The Basics of Stoichiometry and Mole Calculations

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1 The Basics of Stoichiometry and Mole Calculations References and Resources Our TB: Ch. 3 of Chemistry: The central Science AP version (10 th edition) Powerpoint *and in-class work POGIL activities Online resources for our TB (in particular: Practice Quiz and e-book) Concept map for Stoichiometry and problem solving (handout) Video lectures from chem guy (see next page) Chemtours from ch. 3 of the W.W. Norton online book by Gilbert: *Some figures and examples in this PPT file have been taken from Melissa Brophy spresentation at

2 Resources: Video lectures for Chemistry Chemguy videos (google AP chemistry and chemguy videos; use in sequence) (good, short, video lectures that are appropriate for regular, honors, or AP chemistry) chemguy video for mole 1 n_order Mole 2 chemguy: next=1&playnext_from=pl&index=19 Mole 3 (Junior) : xt=1&playnext_from=pl&index=20 Stoichiometryvideo with worked example for predicting amount of product from 2.6 mol of one of reactant: see Empirical formula for AP chemwith chem. Guy:

3 The Basics of Stoichiometryand Mole Calculations Topics (review and new) Reading and writing chemical equations Balancing Equations Types of reactions Formula Weights Formula Weights and Molar Masses Percent composition Avagadro s Number (continued) Mole calculations Solving problems using dimensional analysis using (1) Avagadro snumber, (2) GFM, or (3) mole ratios from coefficients of balanced equations Determining Empirical formulas and Molecular formulas Limiting reagents(reactants)

4 Chemical Equations 2 H 2 (g) + O 2 (g) 2 H 2 O (l) Symbolic representations or descriptions of chemical reactions Meaningof the symbols, subscripts, parenthesis, + signs, arrows, co-efficients, states of matter, etc. Vocab. terms: reactants, products, catalyst, reaction conditions, etc. Resources: POGIL activity Chemical Reaction Equations (To Do) Use of particle diagrams (to represent) Conservation of Mass

5 Reading Balanced Chemical Equations 2 H 2 (g) + O 2 (g) 2 H 2 O (l) Molecules Particle diagrams Mass (amu) Amount (mol) Mass (g)

6 Balancing Equations NaN 3 (s) Na (s) + N 2 (g) Objective:find the smallest whole number ratio of coefficients for reactants and products that respects the law of conservation of mass. How? Trial and error to some extent Can Not change subscripts in chemical formulas but you Can change co-efficients in front of the formulas Start by balancing those elements that occur in the fewest chemical formulas Move back and forth checking the atom count each time a co-efficient is changed

7 Practice Balancing Equations (example of a decomposition reaction that is used in air bags in cars) NaN 3 (s) Na (s) + N 2 (g)

8 Practice Balancing Equations (Index card activity to hand in) (example of a combustion reaction) C 3 H 8 (g) + O 2 (g) CO 2 (g) + H 2 O (g) (Hint: Start with the element that occurs in the fewest chemical formulas)

9 Formula Weights Atomic masses of atoms, molecules and formula unit Calculating formula weights in amu using chemical formula and the atomic masses from the periodic table Ex: NaCl Atoms: Ex: Au (gold) One atom of Au has a mass of amu One formula unit of NaCl has a mass of amu Ex: C6H12O6 (glucose) One molecule of glucose has a mass of amu

10 Percent Composition What is the percent carbon in C 5 H 8 NO 4 (the glutamic acid used to make MSG monosodium glutamate), a compound used to flavor foods and tenderize meats?

11 Percent Composition What is the percent carbon in C 5 H 8 NO 4? Step 1: First calculate the Formula weight for the compound (FW for C 5 H 8 NO 4 is amu) Step 2: % composition = (mass of part mass of whole) X 100 or % element = (# atoms of that element)(atomic weight of the element) x 100 Formula weight of the compound

12 What is the % carbon in MSG? Step 1: FW for C 5 H 8 NO 4 is amu C H N O

13 % Composition of Carbon in C 5 H 8 NO 4 Step 2 % C = (mass of part mass of whole) X 100 or % element = (# atoms of element)(atomic weight of the element) x 100 Formula weight of the compound Answer?

14 Percent Composition What is the percent carbon in C 5 H 8 NO 4 (the glutamic acid used to make MSG monosodium glutamate), a compound used to flavor foods and tenderize meats? a) 8.22 %C b) 24.3 %C c) 41.1 %C

15 Avagadro s number and The Mole 6.02 x ,000,000,000,000,000,000,000

The Mole A counting unit Similar to a dozen, except instead of 12, it s 602 billion trillion 602,000,000,000,000,000,000,000 6.

16 The Mole A counting unit Similar to a dozen, except instead of 12, it s 602 billion trillion 602,000,000,000,000,000,000, X (in scientific notation) This number is named in honor of Amedeo ( ), who studied quantities of gases and discovered that no matter what the gas was, there were the same number of molecules present

17 How big of a unit is a mole? If you had Avogadro's number of unpopped popcorn kernels, and spread them across the United States of America, the country would be covered in popcorn to a depth of over 9 miles.

18 The Mole (abbreviated as mol) 1 dozen cookies = 12 cookies 1 mole of cookies = 6.02 X cookies 1 dozen cars = 12 cars 1 mole of cars = 6.02 X cars 1 dozen Al atoms = 12 Al atoms 1 mole of Al atoms = 6.02 X atoms Note that the NUMBER is always the same, but the MASS is very different!

19 A Mole of Particles Contains 6.02 x particles 1 mole C 1 mole H 2 O 1 mole NaCl = 6.02 x C atoms = 6.02 x H 2 O molecules = 6.02 x NaCl formula units (technically, ionic compounds are not molecules so they are called formula units) 6.02 x Na + ions and 6.02 x Cl ions

20 Avogadro s Number as Conversion Factor 6.02 x particles 1 mole or 1 mole 6.02 x particles Note that a particle could be an atom OR a molecule!

21 Learning Check 1. Number of atoms in mole of Al a) 500 Al atoms b) 6.02 x Al atoms c) 3.01 x Al atoms 2.Number of moles of S in 1.8 x S atoms a) 1.0 mole S atoms b) 3.0 mole S atoms c) 1.1 x mole S atoms

22 Practice Problems with /without using your calculator How many hydrogen atoms are in 2.5 moles of water? (Analyze, plan, solve, check)

23 Molar Mass or Gram-Formula Mass (GFM) The Mass of 1 mole (in grams) Equal to the numerical value of the average atomic mass (get from periodic table) 1 mole of C atoms = 12.0 g 1 mole of Mg atoms = 24.3 g 1 mole of Cu atoms = 63.5 g

24 Other Names Related to Molar Mass or Gram-Formula Mass (GFM) Molecular Mass/Molecular Weight: If you have a single molecule, mass is measured in amu s instead of grams. But, the molecular mass/weight is the same numerical value as 1 mole of molecules. Only the units are different. (This is the beauty of Avogadro s Number!) Formula Mass/Formula Weight: Same goes for compounds. But again, the numerical value is the same. Only the units are different. THE POINT: You may hear all of these terms which mean the SAME NUMBER just different units

25 Learning Check! Find the molar mass or gram-formula mass (usually we round to the tenths place) A.1 mole of Br atoms B. 1 mole of Sn atoms = 79.9 g/mole = g/mole

26 Practice Problems use mole road map or concept map to help you solve How many atoms of copper are there in a 3 gram sample of copper? (Interconverting mass and numbers of particles)

27 Problems involving Empirical Formulas Resources: PPT POGIL and Molecular Formulas Practice problems (in-class, chapter problem set, online practice quiz questions)

28 Chemical Formulas of Compounds Formulas give the relative numbers of atoms or moles of each element in a formula unit - always a whole number ratio (the law of definite proportions). NO 2 2 atoms of O for every 1 atom of N 1 mole of NO 2 2 mol of O atoms to every 1 mol of N atoms If we know, or can determine, the relative number of moles of each element in a compound, we can determine a formula for the compound.

29 Types of Formulas Empirical Formula The formula of a compound that expresses the smallest whole number ratio of the atoms present. Ionic formula are always empirical formula Molecular Formula The formula that states the actual number of each kind of atom found in one molecule of the compound.

30 To obtain an Empirical Formula 1. Determine the mass in grams of each element present, if necessary. 2. Calculate the number of moles of each element. 3. Divide each by the smallest number of moles to obtain the simplest whole number ratio. 4. If whole numbers are not obtained * in step 3), multiply through by the smallest number that will give all whole numbers * Be careful! Do not round off numbers prematurely

31 A sample of a brown gas, a major air pollutant, is found to contain 2.34 g N and 5.34g O. Determine a formula for this substance. require mole ratios so convert grams to moles moles of N = 2.34g of N g/mole = moles of N moles of O = 5.34 g = moles of O g/mole Empirical Formula: N O = NO N0.167 O

32 Calculation of the Molecular Formula A compound has an empirical formula of NO 2. The colourless liquid, used in rocket engines has a molar mass of 92.0 g/mole. What is the molecular formula of this substance?

33 Empirical Formula from % Composition A substance has the following composition by mass: % Na ; % B ; % H What is the empirical formula of the substance? Consider a sample size of 100 grams This will contain grams of B and grams H Determine the number of moles of each element Determine the simplest whole number ratio of each element

34 Determination of Empirical Formulas using combustion analysis data for organic compounds Experimental method that uses the mass of starting materials and masses of water and carbon dioxide produced to solve for the emp. Formula of organic compounds. Steps?

35 Predicting Quantities of Products Based on Quantities of Reactants Use (use concept/road map) Grams of reactant (Given) Moles of reactant Moles of product Grams of product (To Find_

36 How do we do this? Predicting Quantities of Products Based on Quantities of Reactants Start from a balanced equation Use concept/road map Use dimensional analysis with (a) molar mass/ GFM and (b) mole ratios as the basis of conversion factors

37 Practice Problem (Practice ex. P. 103). The decomposition of potassium chlorate is commonly used to prepare small amounts of oxygen gas in the laboratory. How many grams of oxygen gas can be prepared from 4.50 g of potassium chlorate?

38 Limiting reagents(reactants) problems (Start with POGIL Limiting reactants ) Start with the balanced chemical equation Determine moles of reactants Setup tables [(initial, change and end (final)] or ICE charts Identify the limiting reagent/reactant Finish the problem using dimensional analysis

40 Activities and Problem set X TB ch. 3 all sections required for regents, SAT II and AP exams POGIL activities on chemical reaction equations, limiting reactants, etc Lab activities: Determination CuxOy formula Percent composition of water in a hydrate Others -TBD Road/Concept map (provided) Online practice quiz due by Ch 3 Problems TO DO: write out questions and answers & show work all GIST, sample & practice exercises, Visualizing concepts, end of chapter 3 exercises: 10, 11 (b,e,g), 12 (d,g), 14, 15, 17, 19, 23 (b,d), 26d, 27, 29, 33, 35, 37, 41, 43, 47, 53, 59, 63, 67, 68, 71, 73, 75, 80, 105

1 The Mole 6.02 X 10 23

1 The Mole 6.02 X 10 23 2 STOICHIOMETRY - the study of the quantitative aspects of chemical reactions. 3 The Mole A counting unit Similar to a dozen, except instead of 12, it s 602 billion trillion 602,000,000,000,000,000,000,000