 CHEMISTRY 11 UNIT VII STOICHIOMETRY 15 mass of FeCl 2 in excess = 56.8 g 52.7 g = 4.1 g mass of HCl (reacted) = 14.0 g KNO 3 x g x 4 HCl 1 KNO x 36.5 g 3 = 20.2 g mass of HCl in excess = 40.0 g 20.2 g = 19.8 g SAMPLE PROBLEM VII.4 LIMITING REACTANTS AND EXCESS Problem If 35.0 g of H 2 reacts with 4.68 g of N 2 according to the reaction: Solution N 2 + 3H 2 2NH 3 what is the limiting reactant and by how many grams? Assign 26-32

16 CHEMISTRY 11 UNIT VII STOICHIOMETRY 16 E. Percentage Yield and Percentage Purity 1. Sometimes 100% of the expected amount of products cannot be obtained from a reaction. The term PERCENTAGE YIELD is used to describe the amount of product actually obtained as a percentage of the expected amount. There are two major reasons for this reduced yield of products. (i) (ii) The reactants may not all react because not all the pure material reacts or the reactants may be less than 100% pure. Some of the products are lost during experimental procedures such as filtering. PERCENTAGE YIELD = PERCENTAGE PURITY = mass of product obtained mass of product expected x 100% mass of pure reactant mass of impure reactant x 100% 2. Percentage yield calculations fall into three categories. (i) (ii) Find the percentage yield, given the mass of reactant used and mass of product formed. Find the mass of product formed, given the mass of reactant used and the percentage yield. (iii) Find the mass of reactant used, given the mass of product formed and the percentage yield.

17 CHEMISTRY 11 UNIT VII STOICHIOMETRY 17 EXAMPLE VII.11 FINDING PERCENTAGE YIELD Problem When 15.0 g of CH 4 is reacted with an excess of Cl 2 according to the reaction CH 4 + Cl 2 CH 3 Cl + HCl a total of 29.7 g of CH 3 Cl is formed. What is the percentage yield of the reaction? Solution (1) Find expected (theoretical yield) mass of CH 3 Cl mass of CH 3 Cl = 15.0 g CH 4 x 1 CH Cl g x 1 CH x 50.5 g 4 = g (2) now: percentage yield = 29.7 g g x 100% = 62.7% EXAMPLE VII.12 FINDING MASS OF PRODUCT Problem What mass of K 2 CO 3 is produced when 1.50 g of KO 2 is reacted with an excess of CO 2 according to the reaction 4KO 2 + 2CO 2 2K 2 CO 3 + 3O 2 if the reaction has a 76.0% yield? Solution (1) Find expected (theoretical yield) mass of K 2 CO 3 mass of K 2 CO 3 = 1.50 g KO 2 x 2 K CO g x 4 KO x g 2 = g (2) now since the percentage yield is only 76.0% percentage yield = actual expected = actual g actual mass of K 2 CO 3 = g x = 1.11 g

18 CHEMISTRY 11 UNIT VII STOICHIOMETRY 18 EXAMPLE VII.13 FINDING MASS OF REACTANT USED Problem What mass of CuO is required to make 10.0 g of Cu according to the reaction 2NH 3 + 3CuO N 2 + 3Cu + 3H 2 O if the reaction has a 58.0% yield? Solution (1) The actual yield is 10.0 g, we need to expect to produce more than 10.0 g of Cu since only 58.0% of what is expected will actually be produced. percentage yield = actual expected = 10.0 g expected expected = 10.0 g = g This means that we must attempt to produce g of Cu if we are to actually obtain 10.0 g of Cu. (2) Now calculate how much CuO is required to produce 17.2 g of Cu. mass of CuO = g Cu x 63.5 g x 3 CuO 3 Cu x 79.5 g = 21.6 g

19 CHEMISTRY 11 UNIT VII STOICHIOMETRY 19 SAMPLE PROBLEM VII.5 PERCENTAGE YIELD Problem When 25.0 g of H 2 are reacted with excess N 2 according to the reaction: Solution N 2 + 3H 2 2NH 3 a total of g of NH 3 is formed. What is the percentage yield? Assign 33-38

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