Lesson 01: Atomic Masses and Avogadro s Hypothesis. 01 Counting Atoms and Molecules
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1 Chemistry 11, Mole Concept, Unit 04 1 Lesson 01: Atomic Masses and Avogadro s Hypothesis 01 Counting Atoms and Molecules The chemical changes we observe always involve a certain number of atoms that rearrange themselves into new configurations. These numbers are far too large in magnitude for us to visualize let alone count, but they are still numbers, and we need to have a way to deal with them. Because of this we need a computational bridge for these kinds of numbers. The mole concept provides this bridge, and is central to all of quantitative chemistry.
2 Chemistry 11, Mole Concept, Unit 04 0 Avogadro s Number and the Mole 3 Avogadro's Constant or Number, is 6.0x10. It is one of the fundamental constants of chemistry. It permits calculation of the amount of pure substance in chemical reactions on the basis of stoichiometric relationships or relationships between reactants used and products formed. Avogadro s Constant is a number, just as a dozen is number, and thus is dimensionless. You can think of Avogadro's number as the chemist's dozen. This chemists dozen, from this point onwards, is referred to as the mole. 1 individuals 1dozen individuals 3 6.0x10 individuals 1 mole individuals Chemical Reactions and the Mole
3 Chemistry 11, Mole Concept, Unit 04 3 Example 01 If you have a mole or donuts, nails, dogs, atoms, molecules or electrons, how many individuals do you have? 3 6.0x10 donuts 3 6.0x10 nails 3 6.0x10 dogs 3 6.0x10 atoms 3 6.0x10 molecules 3 6.0x10 electrons
4 Chemistry 11, Mole Concept, Unit Atomic Mass Units (AMU) and Molar Mass The atomic mass unit is the standard unit that is used for indicating mass on an individual atomic or molecular scale. The relationship between atomic mass units and mass in grams is... molar mass constant 1 atomic mass unit (u) avogadro's constant 1 g particles / 1 mole particles 3 6.0x10 particles / 1 mole particles 1 g 3 6.0x10 particles Atomic mass units are convenient because an atom or molecule that contains: n protons and neutrons has a mass approximately equal to: n x u 1 g of particles 6.0x10 particles or 1 g of particles 1 mole of particles This is molar mass. 3
5 Chemistry 11, Mole Concept, Unit 04 5 Protons, Neutrons, Atomic Mass and Molar Mass Element Protons Neutrons Atomic Molar Mass (u) Mass (g) 1 H He Li Be B C N O F Ne What does a mole look like? Actual Mass and Atomic Mass g Fe g Cu g Sn g I
6 Chemistry 11, Mole Concept, Unit Molar Mass The molar mass of elements is found by looking at the atomic mass of the element on the periodic table. If we wanted to know the molar mass of say magnesium or carbon we would discover that carbon has a molar mass of 1.011g/mole magnesium has a molar mass of 4.305g/mole
7 Chemistry 11, Mole Concept, Unit 04 7 Example 0 How many moles of carbon atoms are there in 4 g carbon atoms? 1 mol C 4 g C mol C 1 g C Example 03 How many atoms of carbon are there in 4 g carbon atoms? 3 1 mol C 6.0x g C 1.x10 atoms C 1 g C 1 mol
8 Chemistry 11, Mole Concept, Unit 04 8 Lesson 0 and 03: Molar Mass and Unit Conversions and Multiple Unit Conversions 01 Molar Mass of an Element The molar mass of elements is found by looking at the atomic mass of the element on the periodic table.
9 Chemistry 11, Mole Concept, Unit Molar Mass of a Compound For any chemical compound that's not an element, we need to find the molar mass from the chemical formula. To do this, we need to remember that molar masses of chemical compounds are equal to the sum of the molar masses of all the atoms in one molecule of that compound Example 01: Mole NaCl Grams 3 g Na molar mass of NaCl 1 mol Na 1 mol Na 35.5 g Cl 1 mol Cl 58.5 g/mol NaCl 1 mol Cl Example 0: FeCl molar mass of FeCl 1 mol Fe 1 mol Fe 56 g Fe 35.5 g Cl mol Cl 17 g/mol FeCl 1 mol Cl
10 Chemistry 11, Mole Concept, Unit Example 03: Zn NO 3 molar mass of Zn NO3 1 mol Zn 1 mol Zn mol N 6 mol O 3 65 g Zn 14 g N 16 g O 1 mol N 1 mol O 189 g/mol Zn NO
11 Chemistry 11, Mole Concept, Unit Unit Conversions Moles to Mass and Mass to Moles When converting from moles to mass or from mass to moles you need to use one of the following conversion factors 1mole of substance molar mass of substance OR molar mass of substance 1mole of substance Determine the mass of 5.0 moles of oxygen gas ( O ). 3 g O 1mol O 5.0 mol O 160 g O Determine the number of moles in 6.3 grams of 1mol H O 18 gho 6.3 gho 0.35 mol O H O.
12 Chemistry 11, Mole Concept, Unit 04 1 Number of Moles and Gas Volume Remember that Avogadro's Hypothesis states that "equal volumes of gases at the same temperature and pressure contain the same number of molecules regardless of their chemical nature and physical properties". 1 mole of ANY gas at STP has a volume of.4 L Conversion Factor 1mol.4 L OR.4 L 1mol We are going to use this information to help us solve the following two problems
13 Chemistry 11, Mole Concept, Unit What volume is occupied by mol of carbon dioxide, CO, at STP?.4 L 1mol CO mol CO 7.84 L Calculate the number of moles for 5.00 ml of SO gas. 1L 1mol SO 1000 ml.4 L SO ml SO.3x10 mol SO
14 Chemistry 11, Mole Concept, Unit Number of Moles and Number of Molecules or Atoms Conversion Factor 1mol molecules or atoms 6.0x molecules or atoms OR 6.0x10 molecules or atoms 1mol molecules or atoms How many molecules are there in 0.5 moles of 6.0x10 mol mol 3x10 molecules H O? How many atoms are there in 0.5 moles of H O molecules? 0.5 mol H 6.0x10 O molecules 1mol H 3 HO atoms 1HO molecule 5x H HO molecules O molecules O atoms
15 Chemistry 11, Mole Concept, Unit Multiple Unit Conversions Generally speaking, when converting the mass of a reactant to the mass of a product you must follow the procedure below When inter-converting between mass, volume and number of particles (molecules, atoms or ions) you must follow the procedure below
16 Chemistry 11, Mole Concept, Unit Take a look at the following examples where we convert between the units in the previous diagram. Ca OH You have 5.00 grams of calcium hydroxide, 5.00 g Ca OH many atoms do you have? 4.06x10 atoms 1mol Ca OH 74.1g Ca OH of 3 6.0x10 atoms 1mol Ca OH. How What is the volume occupied by 5.0 grams of CO (g) at STP? 1mol CO 44.0 g CO 5.0 g CO.5 L CO.4 L CO 1mol CO What is the volume occupied by.75 moles of methanol, CH 3 OH(l)? ( d g/cm g/ml ).75 mol CH 1ml CH3OH g CH3OH g CH3OH OH 1mol CH3OH 111ml CH 3 OH
17 Chemistry 11, Mole Concept, Unit How many sulphur atoms are contained in litres of SO 3 (g) at STP? L SO 3 1mol SO.4 L SO 1atom S 1molecule SO x10 molecules SO 1mol SO3molecules.69x10 4 S atoms 3 Al O3(s) has a density of 3.97 g/ml. How many atoms of oxygen are in 100 milliliters of Al O3(s)? 3.97 g 1mol Al 100 ml ml 10 g Al 6.0x10 1mol Al 3 3 atoms O 1molecule Al O 3 molecules Al O O molecules x10 4 O O O atoms
18 Chemistry 11, Mole Concept, Unit Lesson 04: Percent Composition Percent composition is percentage by mass of a component in a compound. To calculate the percent composition of a component in a compound Find the molar mass of the compound by adding up the masses of each atom in the compound using the periodic table. Find the mass of the component in the compound for which you are solving. Divide the mass of the component by the total molar mass of the compound and multiply by 100. Percent Composition Formula percent composition mass of component total mass of compound 100
19 Chemistry 11, Mole Concept, Unit Example 01 What is the percentage composition of hydrogen and oxygen in H O? Hydrogen percent composition in H O 1.01 g/mol H % H % H 18.0 g/mol HO Oxygen percent composition in H O g/mol O % O % O 18.0 g/mol HO
20 Chemistry 11, Mole Concept, Unit 04 0 Example 0 What is the percentage composition of each element in Ba NO 3? barium nitrate, Barium percent composition in Ba NO g/mol Ba % Ba % Ba g/mol BaNO3 Nitrogen percent composition in Ba NO g/mol N % N % N g/mol BaNO3 Oxygen percent composition in Ba NO g/mol O % O % O g/mol BaNO3
21 Chemistry 11, Mole Concept, Unit 04 1 Lesson 05: Empirical and Molecular Formulas 01 Types of Chemical Formulas Chemical formulas can be divided into three types empirical formula molecular formula structural formula The first two are discussed in this lesson empirical formula: shows the simplest whole number ratio for the elements in the compounds molecular formula: shows the correct ratio of elements in the compound (as it actually exists in nature) Compound Water Molecular Formula Empirical Formula H O H O Ethanol C 6 H O CH 3 O Hydrogen H Peroxide O HO Glucose C 6H1O6 CH O Methane CH 4 CH 4 Ethane C H6 CH 3
22 Chemistry 11, Mole Concept, Unit 04 The third type is discussed at the end of this course structural formula: shows the correct ratio of elements in the compound and also displays the elements in their proper bonding arrangement in a diagrammatic fashion Compound Molecular Formula Structural Formula Methane CH 4 Propane CH 3 8
23 Chemistry 11, Mole Concept, Unit Determining the Empirical and Molecular Formula of a Compound In determine chemical formulae, assuming the molar mass of the compound is given, the FIRST STEP is to determined the empirical formula SECOND STEP determined the molecular formula If the molar mass is NOT given then it will be necessary to determine it too based on information such as mass of given number of moles molar mass mass x g g moles x mol mol density of a gas at STP g.4l g molar mass densitymolar volume x L mol mol mass and volume of a gas at STP molar mass molar volume mass x g.4l g volume x L mol mol
24 Chemistry 11, Mole Concept, Unit 04 4 Example 01: Empirical Formula Find the empirical formula for a compound containing 15.9% boron (B) and 84.1% fluorine (F). First make an assumption that we have 100 grams 100% 100g 15.9% 15.9g 84.1% 84.1g Therefore we have 1 mol B 15.9 g B 1.47 mol B 10.8 g B 1 mol F 84.1 g F 4.68 mol F 19.0 g F Determine the simplest whole number ratio by dividing by the smallest molar amount 1.47 mol B mol 4.68 mol F mol
25 Chemistry 11, Mole Concept, Unit 04 5 Thus the empirical formula for this compound is BF 3 03 Empirical Formulae and Combustion Analysis Combustion analysis is a technique used to determine the empirical formula of organic compounds, compounds that contain carbon and hydrogen. When an organic compound is subject to combustion with oxygen in a special combustion apparatus all the carbon is converted to CO, and the hydrogen to O H The amount of carbon produced can be determined by measuring the amount of CO trapped by the sodium hydroxide, NaOH
26 Chemistry 11, Mole Concept, Unit 04 6 hydrogen produced by the amount of H O trapped by Mg ClO 4 the magnesium perchlorate, Example 0: Empirical Formula and Combustion Analysis Consider the combustion of isopropyl alcohol. The sample is known to contain only carbon, hydrogen and oxygen. Combustion of 0.55 grams of isopropyl alcohol produces grams of CO and grams of H O. Determine the empirical formula. first determine the amount of carbon present 1 mol CO 1 mol C g C lc g C g CO 44.0 g CO 1 mol CO 1 mo then determine the amount of hydrogen present 1 mol H O mol H g g H g HO 18.0 g H O 1 mol H O 1mol H then determine the missing mass
27 Chemistry 11, Mole Concept, Unit 04 7 given mass mass of C andh mass of O 0.55 g g C g H g O then determine the moles of each atom present 1 mol C g C moles C g C 1 mol H g H moles H g H 1 mol O g O moles O g O convert fractions to whole numbers moles C mo les moles H mo les moles O mo les and finally the empirical formula is CHO 3 8
28 Chemistry 11, Mole Concept, Unit 04 8 Example 03: Molecular Formula (Molar Mass NOT Given) A gas has an empirical formula of POF 3. If L of the gas at STP has a mass 1.6 g, what is the molecular formula of the compound? First determine the empirical mass of POF g Second, determine the molar mass molar mass density molar volume molar mass 1.6 g.4 L L 1 mol 104 g/mol Finally determine how much larger the molecular mass is compared to the empirical mass. This is the number you will multiply all subscripts by.
29 Chemistry 11, Mole Concept, Unit 04 9 molar mass empirical mass 104 g/mol 104 g/mol 1 In this case the molecular formula is the same as the empirical formula and therefore there is no need to change any subscripts. POF 3 Example 04: Molecular Formula (Molar Mass Given) Vitamin C (ascorbic acid) contains 40.9 % C, 4.58 % H, and % O, by mass. The experimentally determined molar mass is 176 g/mol. What is the molecular formula for ascorbic acid? First make an assumption that we have 100 grams. Therefore, we have 1 mol C 40.9 g C mol C g C 1 mol H 4.58 g H mol H g H 1 mol O g O mol O g O Determine the simplest whole number ratio by dividing by the smallest molar amount
30 Chemistry 11, Mole Concept, Unit moles C mole s moles H mole s moles O mole s Since we cannot have fractional amounts of atoms in a compound, we need to multiply the relative amounts of each atom by a number which gets rid of the fractional amounts. In this case multiply by three C H O Thus the empirical formula for ascorbic acid is C3H4O 3 But what about the molecular formula? We are told that the experimentally determined molecular mass is 176 g/mol. What is the molecular mass of our empirical formula? The molecular mass from our empirical formula is
31 Chemistry 11, Mole Concept, Unit g C g H mol C 1 mol H g O 1 mol O g/mol C3H4O3 The molecular mass from our empirical formula is significantly lower than the experimentally determined value 176 g/mol versus g/mol What is the ratio between the two values? g/mol g/mol.00 Thus the molecular formula for ascorbic acid is C H O C H O
32 Chemistry 11, Mole Concept, Unit 04 3 Lesson 06: Molar Concentration 1. Solutions and Concentration Solutions are homogeneous mixtures of two or more substances. Usually the solution is a liquid, but this is not always the case. Normally, a mixture has two components solute: the substance you are dissolving solvent: the substance you are dissolving into
33 Chemistry 11, Mole Concept, Unit Since solutions are a mixture, and therefore the ratio of solute to solvent varies. This ratio of solute to solvent is called concentration.
34 Chemistry 11, Mole Concept, Unit Making a Solution A volumetric flask is used to make up solutions of fixed volume very accurately. To make up a solution first add your solid to a flask with an appropriate volume dissolve the solid completely in a little water by shaking carefully fill the flask to just below the mark
35 Chemistry 11, Mole Concept, Unit add distilled water a drop at a time until the bottom of the meniscus lines up exactly with the mark on the neck of the flask after the final dilution, remember to mix your solution thoroughly, by inverting the flask and shaking.
36 Chemistry 11, Mole Concept, Unit Qualitative Description of Concentration To concentrate a solution, one must either add more solute (most common), or reduce the amount of solvent To dilute a solution, one must either add more solvent (most common) or reduce the amount of solute Example of Dilute and Concentrated Solutions
37 Chemistry 11, Mole Concept, Unit Quantitative Description of Concentration For scientific or technical applications, a qualitative account of concentration is almost never sufficient; therefore quantitative measures are needed to describe concentration. There are a number of ways to express concentration. Which one we choose often depends on convenience. In chemistry 11 and 1 we will deal exclusively with molarity Some Common Concentration Measures 1. Molarity moles of solute L of solution. Molality moles of solute kg of solvent 3. Parts per million mass of solute 6 10 mass of solution 4. Parts per billion mass of solute 9 10 mass of solution 5. Parts per trillion mass of solute 1 10 mass of solution 6. Mass percent mass of solute 100 mass of solution 7. Volume percent ml of solute 100 ml of solution
38 Chemistry 11, Mole Concept, Unit Molarity Calculations Example 01 What is the molarity of a solution made by dissolving 0 grams of NaCl in 100 ml s of water? 1 mol NaCl 1 0 grams NaCl 58.5 grams NaCl 0.1 L 3.4 moles/l NaCl or 3.4 M NaCl Example 0 Calculate the number of moles of CaCl in 0.78 litres of a 3.5 M solution 3.5molesCaCl 1L 0.78 L.73 moles CaCl Example 03 How many litres of a.0 M solution of HNO 3 do we need to have 5 moles of HNO 3? 1L moles 5 moles.5 L HNO3
39 Chemistry 11, Mole Concept, Unit Dilution For convenience, solutions are either purchased or prepared in concentrated stock solutions which must be diluted prior to use. When we take a sample of a stock solution we have a certain number of moles of molecules in that sample. Dilution alters the molarity or concentration of the solution but not the total number of moles of molecules in the solution. One of the standard equations for determining the effects of dilution upon a sample is to set up an equation comparing concentration and volume before and after dilution. Since concentration times volume gives us the total number of moles in the sample, and since this does not change, this value before and after dilution are equal Mbefore Vbefore Mafter Vafter moles before moles after
40 Chemistry 11, Mole Concept, Unit Example 04 How much of a 5 M stock solution of NaCl will you need to make up 50 ml of a 1.5 M solution? M V before before M after V after V before M after M V before after 1.5 M0.5 L 5 M L Thus, we would need litres of our 5 M NaCl stock solution. The rest of the 0.5 L volume is made up by the addition of water 0.5 L L L So we would take litres of stock 5M NaCl solution and add that to litres of water for a final volume of 0.5 litres with a final concentration of 1.5 M.
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