Chapter 7 Chemical Quantities Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings Atomic Mass Atomic mass is the • Mass of a single atom in atomic mass units (amu) • Mass of an atom compared to a 12C atom • Number below the symbol of an element Periodic Table and Atomic Mass Ag has atomic mass = 107.9 amu S has atomic mass = 32.07 amu C has atomic mass = 12.01 amu Atomic Mass Factors The atomic mass, • Can be written as an equality Example: 1 P atom = 30.97 amu • Can be written as two conversion factors Example: 1 P atom and 30.97 amu 30.97 amu 1 P atom Uses of Atomic Mass Factors The atomic mass is used to convert: • A specific number of atoms to mass (amu) • An amount in amu to number of atoms Learning Check What is the mass in amu of 75 silver atoms? 1) 107.9 amu 2) 8093 amu 3) 1.439 amu Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings Learning Check How many gold atoms have a mass of 1.85 x 105 amu? 1) 939 Au atoms 2) 3.64 x 107 Au atoms 3) 106 Au atoms Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings Formula Mass The formula mass is • The mass in amu of a compound • The sum of the atomic masses of the elements in a formula Calculating Formula Mass To calculate formula mass of Na2SO4, • Multiply the atomic mass of each element by its subscript • Total the masses 2 Na x 22.99 amu = 45.98 amu 1 1 S x 32.07 amu = 32.07 amu 1S 4 O x 16.00 amu = 64.00 amu 1O Formula mass Na2SO4 142.05 amu Learning Check Using the periodic table, calculate the formula mass of aluminum sulfide Al2S3. Collection Terms A collection term states a specific number of items. • 1 dozen donuts = 12 donuts • 1 ream of paper = 500 sheets • 1 case = 24 cans Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings A Mole of Atoms A mole (mol) is a collection that contains: • The same number of particles as there are carbon atoms in 12.01 g of carbon • 6.022 x 1023 atoms of an element (Avogadro’s number). 1 Mol element Number of Atoms 1 mol C = 6.022 x 1023 C atoms 1 mol Na = 6.022 x 1023 Na atoms 1 mol Au = 6.022 x 1023 Au atoms A Mole of a Compound A mole • Of a covalent compound has Avogadro’s number of molecules 1 mol CO2 = 6.022 x 1023 CO2 molecules 1 mol H2O = 6.022 x 1023 H2O molecules • Of an ionic compound contains Avogadro’s number of formula units 1 mol NaCl = 6.022 x 1023 NaCl formula units 1 mol K2SO4 = 6.022 x 1023 K2SO4 formula units Samples of One-Mole Quantities Avogadro’s Number Avogadro’s number 6.022 x 1023 can be written as an equality and two conversion factors. Equality: 1 mol = 6.022 x 1023 particles Conversion Factors: 6.022 x 1023 particles 1 mol and 1 mol 6.022 x 1023 particles Using Avogadro’s Number Avogadro’s number is used to convert Moles of a substance to Particles. How many Cu atoms are in 0.50 mol Cu? 0.50 mol Cu x 6.022 x 1023 Cu atoms 1 mol Cu = 3.0 x 1023 Cu atoms Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings Using Avogadro’s Number Avogadro’s number is used to convert Particles of a substance to Moles. How many moles of CO2 are in 2.50 x 1024 CO2 molecules? 2.50 x 1024 CO2 x 1 mol CO2 6.022 x 1023 CO2 = 4.15 mol CO2 Learning Check 1. The number of atoms in 2.0 mol Al is A. 2.0 Al atoms B. 3.0 x 1023 Al atoms C. 1.2 x 1024 Al atoms 2. The number of moles of S in 1.8 x 1024 atoms S is A. 1.0 mol S atoms B. 3.0 mol S atoms C. 1.1 x 1048 mol S atoms Subscripts and Moles The subscripts in a formula state • The relationship of atoms in the formula. • The moles of each element in 1 mol of compound. Glucose C6H12O6 In 1 molecule: 6 atoms C 12 atoms H 6 atoms O In 1 mol: 6 mol C 12 mol H 6 mol O Subscripts State Atoms and Moles The subscripts are used to write conversion factors for moles of each element in 1 mol compound. For aspirin C9H8O4, the following factors can be written: 9 mol C 1 mol C9H8O4 8 mol H 1 mol C9H8O4 4 mol O 1 mol C9H8O4 and 1 mol C9H8O4 9 mol C 1 mol C9H8O4 8 mol H 1 mol C9H8O4 4 mol O Learning Check How many O atoms are in 0.150 mol aspirin C9H8O4? Molar Mass The molar mass • Is the mass of one mole of an element or compound • Is the atomic mass expressed in grams Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings Molar Mass from Periodic Table Molar mass • Is the atomic mass expressed in grams Molar Mass of a Compound The molar mass of a compound is the sum of the molar masses of the elements in the formula. Example: Calculate the molar mass of CaCl2. Element Ca Number Atomic Mass Total Mass of Moles 1 40.08 g/mol 40.08 g Cl 2 CaCl2 35.45 g/mol 70.90 g 110.98 g Some One-Mole Quantities 32.07 g 55.85 g 58.44 g 294.20 g 342.30 g Learning Check Prozac, C17H18F3NO, is an antidepressant that inhibits the uptake of serotonin by the brain. What is the molar mass of Prozac? 1) 40.06 g/mol 2) 262.0 g/mol 3) 309.3 g/mol Molar Mass Factors Molar mass conversion factors: • Are written from molar mass • Relate grams and moles of an element or compound. Example: Write molar mass factors for methane CH4 used in gas cook tops and gas heaters. Molar mass: 1 mol CH4 = 16.04 g Conversion factors: 16.04 g CH4 1 mol CH4 and 1 mol CH4 16.04 g CH4 Learning Check Acetic acid C2H4O2 gives the sour taste to vinegar. Write two molar mass conversion factors for acetic acid. Calculations Using Molar Mass Molar mass factors are used to convert between the grams of a substance and the number of moles. Grams Molar mass factor Moles Moles to Grams Aluminum is often used to build lightweight bicycle frames. How many grams of Al are in 3.00 mol Al? Molar mass equality: 1 mol Al = 26.98 g Al Setup with molar mass as a factor: 3.00 mol Al x 26.98 g Al = 80.9 g Al 1 mol Al molar mass factor for Al Learning Check Allyl sulfide C6H10S is a compound that has the odor of garlic. How many moles of C6H10S are in 225 g? Grams, Moles, and Particles A molar mass factor and Avogadro’s number convert: • Grams to particles molar mass (g Avogadro’s number mol particles) • Particles to grams Avogadro’s number (particles molar mass mol g) Learning Check How many H2O molecules are in 24.0 g H2O? 1) 4.52 x 1023 2) 1.44 x 1025 3) 8.02 x 1023 Percent Composition Percent composition • Is the percent by mass of each element in a formula Example: Calculate the percent composition of CO2. CO2 = 1 C(12.01g) + 2 O(16.00 g) = 44.01 g/mol) 12.01 g C 44.01 g CO2 x 100 = 27.29 % C 32.00 g O x 100 44.01 g CO2 = 72.71 % O 100.00 % Learning Check The chemical isoamyl acetate C7H14O2 gives the odor of pears. What is the percent carbon in isoamyl acetate? 1) 7.102% C 2) 35.51% C 3) 64.58% C Types of Formulas The molecular formula • Is the true or actual number of the atoms in a molecule The empirical formula • Is the simplest whole number ratio of the atoms • Is calculated by dividing the subscripts in the molecular formula by a whole number to give the lowest ratio C5H10O5 5 = C1H2O1 = molecular formula CH2O empirical formula Some Molecular and Empirical Formulas • The molecular formula is the same or a multiple of the empirical. Learning Check A. What is the empirical formula for C4H8? 1) C2H4 2) CH2 3) CH B. What is the empirical formula for C8H14? 1) C4H7 2) C6H12 3) C8H14 C. Which is a possible molecular formula for CH2O? 1) C4H4O4 2) C2H4O2 3) C3H6O3 Learning Check A compound contains 7.31 g Ni and 20.0 g Br. • Calculate its empirical (simplest) formula. • If the compound has a molar mass of 437 g/mol, calculate the molecular formula. Learning Check Aspirin is 60.0% C, 4.5% H and 35.5% O. Calculate its empirical (simplest) formula.