Lecture Presentation Chapter 3 Stoichiometry: Calculations with Chemical Formulas and Equations John D. Bookstaver St. Charles Community College Cottleville, MO
A. 1 Mg, 2 O, and 2 H B. 2 Mg, 2 O, and 2 H C. 6 Mg, 6 O, and 6 H D. 3 Mg, 6 O, and 6 H
Balancing Chemical Equation http://phet.colorado.edu/en/simulation/balancing-chemical-equations
Combustion of Magnesium Metal in Air
Decomposition of Sodium Azide
Combination and Decomposition Reactions
Propane Burning in the Air
Formula Weights 2012 Pearson Education, Inc.
Formula Weight (FW) A formula weight is the sum of the atomic weights for the atoms in a chemical formula. So, the formula weight of calcium chloride, CaCl 2, would be Ca: 1(40.08 amu) + Cl: 2(35.453 amu) 110.99 amu Formula weights are generally reported for ionic compounds. 2012 Pearson Education, Inc.
Molecular Weight (MW) A molecular weight is the sum of the atomic weights of the atoms in a molecule. For the molecule ethane, C 2 H 6, the molecular weight would be C: 2(12.011 amu) + H: 6(1.00794 amu) 30.070 amu 2012 Pearson Education, Inc.
Percent Composition One can find the percentage of the mass of a compound that comes from each of the elements in the compound by using this equation: % Element = (number of atoms)(atomic weight) (FW of the compound) x 100% 2012 Pearson Education, Inc.
Practice Calculating Percentage Composition Calculate the percentage of carbon, hydrogen, and oxygen (by mass) in C 12 H 22 O 11. Solution Analyze We are given a chemical formula and asked to calculate the percentage by mass of each element.
Avogadro s Number 6.02 x 10 23 1 mole of 12 C has a mass of 12.000 g. 2012 Pearson Education, Inc.
1 Mole Each of Substance
Practice Estimating Numbers of Atoms Without using a calculator, arrange these samples in order of increasing numbers of carbon atoms: 12 g 12 C, 1 mol C 2 H 2, 9 10 23 molecules of CO 2. Solution
Practice Converting Moles to Number of Atoms Calculate the number of H atoms in 0.350 mol of C 6 H 12 O 6. Solution Plan Avogadro s number provides the conversion factor between number of moles of C 6 H 12 O 6 and number of molecules of C 6 H 12 O 6 : 1 mol C 6 H 12 O 6 = 6.02 10 23 molecules C 6 H 12 O 6. Once we know the number of molecules of C 6 H 12 O 6, we can use the chemical formula, which tells us that each molecule of C 6 H 12 O 6 contains 12 H atoms. Thus, we convert moles of C 6 H 12 O 6 to molecules of C 6 H 12 O 6 and then determine the number of atoms of H from the number of molecules of C 6 H 12 O 6 : Moles C 6 H 12 O 6 molecules C 6 H 12 O 6 atoms H
Mole Relationships
Procedure for Interconverting Mass and Number of Formula Units
Practice Converting Grams to Moles Calculate the number of moles of glucose (C 6 H 12 O 6 ) in 5.380 g of C 6 H 12 O 6. Solution
Practice Converting Moles to Grams Calculate the mass, in grams, of 0.433 mol of calcium nitrate. Solution
Practice Calculating Numbers of Molecules and Atoms from Mass (a) How many glucose molecules are in 5.23 g of C 6 H 12 O 6? (b) How many oxygen atoms are in this sample? Chemistry, The Central Science, 12th Edition Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward 2012 Pearson Education, Inc.
How to Calculate an Empirical Formula from Percentage?
Practice Determining a Empirical Formula A molecule containing only nitrogen and oxygen contains (by mass) 36.8% N. a) How many grams of N would be found in a 100 g sample of the compound? How many grams of O would be found in the same sample? b) How many moles of N would be found in a 100 g sample of the compound? How many moles of O would be found in the same sample? c) What is the ratio of the number of moles of O to the number of moles of N? d) What is the empirical formula of the compound? Chemistry, The Central Science, 12th Edition Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward 2012 Pearson Education, Inc.
Practice Determining a Molecular Formula Mesitylene, a hydrocarbon found in crude oil, has an empirical formula of C 3 H 4 and an experimentally determined molecular weight of 121 amu. What is its molecular formula? Chemistry, The Central Science, 12th Edition Theodore L. Brown; H. Eugene LeMay, Jr.; Bruce E. Bursten; Catherine J. Murphy; and Patrick Woodward 2012 Pearson Education, Inc.
Combustion Analysis Compounds containing C, H, and O are routinely analyzed through combustion in a chamber like the one shown in Figure 3.14. C is determined from the mass of CO 2 produced. H is determined from the mass of H 2 O produced. O is determined by difference after the C and H have been determined. 2012 Pearson Education, Inc. Stoichiometry
Quantitative Information from Balanced Equations Stoichiometry
Rules for Balancing Chemical Equations Double check formulas NEVER change subscripts. ONLY use coefficients Save single atoms for last Start with the most complex component OR the odd numbered atoms Fractions can be used, but remember to multiply out. Coefficients should be integers Double check atom balance on reactant & product side PRACTICE! Stoichiometry
Sample Problem - Balancing Chemical Equations As 2 S 3 + O 2 As 2 O 3 + SO 2 Ca(OH) 2 (aq) + H 3 PO 4 (aq) H 2 O (l) + Ca 3 (PO 4 ) 2 (s) HClO 4 + P 4 O 10 H 3 PO 4 + Cl 2 O 7 Stoichiometry
Stoichiometric Calculations The stoichiometric coefficients in a balanced equation may be interpreted as: relative numbers of molecules involved in the reaction relative numbers of moles involved in the reaction Stoichiometry
Sample Problem A. 1.57 mol B. 3.14 mol C. 6.28 mol D. 9.42 mol Stoichiometry
Stoichiometric Calculations Starting with the mass of Substance A, you can use the ratio of the coefficients of A and B to calculate the mass of Substance B formed (if it s a product) or used (if it s a reactant). Stoichiometry
Sample Problem Consider the following reaction: Na 2 SiO 3 (s) + HF ( aq) H 2 SiF 6 (aq) + NaF (aq) + H 2 O (l) Looks complicated? 1) How many mols of HF are needed to react with 0.300 mols of the silicate Na 2 SiO 3? 2) How many grams of NaF forms when 0.500 mols of HF reacts with the silicate Na 2 SiO 3? 3) Calculate the mass of silicate Na 2 SiO 3 that reacts with 0.800 g HF. Stoichiometry
Practice Example C 6 H 12 O 6 + 6 O 2 6 CO 2 + 6 H 2 O Find the mass of water produced from 1.00 g glucose. Stoichiometry
LIMITING REACTANTS Stoichiometry
The Scenario Let s make some cheese sandwiches. 1 sandwich requires: 2 slices of bread (Bd) and 1 slice of cheese (Ch). The balanced equation: 2Bd + Ch Bd 2 Ch + Yummy! What we have: 10 slices of bread and 7 slices of cheese. How many sandwiches can we make? What limited you from making more sandwiches? So what is the limiting reactant/reagent? Which is the excess reactant? Stoichiometry http://phet.colorado.edu/en/simulation/reactants-products-and-leftovers http://funnycrave.com/7-products-you-should-never-buy-generic/7912/; http://kathyscookbook.wordpress.com/2011/03/25/grilled-cheese-sandwiches-for-two/; http://www.istockphoto.com/file_closeup/baked_goods/bread/whole_wheat/1867186_two_slices_of_bread.php?id=1867186
Limiting Reactants The limiting reactant in a chemical reaction is the reactant present in the...... smallest stoichiometric amount The one that gets used up first stops the reaction from proceeding. What is the limiting reagent in reaction below?.... H 2 would be the limiting reagent What is the reagent in excess? O 2 Stoichiometry
Problem-Solving Tips xa + yb product(s) x and y are the stoichiometric coefficients moles of A moles of B If (mol A / mol B) is > than x/y, then A is the excess reagent & B is the limiting reactant. If (mol A / mol B) is < than x/y, then B is the excess reagent & A is the limiting reactant. Stoichiometry
Sample Problem - To Find the Limiting Reactant NaOH (s) reacts with CO 2 (g) to produce sodium carbonate (s) and water. 1)Which is the limiting reactant when 1.85 mols NaOH and 1.00 mols CO 2 react? 2)How many mols of carbonate can be produced? 3)How many mols of excess reactant remain after the reaction is complete? The PLAN for qu 1: Write the balanced equation. Determine limiting reagent. The PLAN for qu 2: MUST use stoichiometry (mole ratio) based on the limiting reagent. Mols NaOH mols Na 2 CO 3 The PLAN for qu 3: Calculate the # mols excess reactant used Subtract initial mols of CO 2 from # mols CO 2 consumed in the reaction. Stoichiometry Ans: 1) NaOH 2) 0.925 mols Na 2 CO 3 3) 0.075 mols CO 2
Sample Problem 2.00 g of Zn metal reacts with 2.50 g silver nitrate to produce silver metal and zinc nitrate. 1)What reactant is limiting? 2)How many grams of Ag and zinc nitrate is formed? 3)What mass of the excess reagent is left after the reaction? Follow the similar steps as before but must now use Molar Mass (MM) to convert from g mols Stoichiometry
Theoretical Yield The theoretical yield is the maximum amount of..... product that can be made as calculated through the stoichiometry problem This is different from the actual yield, which is the amount one...... actually produces and measures by actually doing the experiment/reaction Percent yield: relates theoretical and actual yields Tells how much product was formed or recovered from the rxn. Actual Yield Percent Yield = x 100% Theoretical Yield Stoichiometry
The effect of side reactions on yield. A + B (reactants) C (main product) D (side products) Stoichiometry
Calculating % Yield Silicon carbide (SiC) is an important ceramic material that is made by allowing sand (silicon dioxide, SiO 2 ) to react with powdered carbon at high temperature. Carbon monoxide is also formed. When 100.0 kg of sand is processed, 51.4 kg of SiC is recovered. What is the percent yield of SiC from this process? NOTE: Unless already stated or a quantity for a reactant is not given, you will have to determine the limiting reagent. Stoichiometry
Solution write balanced equation find moles of reactant & product find mass of product predicted actual yield/theoretical yield x 100 = percent yield Stoichiometry