What is a Mole? An Animal or What?

Transcription

1 Unit 7: (Chapter 9) Chemical Quantities What is a Mole? An Animal or What? Section 9.1 The Mole: A Measurement of Matter Describe how Avogadro s number is related to a mole of any substance. Calculate the mass of a mole of any substance. What is a Mole? You can measure mass, or volume, or you can count pieces. We measure mass in grams. We measure volume in liters. We count pieces in MOLES. Moles (abbreviated: mol) Defined as the number of carbon atoms in exactly 12 grams of carbon mole is 6.02 x particles. Treat it like a very large dozen 6.02 x is called Avogadro s number. What are Our Representative, or Basic Particles? They are the smallest pieces of a substance. For a molecular compound: it is the molecule. For an element: it is the atom. Remember the 7 diatomic elements (made of molecules) Br I N Cl H O F Let s Check your Understanding? How many oxygen atoms are in each of the following molecules? CaCO 3 Al 2 (SO 4 ) 3 O 2 How many oxygen atoms in 1 mole of the following? CaCO 3 Al 2 (SO 4 ) 3 O 2 1

2 Mole Questions How many molecules of CO 2 are there in 4.56 moles of CO 2? How many moles of water is 5.87 x molecules? How many atoms of carbon are there in 1.23 moles of C 6 H 12 O 6? What s the Significance of the Mole? Let s look at Molar Masses Gram Atomic Mass (gam) Equals the mass of 1 mole of an element in grams grams of C has the same number of atoms as grams of H and grams of iron. Each of these has 6.02 x atoms We can write this as grams C = 1 mole C We now have a Factor of g/mole of Carbon Now we can count things by weighing them. Examples Using GAM How much would 2.34 moles of carbon weigh? How much would 1.50 moles of bromine weigh? How many moles of magnesium is g of Mg? How many moles are in 56 grams of Nitrogen? How many atoms of lithium is 1.00 g of Li? How much would 3.45 x atoms of U weigh? Is There a Similar Measure for Compounds? in 1 mole of H 2 O molecules there are two moles of H atoms and 1 mole of O atoms To find the mass of one mole of a compound determine the moles of the elements they have Find out how much they would weigh add them up Gram Molecular Mass (gmm) What is the mass of one mole of CH 4? 1 mole of C = g 4 mole of H x 1.01 g = 4.04g 1 mole CH 4 = = 16.05g The Gram Molecular Mass (gmm) of CH 4 is 16.05g/mol this is the mass of one mole of a molecular compound. 2

3 Mole-Mass and Mole-Volume Relationships Use the molar mass to convert between mass and moles of a substance. Use the mole to convert among measurements of mass, volume, and number of particles. Another New Term Molar Mass Molar mass is the generic term for the mass of one mole of any substance (in grams) The same as: 1) gram molecular mass, 2) gram formula mass, and 3) gram atomic mass- just a much broader term. Calculate the molar mass of the following: Na 2 S N 2 O 4 C Ca(NO 3 ) 2 C 6 H 12 O 6 (NH 4 ) 3 PO 4 Molar Mass The number of grams of 1 mole of atoms, ions, or molecules. Let s take a closer look at making conversion factors to change grams of a compound to moles of a compound. How many moles is 5.69 g of NaOH? How many moles is 5.69 g of NaOH? g 3

4 How many moles is 5.69 g of NaOH? g mole g need to change grams to moles How many moles is 5.69 g of NaOH? g mole g need to change grams to moles for NaOH How many moles is 5.69 g of NaOH? g mole g need to change grams to moles for NaOH 1mole Na = 22.99g 1 mol O = g 1 mole of H = 1.01 g How many moles is 5.69 g of NaOH? g mole g need to change grams to moles for NaOH 1mole Na = 22.99g 1 mol O = g 1 mole of H = 1.01 g 1 mole NaOH = g How many moles is 5.69 g of NaOH? g 1 mole g need to change grams to moles for NaOH 1mole Na = 22.99g 1 mol O = g 1 mole of H = 1.01 g 1 mole NaOH = g How many moles is 5.69 g of NaOH? g 1 mole g = mol NaOH need to change grams to moles for NaOH 1mole Na = 22.99g 1 mol O = g 1 mole of H = 1.01 g 1 mole NaOH = g 4

5 Examples Using Molar Mass How many moles is 4.56 g of CO 2? moles How many grams is 9.87 moles of H 2 O? 178 grams How many molecules is 6.8 g of CH 4? 2.55 x molecules 49 molecules of C 6 H 12 O 6 weighs how much? 1.47 x grams Worksheets on Moles, Mass, Avogadro s Number, & Moles/Molecules/Grams What About Gases? Many of the chemicals we deal with are gases. They are difficult to weigh. Need to know how many moles of gas we have. Two things effect the volume of a gas Temperature and pressure We need to compare them at the same temperature and pressure. What is Standard Temperature and Pressure? 0ºC (or 273 K) and 1 atm pressure is abbreviated as STP At STP 1 mole of gas occupies 22.4 L Called the molar volume 1 mole = 22.4 L of any gas at STP: 1 mole/22.4 Liters Molar Volume Problems What is the volume of 4.59 mole of CO 2 gas at STP? 103 Liters How many moles is 5.67 L of O 2 at STP? moles What is the volume of 8.8 g of CH 4 gas at STP? 12.3 Liters Density of a gas D = m / V for a gas the units will be g / L We can determine the density of any gas at STP if we know its formula. To find the density we need the mass and the volume. If you assume you have 1 mole, then the mass is the molar mass (from PT) At STP the volume is 22.4 L. 5

6 Molar Density Problems Find the molar density of CO 2 at STP. 44 grams / 22.4 L = 1.96 g/l Find the molar density of CH 4 at STP. 16 grams / 22.4 L = g/l Can we Find the Molar Mass, given the density of 1 Mole of Gas at STP? Pretend you have 1 mole at STP, so V = 22.4 L. Rearranging, MM = D x MV MM is the molar mass of 1 mole, and MV is the Molar Volume (22.4 L at STP). What is the molar mass of a gas with a density of g/l? 44.0 grams Have We Learned Anything?? These four items are all equal: a) 1 mole b) molar mass (in grams) c) 6.02 x representative particles d) 22.4 L at STP Thus, we can make conversion factors from them x molecules/mole Grams/mole 1 mole/22.4 L 6.02 x molecules/22.4 L Is There an Easy Way to Remember these Mole Conversions? Introducing Moletown! Gram Town Molar Mass X Mole Town The Mole X Volume City (L) 22.4 L/mole Avacado s Number X Particle City (molecules) We Can use Moletown for all of our conversions! The Mole and Volume, the Comprehensive Mole (1-46), & Mixed-Mole Worksheets 6

7 Percent Composition and Chemical Formulas Calculate the percent composition of a substance from its chemical formula or experimental data. Derive the empirical formula and the molecular formula of a compound from experimental data. Calculating Percent Composition of a Compound Like all percent problems: Part whole x 100 % Find the mass of each component, then divide by the total mass. What is the Percent Composition? Calculate the percent composition of a compound that is 29.0 g of Ag with 4.30 g of S. = 29.0g/(29.0g g) x 100% = 87.1 % Ag What is the Percent Composition? 8.20 grams of Magnesium makes up 60.3% composition of a compound of Mg and Oxygen. What is the weight of Oxygen in the sample? = 8.20 g/(8.20 g + x) x = 5.4 grams Finding the Percent Composition from the Formula Use the ratio from the formula, and assume that you have 1 mole of each element. Then you would use the Molar Mass of each element to determine the percent composition of each. : What is the percent composition of hydrogen in Methane, CH 4? Finding the Percent Composition from the Formula What is the percent composition of hydrogen in Methane, CH 4? 1 mole of Carbon = 12 grams There s 1 C, so the mass of C = 12 grams 1 mole of Hydrogen = 1.01 grams There are 4 H s, so the total mass of H is 4.04 grams. % C = [12 g/(16.04 g)] x 100% = 74.8 % 7

8 More Percent Composition Problems Calculate the percent composition of Hydrogen & Carbon in C 2 H 4? C: 85.6 % H: 14.4% Calculate the percent composition of each element in Aluminum carbonate? Al: 23.1 % C: 15.4 % O: 61.5 % Percent Composition As a Conversion Factor We can also use the percent as a conversion factor: For Example Sulfur makes up 26.7 % of the mass of NaHSO 4. What is the mass of the sulfur in 16.8 grams of NaHSO 4? Mass of Sulfur = 16.8 grams NaHCO 3 x (26.7 grams S/100 grams NaHCO 3 ) Mass of Sulfur = 4.49 grams Percent Composition As a Conversion Factor A bit more difficult: Calculate the mass of carbon in 82.0 grams of C 3 H 8? The Solution: % Composition of C in C 3 H 8 = (3x12g)/[(3x12g)+(8x1g)] % Composition of C in C 3 H 8 = 81.8 % Mass of Carbon = 82.o grams C 3 H 8 x (81.8 grams C/100 grams C 3 H 8 ) Mass of Carbon = 67.1 grams Percent Composition Problems for those times when you have difficulty getting to sleep Please try these problems in the textbook: Page 191, practice problems 31, 32, 33, & 34 And the Last Tidbit in Chapter 7: The Empirical Formula vs. the Molecular Formula The Empirical Formula is the lowest whole number ratio of elements in a compound vs. The Molecular Formula is the actual ratio of elements in a compound CH 2 C 2 H 4 C 8 H 16 8

9 The Molecular Formula and Empirical Formula can be the same H 2 O CO 2 CO How Can we Calculate the Empirical Formula? The Empirical Formula is not just the ratio of atoms, it is also the ratio of moles of atoms. In 1 mole of CO 2 there is 1 mole of carbon and 2 moles of oxygen. In one molecule of CO 2 there is 1 atom of C and 2 atoms of O. Calculating the Empirical Formula We can get a ratio from the percent composition. Assume you have a 100 g. The percentages become grams. Convert grams to moles. Find lowest whole number ratio by dividing by the smallest. Calculating the Empirical Formula Confused? Let s try an example Here s an Example The Problem: Calculate the empirical formula of a compound composed of % C, % H, and %N. 1. Assume 100 g so g C g H g N Now, convert to moles Here s an Example The Problem: Calculate the empirical formula of a compound composed of % C, % H, and %N g C x 1mol C = mole C g C g H x 1mol H = mole H 1.01 g H g N x 1mol N = mole N g N 9

10 Here s an Example g C x 1mol C = mole C g C g H x 1mol H = mole H 1.01 g H g N x 1mol N = mole N g N Find the lowest whole number ratio: 1. Mole C = / = 1 2. Mole H = / = 5 3. Mole N = / = 1 C 1 H 5 N 1 A compound is % P and % O. What is the empirical formula? Moles of P = g/ 31 g/mol = 1.40 mol Moles of O = g / 16 g/mol = 3.50 mol Dividing by 1.40, we get P 1 O 2.5 Multiply by 2 to get P 2 O 5 More Examples Caffeine is 49.48% C, 5.15% H, 28.87% N and 16.49% O. What is its empirical formula? Moles of C = 4.12 Moles of H = 5.10 Moles of N = 2.06 Moles of O = 1.03 C 4 H 5 N 2 O More Examples What is the Empirical Formula of a compound that has 1.04g K, 0.70g Cr, and 0.86g O? Hint: Convert to % first, then follow previous example A Slightly Different Twist? Ans. K 2 CrO 4 Can I Go from Empirical to Molecular Formulas? Absolutely Can I Go from Empirical to Molecular Formulas? Since the empirical formula is the lowest ratio, the actual molecule would weigh more. By a whole number multiple. Divide the actual molar mass by the empirical formula mass. 10

11 Can I Go from Empirical to Molecular Formulas? Caffeine has a molar mass of 194 g. what is its molecular formula? The Empirical Formula for Caffeine is C 4 H 5 N 2 O The empirical formula mass is 97 grams. 194 g / 97 g = 2 The molecular formula is 2 x the Empirical formula, or C 8 H 10 N 4 O 2. Here s Another A compound is known to be composed of % Cl, 24.27% C and 4.07% H. Its molar mass is known (from gas density) to be g. What is its molecular formula? Moles of Cl = g / g/mol = 2 Moles of C = g / 12 g/mol = 2 Moles of H = 4.07 g / 1 g/mol = 4 Empirical Formula = CH 2 Cl Empirical Molar Mass = g / mol g / g = 2 Molecular Formula = C 2 H 4 Cl 2 Percent Composition & Molecular Formula Worksheet Chapter 7 Practice Problems Worksheet 11