Weights on the Periodic Table ATOMIC MASSES MASS SPECTROMETER. Average Weights Relative Weights C is defined as weighing 12 AMU MASS O = 1.

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1 3.1. Relate weight to moles, moles to weights, mole to mole ratio, and numbers of particles in a chemical formula. [Readings Problems 18, 19, 21, 27, 33, 35, 39, 41, 45, 47, 49, 51, & 53] Weights on the Periodic Table Average Weights Relative Weights C is defined as weighing 12 AMU ATOMIC MASSES MASS SPECTROMETER LEAST MASS DETECTOR ACCELERATED ION BEAM MOST MASS MAGNETIC FIELD MASS O MASS 12 C = /3 1

2 Weights on the Periodic Table Average Weights Relative Weights C is defined as weighing 12 AMU 1 H atom weighs 1/ 12 times 1 C atom 1 O atom is 16/12 the weight of 1 C atom Formula Weights The relative weight of a molecule or formula unit is the sum of the AWs. What is the mass of 1 molecule of HNO 3 : 1 H atom = 1.01 amu 1 N atom = amu 3 O atom = amu amu Molar Mass The masses of individual atoms and molecules is not on a practical scale. In the laboratory chemists measure massive quantities of atoms. amu are too small to measure. solution: use enough atoms so a measurable amount is obtained. 2

3 Molar Mass rationale: 1 H atom weighs 1 amu. 1 O atom weighs 16 amu. O atoms weigh 16 times as much as H atoms. Also, 1 million O atoms would weigh 16 times as much as 1 million H atoms. If I had enough atoms of H to weigh 1 gram; then the same number of O atoms would weigh 16 grams Molar Mass Since the weights listed on the periodic table are relative weights; they can be expressed in grams instead of amu. The number of atoms necessary to scale up from amu to grams is called a mole and the weight is called a molar mass. The actual number atoms necessary to have 1 gram of H or 16 grams of O is x This number is called Avogadro s Number. Molar masses can be applied to molecules as well as to atoms. Summary 1 atom of H weighs 1 amu x atoms of H weigh 1 g. 1 atom of Cu weighs amu x atoms of Cu weigh g. 1 molecule of HNO 3 weighs amu x molecules of HNO 3 weigh g. 3

4 Mole Mole (SI unit for amount of a chemical substance) One mole of any element or compound has the same number of formula units as there are atoms in exactly 12 g of carbon-12. (6.022 x ) Samples of elements containing one mole (6.022x10 23 ) atoms. Zn, Hg, Cu, S One mole size samples of: NaCl (white), CuSO 4 5H 2 O (blue) H 2 O (colorless), K 2 CrO 4 (yellow) Practice Problem What is the molecular weight of ethanol, C 2 H 5 OH wt of C = 2 x 12.0 amu = 24.0 amu wt of H = 6 x 1.0 amu = 6.0 amu wt of O = 1 x 16.0 amu = 16.0 amu MW of C 2 H 5 OH = 46.0 amu Molar mass of C 2 H 5 OH =??? The Mole Concept Stoichiometric Equivalencies: C 2 H 5 OH (ethanol) 2 C atoms; 6 H atoms; & 1 O atom 4 mol C; 6 mol H; & 1 mol O atoms Calculate the mols of C atoms contained in 5.25 mol of ethanol 4

5 The Mole Concept Stoichiometric Equivalencies: C 2 H 5 OH (ethanol) 2 C atoms; 6 H atoms; & 1 O atom 4 mol C; 6 mol H; & 1 mol O atoms Calculate the mols of C atoms contained in 5.25 mol of ethanol 2 molc mol C = 0.52 molc 2 H 5 OH =1.04 molc 1molC 2 H 5 OH Practice Problem caproic acid smells like goats and has this chemical formula CH 3 (CH 2 ) 4 CO 2 H Calculate the number of moles of caproic acid contained in g of caproic acid Practice Problem CH 3 (CH 2 ) 4 CO 2 H Calculate the number of moles of caproic acid contained in g of caproic acid molscap= 65.24gcap 1molcap = molcap gcap 5

6 Correct Conversion Pathway Grams A Grams B x mol A/ gram A x grams B/mole B Moles A Moles B x mol B/mol A Stoichiometric Calculations All stoichiometric calculations involve center moles A to moles B step. Mass A to moles A step and mass B to moles B steps are used as needed. A and B are any species, reactant or product, in reaction. Practice Problem Calculate the number of copper atoms contained in.635 g of copper 6

7 Thermite Reaction Stoichiometry 2Al(s) + Fe 2 O 3 (s) Al 2 O 3 (s) + Fe(l) This reaction generates so much heat that the iron formed is a liquid. In order to produce 100 g of Fe find: 1. Mass of Fe 2 O 3 required. 2. Mass of Al required. 3. Mass of Al 2 O 3 produced Calculate the percent composition of a compound. [Readings 3.3 Example 3.9, Practice ex. 10, Problems 55, 57, & 59] Quantitative Decomposition of HgO HgO is a red solid. 2HgO(s) 2Hg(l) + O 2 (g) Upon heating HgO decomposes to Hg and O 2. After heating the Hg(l) is observed in this U bend which was in a 0 o C ice bath. Click picture to view animation 7

8 Percent Composition Percentage of one element in a compound %element = mass of element mass of sample *100% Percent Composition A g sample of a liquid is found to contain g carbon, g of hydrogen, and g of oxygen. What is the percent composition of each element? 3.3. Given data, calculate the empirical and the molecular formulas of a compound. [Readings 3.4 Problems 63, 65, 67, 71, 75, & 79] 8

9 Empirical Formula Empirical formula gives the smallest whole number ratio of moles of atoms in a formula unit. e.g. a g sample of hydrazine contains g of nitrogen and the rest is hydrogen. What is the empirical formula of hydrazine? Molecular formula The molecular formula results from multiplying the Empirical formula by a whole number scalar. You must know the molecular mass of the compound in order to calculate the molecular formula From previous problem: Hydrazine could be (NH 2 ) n N 2 H 4 or N 3 H 6 N n H 2n Sample problem The empirical formula of hydrazine is NH 2 and its molecular mass is What is the molecular formula of hydrazine? n = molecular formula weight empirical formula weight 9

10 Sample problem The empirical formula of hydrazine is NH 2 and its molecular mass is What is the molecular formula of hydrazine? Molecular formula is (NH 2 ) n N 2 H Write and balance chemical equations. [Readings 3.5 Problems 83, 85, 87,& 89] Chemical Equations A balanced chemical equation describes symbolically a chemical reaction. An equation is balanced when all of the reactant atoms are also present in the products. Reactions are balanced by adjusting the coefficients - NOT the chemical formulas 10

11 Chemical Equations Balancing Chemical Equations Step 1: Write the unbalanced equation. Make sure that each formula is written correctly. Step 2: Adjust the coefficients to get equal numbers of each kind of atom on both sides of the arrow. Chemical Equations Tips 1. Balance for elements other than O and H first. 2. Balance polyatomic ions as a group 3. When elements appear in the equation separately, balance them separately. The process is TRIAL AND ERROR!! Chemical Equations A chemical equation represents a chemical reaction: Before After Reactants Products Atoms in Before Atoms in After Arrangement Arrangement Conservation of atoms: there are the same number and kind of atoms after a chemical reaction as before the reaction. Since atoms have a consistent weight: mass is also conserved. 11

12 Balancing Equations From Experiment Co(NO 3 ) 2 + Na 3 PO 4 -->? Co(NO 3 ) 2 + Na 3 PO 4 --> Co 3 (PO 4 ) 2 + NaNO 3 Conservation of Atoms: How? Adjust coefficients. Method: systematic trial and error. Co(NO 3 ) 2 + Na 3 PO 4 --> Co 3 (PO 4 ) 2 + NaNO 3 Examples S 8 + O 2 --> SO 2 Pb(NO 3 ) 2 + K 2 CrO 4 --> PbCrO 4 + KNO 3 Cu + HNO 3 --> NO 2 + Cu(NO 3 ) 2 + H 2 O Rules for Balanced Equations Same number and kind of atoms on both sides. Same overall net electrical charge. Cannot change the subscripts of compounds: Only change the coefficients. Coefficients should be the lowest ratio integers. Mathematically they are similar to algebraic equations. 12

13 3.5. Calculate the yield of a reaction using a chemical equation. [Readings 3.6 Problems 91, 93, 95, & 96] Correct Conversion Pathway Grams A Grams B x mol A/ gram A x grams B/mole B Moles A Moles B x mol B/mol A Calculations & Equations NH 3 (g) + O 2 (g) --> H 2 O(l) + N 2 (g) 4NH 3 (g) + 3O 2 (g) --> 6H 2 O(l) + 2N 2 (g) (a) 3 moles of NH 3 will produce? moles N 2? (b) 1.7 g of NH 3 will produce? moles H 2 O? (c) 1.6 g of O 2 will produce? grams H 2 O? (d) 2 moles of NH 3 and 3 moles of O 2 will produce? moles N 2? (e) 5.0 g of NH 3 and 5.0 g of O 2 will produce? g of H 2 O? How many H 2 O molecules? 13

14 3.6. Calculate the yield of a reaction when one of the reactants limits the reaction. [Readings 3.7 Problems 97, 99, & 100] Assumptions in Stoichiometry 1. All reactions go to completion 2. There are ample quantities of all reactants 3. Only the main reaction takes place These assumptions are rarely true Limiting Reactant Suppose you had 10 bicycle wheels and 50 frames. How many bicycles could you produce? One frame yields one bicycle, thus, 50 bicycles? The amount of wheels limits the number of bicycles which can be produced! How many excess frames are there? 14

15 Limiting Reactant 2AgNO 3 (aq) + K 2 CrO 4 (aq) AgNO 3 is limiting Ag 2 CrO 4 (s) + 2KNO 3 (aq) K 2 CrO 4 is limiting Click picture to play movie. Limiting Reactant - The reactant that is completely consumed limits the amount of product formed. HC 2 H 3 O 2 (aq) + NaHCO 3 (aq) Limiting Reactant CO 2 (g) + H 2 O + NaC 2 H 3 O 2 (aq) Adding NaHCO 3 (s) to 100 ml of vinegar After addition of known masses of NaHCO 3 (s) Click picture to view animation How is limiting reactant concept shown in animation and experiment? Example Problem Suppose you add 125g of C and 125g of Cl 2 to an excess amount of TiO 2, how much TiCl 4 can be produced according to the following reaction. TiO 2 (s) + 2 Cl 2 (g) + C(s) > TiCl 4 (l) + CO 2 (g) Step 1: Calculate the moles of product formed from each reactant Step 2: The lesser number is the maximum product. 15

16 Example Problem Methanol is produced according to the following reaction CO(g) + 2 H 2 (g) > CH 3 OH(l) If 356g CO are mixed with 65.0g hydrogen, how much methanol can be produced? What is the limiting reagent.? How much of the excess remains after the reaction? 3.7. Calculate the percent yield of a reaction. [Readings 3.8 Problems 101, 103, & 105] Percent Yield Stoichiometry gives the theoretical amount of product produced by a reaction Actual yield is usually measured Percent Yield = actual yield theoretical yield *100% 16

17 Example Aspirin can be made according to the following reaction C 7 H 6 O 3 (s) + C 4 H 6 O 3 (l) > C 9 H (S) + CH 3 CO 2 H(l) salicylic acid+ acetic anhydride > aspirin acetic acid You begin with 14.4 g of salicylic acid and excess acetic anhydride 6.26g of aspirin are produced Calculate the percent yield Quantitative Decomposition of HgO Mass empty Mass with HgO Mass with Hg 2 HgO(s) 2 Hg(l) + O 2 (g) Use data to find: 1. Empirical formula of Hg x O y compound assuming the formula of the red solid is unknown. 2. The percent yield assuming compound is HgO Calculate the concentration of a solution made by mixing a solid in solution and by dilution. [Readings Problems 109, 111, 113, 115, & 117] 17

18 Concentrations It is convenient to measure amounts of dissolved materials by volume of the solution. Concentration: a measure of how much solute dissolved in what amount of solvent or solution. Most common concentration unit in chemistry: Molar Unit = M = # moles solute/# L solution Problem 250 ml volumetric flask What is the concentration of a solution made by dissolving 55.3 g of Cu(NO 3 ) 2 in enough water to make 250 ml of solution? Problem CuNO ml volumetric flask What is the concentration of a solution made by dissolving 55.3 g of Cu(NO 3 ) 2 in enough water to make 250 ml of solution? 18

19 H 2 O Problem 250 ml volumetric flask What is the concentration of a solution made by dissolving 55.3 g of Cu(NO 3 ) 2 in enough water to make 250 ml of solution? Problem 250 ml volumetric flask What is the concentration of a solution made by dissolving 55.3 g of Cu(NO 3 ) 2 in enough water to make 250 ml of solution? H 2 O Problem 250 ml volumetric flask What is the concentration of a solution made by dissolving 55.3 g of Cu(NO 3 ) 2 in enough water to make 250 ml of solution? 19

20 Sample Problem What is the concentration of a solution of 1.70g of AgNO 3 dissolved in enough water to make 250 ml? How many grams of NaOH will be contained in 50 ml of a M solution? Problem How would you make 150 ml of a 2.70 M solution of NaC 2 H 3 O 2? Problem How many grams of CaCl 2 would react with 100 ml of a 0.15 M solution of AgC 2 H 3 O 2? 20

21 Dilution Problems How much water needs to be added to 100mL of a M NaBr(aq) solution to make a M solution? # moles NaBr before dil =# moles NaBr after dil # moles = M V M i V i = M f V f Dilution Problems Calculate the concentration of a HCl solution when 25.0 ml of water is added to 30.0 ml of a 0.05M HCl solution How much water would you need to add to 10.0 ml of a 16.0 ml solution in order to make a M HCl solution? 3.9. Calculate the yield of reactions in solution using molarity. [Readings 3.11 Problems 119, 121, 123] 21

22 Solution Stoichiometry Calculate the maximum amount of silver carbonate that could be produced if ml of 0.01M AgNO 3 is mixed with 75 ml of a 0.01 M Na 2 CO 3 solution according to the following reaction. 2 AgNO 3 (aq)+ Na 2 CO 3 (aq) > Ag 2 CO 3 (s) + 2 NaNO 3 (aq) Solution Stoichiometry 2 KI(aq) + Pb(NO 3 ) 2 (aq) PbI 2 (s) + 2 KNO 3 (aq) Click pictures to play movies. If the beaker contains 75.0 ml of a M Pb(NO 3 ) 2 solution, how many ml of a 5.00 M KI solution must be added to precipitate as much PbI 2 as possible? Stoichiometry Flowchart A and B are any species, reactant or product. Central mole to mole step involved in all stoichiometric calculations. Green/red steps used as needed. 22