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THE MOLE. Return to dimensional analysis. THE MOLE. What is a mole? Not a furry animal that burrows in the ground What it is? . . . A unit of measure that tells how much of something you have. THE MOLE. Examples: 1 dozen eggs is how many eggs? A couple of dollars is how many dollars?
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THE MOLE Return to dimensional analysis
THE MOLE • What is a mole? • Not a furry animal that burrows in the ground • What it is? . . . A unit of measure that tells how much of something you have
THE MOLE • Examples: • 1 dozen eggs is how many eggs? • A couple of dollars is how many dollars? • 1 score of people is how many people? • The mole is another example of a word that describes a number of something
THE MOLE • How much is a mole? • REMEMBER: In chemistry, we are talking about atoms and there are a LOT of atoms • Therefore the mole is a VERY BIG number • 1 mole of stuff is 6.02 x 1023 items of the thing • For example if you have 1 mole of pennies this equals 6.02 x 1023 pennies
THE MOLE • Where did we get this number? • 6.02 x 1023 • Discovered by a scientist by the name of Avagadro. • Therefore, it is called AVAGADRO’S NUMBER • He discovered how many atoms made up 12 grams of carbon. • Became the basis for the number of any number of particles in 1 mole
THE MOLE • Just how big is the mole? • If you had 1 mole of pennies (6.02 x 1023 pennies), then you could cover the entire earth with pennies 100 feet deep. • Let’s look at some other examples
THE MOLE • Now that we have the mole as a measurement, we can now find the number of particles in a sample • For example: • 1 mole of a compound is 6.02 x 1023molecules • 1 mole of an element is 6.02 x 1023atoms
EXAMPLE • If you have 2 moles of iron, how many iron atoms do you have? (We will continue to use dimensional analysis) • 1 mole = 6.02 x 1023 atoms • 2 moles | 6.02 x 1023 atoms = 1.20 x 1024 atoms | 1 mole
TRY THESE • If you have 3.5 moles of copper, how many copper atoms do you have? • If you have 0.0235 moles of water, how many water molecules do you have? • If you have 3.31 x 1024 atoms of potassium, how many moles do you have?
ANSWER • 2.11 X 1024 atoms of copper • 1.41 x 1022 molecules of water • 5.5 moles of potassium
MOLAR MASS • Yay! We get to use the periodic table again. • We talked earlier in the year about the mass number • The number located at the bottom of each element • For example: • Mass number of Lithium is 6.94 • Mass number of Bromine is 79.90
MOLAR MASS • Before the mass number was the measure of the number of protons + neutrons • It also accounted for all the isotopes (that’s why some numbers have decimals)
MOLAR MASS • It turns out the molar mass has another value • Molar mass: the mass in grams of one mole of any pure substance. • For example • 1 mole of Lithium = 6.94g/mole • 1 mole of Bromine = 79.90g/mole
WHAT HAPPENS WHEN YOU HAVE A COMPOUND? • How do we find the molar mass of water (H2O)? • TRY IT
SOLUTION • To find the molar mass, you add the molar mass of ALL the elements in the compound • H2O • You have 2 H 2 (1.01 g/mole) = 2.02g/mole • You have 1 O 16.0 g/mole • Total molar mass = 16.0 g/mole + 2.02 g/mole • Therefore the molar mass of water is: • 18.02 g/mole H2O
TRY THESE – Be careful of the subscript numbers • CCl4 • NaBr • Pb(SO4)2 • AgNO3
SOLUTIONS • 153.81 g/mole CCl4 • 102.89 g/mole NaBr • 399.32 g/mole Pb(SO4)2 • 169.88 g/mole AgNO3 NOTICE: The units for molar mass are g/mole (this is a conversion unit)
HOW TO USE MOLAR MASS • We can now measure the mass of substances and determine how many moles and how many particles we have? • For example: You measure 22.0g of boron (B), what is the number of moles you have? What is the number of boron atoms you have?
EXAMPLE • 22.0g of B | 1 mole of B = 2.04 moles of B |10.81 g of B • 2.04 moles of B | 6.02x1023 atoms of B = | 1 mole of B 1.23 x 10 24 atoms of B
TRY THESE • If you have 13.8g of lithium, how many moles is this? How many atoms? • If you have 88.5g of chlorine, how many moles is this? How many atoms?
ANSWERS • 1.99 moles; 1.20 x 1024 atoms of Li • 2.50 moles; 1.50 x 1024 atoms of Cl
FINDING THE MOLES FOR A COMPOND • Now that we know how to find the molar mass of a compound, we can calculate the number of moles and the number of molecules of a compound • The procedure is just like an element, except you have to calculate the molar mass first. • YOU MUST ALSO KNOW HOW TO NAME COMPOUNDS
EXAMPLE • Everyone try this out: You start with 261 g of manganese (IV) oxide. What is the molar mass of manganese (IV) oxide? How many moles is this? How many molecules is this?
SOLUTION • To find the molar mass, you need to use the periodic table: • manganese (IV) oxide = MnO2 • 1 Mn = 54.94 g/mole • 2 O = 2(16 g/mole) = 32 g/mole • Molar mass = 54.94g/mole + 32 g/mole • Total molar mass = 86.94 g/mole
SOLUTION (CONT) • Starting mass: 261g • Molar mass: 86.94g/mole 261g of MnO2 | 1 mole = | 86.49 g of MnO 3.02 moles of MnO2
SOLUTION (CONT) • 3.02 moles | 6.02 molecules = | 1 mole 1.82 x1024 molecules of MnO2
TRY THESE – You can do dimensional analysis all at once • You start with 361g of MgSO4. How many molecules of magnesium sulfate do you have? • You start with 370.5g of Ca(OH)2. How many molecules of calcium hydroxide do you have in this sample?
SOLUTION • 1.81 x 1024 molecules MgSO4 • 3.010 x 1024 molecules Ca(OH)2
LET’S BRING IN DENSITY • We know that density is a conversion factor between mass and volume • Density has the following units: • Mass/volume • Grams/milliliter (g/mL) • Grams/cubic centimeter (g/cm3) • Kilograms/liter (kg/L) • etc . . .
DENSITY • As long as we know that density is a conversion factor, now we can convert number of molecules to volume or volume to number of molecules • Molecules Moles Mass Volume (atoms) avagadro’s #molar massdensity
EXAMPLE • You have a sample of iron that contains 3.00 x 1025 atoms. If the density of iron is 7.87g/mL, what is the volume that the iron takes up? • To do this, it’s just like any other dimensional analysis problem • Write down what you start with • Cancel out the stinking units
EXAMPLE • Step 1: • You begin with 3.00 x 1025 iron atoms. • What is the next thing we have to get to head towards volume? Molecules/Atoms Moles • 3.00 x 1025 atoms | 1 mole_______ 6.02 x 1023 atoms
EXAMPLE • Step 2 • After you get to moles where is the next stop on your way to volume? Molecules/Atoms Moles Mass • You need the molar mass of iron (Fe) • 55.85 g/mole • 3.00 x 1025 atoms | 1 mole_____| 55.85 g__ | 6.02 x 1023 atom | 1 mole
EXAMPLE • Step 3 • The final conversion we need is density Molecules/Atoms Moles Mass Volume • Density of iron is 7.87 g/mL (you will not be able to do the problem unless density is given • 3.00 x 1025 atoms | 1 mole_____| 55.85 g_| 1mL__ | 6.02 x 1023 atom | 1 mole | 7.87 g
SOLUTION • 353 mL • NOTICE: • Each time we needed to do a conversion, we made sure the units cancelled out • You have to follow the correct pathway to get from the unit you start with to the unit you are trying to find.
TRY THESE • You start with 48.1 mL of H2O. The density of water is 1.00g/mL. What is the number of moles of water you have? • You have 3.5 moles of mercury. What volume does this take up if the density of mercury is 13.6 g/mL? • You start with 9.90 x 1020 molecules of ethyl alcohol (C2H6O). If the density of ethyl alcohol is 0.791 g/mL, what volume does this take up?
SOLUTION • 2.67 moles of H2O • 52 mL of Hg • 0.0958 mL of ethyl alcohol