#### Transcript Sec. 3.4 - Morrison Community Unit District 6

Wednesday, Nov. 6th: “A” Day Thursday, Nov. 7th: “B” Day (11:45 release) Agenda Collect “Introduction to the Elements” Worksheet Section 3.4: “Counting Atoms” Atomic mass unit, atomic mass, mole, molar mass, Avogadro’s number, conversions Homework: Sec. 3.4 review, pg. 104: #1-10 Concept Review: “Counting Atoms” Mole Project/Mole Movie *Quiz next time over section 3.4* Section 3.4: “Counting Atoms” You wouldn’t expect something as small as an atom to have much mass. For example, copper atoms have an average mass of 1.0552 X 10-25 Kg. Since atoms are so small, kilograms or even grams are not convenient units to use for their masses. This is where the atomic mass unit comes into play… Atomic Mass Unit Scientists created a new unit, called the atomic mass unit, or amu. Atomic mass unit: 1/12 the atomic mass of the C-12 isotope. Protons and neutrons each have a mass of about 1 amu. So, an atom’s mass number (from the periodic table) is equal to the atom’s mass in amu’s. mass # = mass in amu’s (remember, mass # = protons + neutrons) Masses of Atoms are Expressed in Atomic Mass Units (amu’s) Atomic mass: the mass of an atom expressed in atomic mass units (amu’s) How can you tell the atomic mass of an element? Check the periodic table: the mass number shown on the periodic table is an average of the atomic masses of the naturally occurring isotopes of that element Introduction to the Mole Words that stand for numbers: Pair: 2 Dozen: 12 Decade: 10 Gross: 144 Ream: 500 Mole: 1 Mole = 602,200,000,000,000,000,000,000 (6.022 X 10 23) Mole Most samples of elements contain huge numbers of atoms. To make working with these numbers easier, chemists created a new unit call the mole (mol) Mole: the number of atoms in exactly 12 grams of carbon-12. (Remember, the mole is the SI base unit for the amount of a substance.) Avogadro’s Number Avogadro’s Number: 6.022 X 1023, the number of particles, atoms, or molecules in 1 mole of a substance. 1 mole = 6.022 X 1023 particles, atoms, or molecules 6.022 x 1023 is called “Avogadro’s Number” Just How Big is a Mole? A mole of marbles stacked over the United States would cover the country to a depth of 70 miles! Just How Big is a Mole? If 1 mole of dollars were equally distributed to every person in the world, about 7 billion people, each person would receive… $86,000,000,000,000 How do we Measure Moles? Calculations in chemistry require moles, but can you go to one of the balances in our lab and measure the number of moles? NO! That’s where the molar mass comes into play… Molar Mass To convert between moles and grams, chemists use the molar mass of a substance. Molar mass: the mass, in grams, of 1 mole of a substance. Units: g/mol (grams per mole) The molar mass of an element is equal to the element’s mass number found on the periodic table. Molar Mass What is the molar mass of iron? 55.85 g/mol What is the molar mass of rubidium? 85.47 g/mol What is the molar mass of gold? 196.97 g/mol What is the molar mass of silver? 107.87 g/mol What is the molar mass of calcium? 40.08 g/mol Molar Mass How do you find the molar mass of a compound, like water? Simply add up the atomic masses of the elements that make up the compound. Water, H2O, consists of 2 hydrogen atoms and 1 oxygen atom: H: 1.01 g/mol H: 1.01 g/mol O: 16.00 g/mol 18.02 g/mol Molar Mass What is the molar mass of salt, NaCl? Na: 22.99 g/mol Cl: 35.45 g/mol 58.44 g/mol What is the molar mass of sodium hydroxide, NaOH? Na: 22.99 g/mol O: 16.00 g/mol H: 1.01 g/mol 40.00 g/mol ‘A Mole in One’ 1 mole = 6.022x1023 atoms = an atom’s mass # in grams Converting from Moles Mass Sample Problem D, pg. 102 What is the mass, in grams, of 3.50 mole of copper, Cu? Use the periodic table to find the molar mass of copper, Cu. The conversion factor needed to solve the problem is: 1 mole Cu = 63.55 g Cu 3.50 mole Cu X 63.55 g Cu = 1 mole Cu 222 g Cu (3 sig figs) Practice Problem #3, pg 102 Calculate the number of moles of 0.850 g of hydrogen atoms. What is the mass in grams of 0.850 mol of hydrogen atoms? Use the periodic table to find the molar mass of hydrogen, H. The conversion factor needed to solve the problem is: 1 mole H = 1.008 g H 0.850 g H X 1 mol H = .843 mol H 1.008 g H 0.850 mol H X 1.008 g H = .857 g H 1 mol H (3 sig figs each) Converting from Moles # of Atoms Sample Problem E, pg. 103 Determine the number of atoms in 0.30 mol of fluorine atoms. The conversion factor needed to solve the problem is: 1 mole F = 6.022 X 10 23 atoms F 0.30 mol F atoms X 6.022 X 1023 atoms F 1 mole F 1.8 X 1023 atoms F (2 sig figs) Practice Problem #2, pg. 103 How many moles of silver are represented by 2.888 X 1023 atoms? The conversion factor needed to solve the problem is: 1 mole Ag = 6.022 X 10 23 atoms Ag 2.888 X 1023 atoms X 1 mole Ag 6.022 X 1023 atoms .4796 mol Ag (4 sig figs) Homework (you may work with a partner) Section 3.4 review pg 104: 1-10 Concept Review: “Counting Atoms” Keep updating your KWL! Remember: Quiz next time over section 3.4 Mole Project Your project is to create a mole to share with the class. The project will be done outside of regular class time. The projects will be due and presented to the class on: “A” Day: Wednesday, Nov. 20th “B” Day: Thursday, Nov. 21st