Adv chem chapt 3

Advanced Chemistry Chapter Three Stoichiometry

3.1 Counting by Weighing Average Mass = total mass/ number of objects For purposes of counting, objects behave as though they are identical

Atomic Masses The modern system of atomic masses is based on 12 C, as the standard. Developed in 1961

Mass Spectrometer An instrument that passes atoms or molecules through a beam of high-speed electrons, which in turn knock electrons off the atoms or molecules being analyzed and change them into positive ions.

Mass Spectrometer An applied electric field accelerates the ions into a magnetic field. The amount of deflection that occurs with each ion depends upon its mass.

Mass Spectrometer The most massive ions are deflected the smallest amount. A comparison of the positions where the ions hit the deflector plate provides accurate values of relative masses.

A Scientist Injecting a Sample into a Mass Spectrometer. (right) Schematic Diagram of a Mass Spectrometer

Atomic masses Naturally occurring isotopes are averaged to reflect the percent of abundance of those isotopes. Counting by averaging the mass of atoms allows for an accurate atomic mass for chemical calculations.

Mass Spectrum of Natural Copper What is the average mass of natural copper?

Mass Spectrum of Natural Copper What is the average mass of natural copper? 63.55 amu/atom

Mole The number equal to the number of carbon atoms in exactly 12 grams of pure 12 C. 6.022 x 10 23 A sample of a natural element with a mass equal to the element’s atomic mass expressed in grams.

Question? What is the mass, in grams, of 12 atoms of Aluminum?

Answer What is the mass, in grams, of 12 atoms of Aluminum?

Question? How many moles and number of atoms are in a 10.0g sample of Aluminum?

Answer How many moles and number of atoms are in a 10.0g sample of Aluminum?

Molar Mass Is the mass in grams of one mole of the compound. “ molecular weight”

Question? The formula for juglone, a dye, is C 10 H 6 O 3 . What is the molar mass?

Answer The formula for juglone, a dye, is C 10 H 6 O 3 . What is the molar mass?

Question? If the molar mass of juglone is 174.1g, How many moles of Juglone are in a 1.56 x 10 -2 g sample?

Answer If the molar mass of juglone is 174.1g, How many moles of Juglone are in a 1.56 x 10 -2 g sample?

3.5 Learning to Solve Problems

Conceptual Problem Solving Read the problem and decide on final goal. Gather facts and state the problem as simply as possible. Where are we going?

Conceptual Problem Solving Work backwards from the final goal to decide where to start. How do we get there?

Conceptual Problem Solving Once a solution is obtained, check to see in answer is reasonable. Does it make sense?

3.6 Percent Composition of Compounds

Mass Percentage Compare the mass of each element in one mole to the molar mass of the compound.

Question? What is the mass percentage of C, H and O in the following molecule: C 10 H 14 O?

Answer What is the mass percentage of C, H and O in the following molecule: C 10 H 14 O?

3.7 Determining the Formula of a Compound

Empirical vs. Molecular Formula Empirical formula is the formula of a molecule in its smallest whole number ratio. Molecular formula is the exact formula of the molecule as it exists. Example: Empirical = CH 5 N Molecular = (CH 5 N) n

Substances Whose Empirical and Molecular Formulas Differ

Determining the Empirical Formula Use percent composition as a 100g molecule sample. Divide the element mass sample by the molar mass of each element to determine the molar ratios between elements. Divide the molar ratios by the smallest ratio. If needed, multiply all ratios by the same number to obtain low whole numbers.

Determining the Molecular Formula Determine the empirical formula by using mole ratios between elements. Divide the molar mass of the molecular formula by the molar mass of the empirical formula. (n) Multiply every element in the formula by (n).

Question? Determine the empirical and molecular formulas for a compound that gives the following percentages on analysis (in mass percents): 71.65% Cl 24.27% C 4.07% H The molar mass is 98.96 g/mol

Answer 71.65% Cl 24.27% C 4.07% H molar mass = 98.96 g/mol Empirical: ClCH 2 Molecular: Cl 2 C 2 H 4

Chemical Equations Representation of the chemical reaction process Reactants – left side Products – right side

Chemical Equations Atoms are reorganized. Bonds have been broken, and new ones have been formed. Atoms are neither created nor destroyed therefore all atoms present in the reactants must be accounted for among the products.

Chemical Equations Subscripts apply to an atom or atoms in parenthesis. Coefficients apply to entire molecule/compound.

Chemical Equations Physical states should be given. Solid – (s) Liquid – (l) Gas – (g) Dissolved in water – (aq)

3.9 Balancing Chemical Equations

Writing and Balancing Equations Determine what reaction is occurring. What are the reactants, the products, and the physical states involved. Write the unbalanced equation that summarizes the reaction.

Writing and Balancing Equations Balance the equation by inspection, starting with the most complicated molecules. Determine what coefficients are necessary so that the same number of each type of atom appears on both reactant and product sides. Do not change the identities (formulas) of any of the reactants or products.

Question? (NH 4 ) 2 Cr 2 O 7(s) Cr 2 O 3(s) + N 2(g) + H 2 O (g)

Answer (NH 4 ) 2 Cr 2 O 7(s) Cr 2 O 3(s) + N 2(g) + H 2 O (g) 1 1 + 1 + 4

Question? NH 3(g) + O 2(g) NO (g) + H 2 O (g)

Answer NH 3(g) + O 2(g) NO (g) + H 2 O (g) 4 + 5 4 + 6

3.10 Stoichiometric Calculations: Amounts of Reactants and Products

Stoichiometry Write a balanced equation. Coefficients in the balanced equation provide the mole ratios used in the conversion of mass and other quantities from one molecule/compound to another.

Calculating Mass of Reactants and Products

Question? NaHCO 3(s) + HCl (aq) NaCl (aq) + H 2 O (l) + CO 2(aq) Mg(OH) 2(s) + 2HCl (aq) 2H 2 O (l) + MgCl 2(aq) Which one of these antacids neutralizes more acid with a 1.0g sample?

Answer NaHCO 3(s) + HCl (aq) NaCl (aq) + H 2 O (l) + CO 2(aq) Mg(OH) 2(s) + 2HCl (aq) 2H 2 O (l) + MgCl 2(aq) Which one of these antacids neutralizes more acid with a 1.0g sample? NaHCO 3 : 1.19 x 10 -2 mol HCl neutralized Mg(OH) 2 : 3.42 x 10 -2 mol HCl neutralized

3.11 The Concept of Limiting Reagent

Stoichiometric Mixtures A stoichiometric mixture is one that contains the relative amounts of reactants that match the numbers in the balanced equation. Assuming the reaction goes to completion, all reactants will be consumed to form products.

Limiting Reactant The reactant that runs out first and therefore limits the amount of product that can form. To determine how much product can be formed from a given mixture of reactants, the limiting reactant must first be determined.

Solving a Stoichiometry Problem Involving Masses of Reactants and Products

Figure 3.5 A Schematic Diagram of the Combustion Device Used to Analyze Substances for Carbon and Hydrogen

Figure 3.9 Three Different Stoichiometric Mixtures of Methane and Water, which React One-to-One

Figure 3.10 A Mixture of CH4 and H20 Molecules

Figure 3.11 Methane and Water Have Reacted to Form Products

Figure 3.12 Hydrogen and Nitrogen React to Form Ammonia

Jellybeans Can be Counted by Weighing

Figure 3.4 Samples Containing One Mole Each of Copper, Aluminum, Iron, Sulfur, Iodine, and Mercury

Bee Stings Cause the Release of Isopentyl Acetate

Penicillin is Isolated from a Mold that Can be Grown in Large Quantities in Fermentation Tanks

Figure 3.7 The Two Forms of Dichloroenthane

Figure 3.8 The Structure of P4O10.

Computer-Generated Molecule of Caffeine

Methane Reacts with Oxygen to Produce Flame

Hydrochloric Acid Reacts with Solid Sodium Hydrogen Carbonate to Produce Gaseous Carbon Dioxide

Decomposition of Ammonium Dichromate

Race Cars use Methanol as a Fuel

Table 3.1 Comparison of 1 Mole Samples of Various Elements

Table 3.2 Information Conveyed by the Balanced Equation for the Combustion of Methane

Adv chem chapt 3

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