Introduction to the Mole. Advanced Chemistry Ms. Grobsky

Transcription

1 Introduction to the Mole Advanced Chemistry Ms. Grobsky

2 Daily Objective To understand the concept of average atomic mass To understand the mole concept and Avogadro s number

3 Counting Units of Measurement Many items require exact counting units One dozen = 12 items A pair = 2 items One gross = 12 dozen = 144 items One ream = 500 items

4 The Mole as a Unit of Measure A mole is the SI unit that describes the amount of a substance One mole = x objects In chemistry, the objects are atoms and molecules 1 mole = x atoms (molecules) Avogadro s Number, N A

5 Exactly How Big is a Mole? A mole of marshmallows would cover the planet Earth 12 miles high A mole of marbles would fill the entire Grand Canyon There would still be enough left over to displace all the water in Lake Michigan and a few other lakes! Computers can count at the rate of over 800 million counts per second At this rate it would take a computer over 25 million years to count to 6.02 x A mole of hockey pucks would be equal to the mass of the Moon. Assume that each human being has 60 trillion body cells (6.0 x ) and the Earth's population is 6 billion (6 x 10 9 ) The total number of living human body cells on the Earth at the present time is 3.6 x or a little over half of a mole

6 The Mole as a Unit of Measure But why would we ever use such a large unit as the mole? Because ATOMS ARE tiny Atoms are too small to count individually so how would we count the number of them?

7 How Many Corn Flakes in a Box of Cereal? With a partner, estimate the number of corn flakes in a box of cereal

8 Counting by Weighing So, how do we determine the number of items when we can t directly count them? We weigh them in a bigger sample! Objects do not need to have identical masses to be counted by weighing Simply need to know the average mass of the objects For purposes of counting, the objects behave as though they were all identical Gumball example on page 60

9 The Relationship Between Mass and Moles The atomic mass shown on the Periodic Table is a weighted average of masses of all isotopes of an element Protons and neutrons have roughly the same mass of about 1.66 x10-24 grams Scientists defined this mass as 1 amu (atomic mass units) 1 amu = 1.66 x g Carbon-12 has a mass of 12 amu

10 The Relationship Between Mass and Moles More useful to associate mass in atomic mass units with a mass in grams Scientist set out to develop a basic unit of measurement to convert from atomic mass units to grams Used Carbon-12 to set the standard Experiments show that there are x carbon atoms in 12 grams of carbon-12 Scientists set one mole = the number of atoms of C-12 in 12 grams of C-12

11 Conversion Between AMU and Molar Mass Molar mass is the mass of 1 mole of eggs marbles in grams atoms ATOMIC MASS IS EQUAL TO MOLAR MASS

12 Molar Mass Scientists use the Periodic Table to determine the mass of a mole of atoms 1 mole 12 C atoms = x atoms = g 1 12 C atom = amu 1 mole 12 C atoms = g 12 C = amu For any element, atomic mass (amu) = molar mass (grams) 1 mole lithium atoms = g of Li

13 Molar Mass of an Element The mass of 1 mole of Strontium (Sr) is g The mass of 1 mole of Silver (Ag) is g

14 Molar Mass of a Compound Scientists also use the Periodic Table to determine the molar mass (formula weight) of compounds. The molar mass of the compound is the sum of the molar masses of each element times the number of atoms of that element in the compound

15 Molar Mass of a Compound Practice The molar mass of 1 mole of SO 2 is g The molar mass of 1 mole of NaBr is g The molar mass of 1 mole of (NH 4 ) 2 CO 3 is g The molar mass of 1 mole of Zn(C 2 H 3 O 2 ) 2 is g

16 The Mole as a Unit of Measurement Which has more molecules, a mole of O 2 or a mole of CO 2?

17 The Relationship Between Mass and Moles Moles are the HEART of chemistry!

18 The Relationship Between Mass and Moles

19 Conversion Between Mass and Moles Practice, Practice, Practice Convert 25.0 g of KMnO 4 to moles Step One The problem will tell you how many grams are present. Look for the unit of grams. The problem gives us 25.0 grams Step Two You need to know the molar mass of the substance. The molar mass of KMnO4 is grams/mole. Step Three Potassium (K) = x 1 = g Manganese (Mn) = x 1 = g Oxygen (O) = x 4 = g g You divide the grams given by the substance's molar mass (25.0/158.04) The answer of 0.16 mole has been rounded off

20 Conversion Between Mass and Moles Practice, Practice, Practice How many grams of Mg(NO 3 ) 2 are in 0.4 moles?

21 Conversion Between Moles and Number of Atoms Practice, Practice, Practice How many moles of PF5 are in 5.30 x 1018 molecules?

22 Conversion Between Mass and Number of Atoms How many boron atoms are there in 2.00 g of boron?