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Chapter 6: Moles, Molar Mass, Percent Composition and Formulas. From moles to mass and to the moon!. AMU (Atomic Mass Units). The mass of Carbon-12 is 12 AMU. But wait, when I look on the periodic table, the atomic mass is listed as 12.01078 AMU??? WHY? Why, cruel world?
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Chapter 6: Moles, Molar Mass, Percent Composition and Formulas From moles to mass and to the moon!
AMU (Atomic Mass Units) • The mass of Carbon-12 is 12 AMU. • But wait, when I look on the periodic table, the atomic mass is listed as 12.01078 AMU??? WHY? Why, cruel world? • WwWWwwhhHHHhhyyYYYYyy???
6.1 Atoms and Moles Amedeo Avogadro Count Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e Cerreto Avogadro’s Number 6.022 x 1023 Avogadro discovered that there are 6.022 x 1023 atoms in 1 gram of hydrogen.
Be able to explain and use the concept of the “mole” Hmmm… I shall call 6.022 x 1023... a “mole”. Yes…that has a nice ring to it. This number is called a “mole.” The word “mole” is just like the word “dozen”. Dozen means “12”. You can have a dozen of anything. You can also have a mole of anything.
So How Big is a “MOLE” Don’t be cruel now… Aaaiiiee Ummm… NO! Here it is written out 602,200,000,000,000,000,000,000 That’s 602 billion groups of a trillion! Let’s just do an example with paper clips. If you have a mole of paper clips and made them into a chain, how many times could you go to the moon and back with your chain?
Assume a paper clip ( still folded) is about 3 cm long. To find the total distance of the paper clips we use the following equation: Notice the unit “clips” cancels!!! Isn’t that Great… Anyone… Anyone see the greatness??? Man I love Conversions!
The moon is 382,171 km from Earth, so to the moon and back would be 764,342 km. So we need to convert our cm into km… … oh how fun… this is a metric conversion This of course is a “2-step conversion” because both units have a prefix I love conversions!
We’re almost done!!! That’s 23 trillion trips!! Mole-tastic! Marshmallow example: A bed of marshmallows covering the U.S. would be 776 miles deep
Convert moles to # of atoms • How many atoms are in 3.2 mol potassium (K)? • Remember: 1 mol = 6.02 x 1023 atoms • This can be written as a conversion factor:
How do we use the “Mole” in chemistry? • The atomic mass of an element is the grams of 1 mole of that atom • Why do chemists use moles? • It’s fun. • It’s impossible to count atoms with your hands. • You can easily measure the mass (in grams) of a chemical.
Convert moles of an atom to grams • I need 2.0 moles of copper (Cu) for an experiment. How many grams is that? • Atomic mass of Cu = 63.55 g/mol (round to 2 decimals) • “mol” is the abbreviation of “Mole”… I knowit’s only one letter different… chemists!!!
Converting grams to moles • I have 302 grams of silver (Ag). How many moles of silver do I have? • Step 1: Atomic mass of Ag = 107.87 g/mol • Step 2: Calculate
6.2 Molar Mass and Percent Composition Atomic Mass = mass of one mole of an atom Molar Mass= mass of one mole of a substance
Calculate Molecular Weights Example: Calculate the Molecular Weight (MW) of RbI2 Step 1: Assume you have 1 mole of this molecule and determine how much each element weighs from the periodic table. Step 2: Determine how many of each element you have Step 3: Add all the masses together
Step 1: Find how much each element weighs from the periodic table • Rb is atomic # 37. How much does each mole of Rb weigh? • 85.47 grams/mol Rb • I is atomic # 53. How much does it weigh? • 126.90 g/mol I
Step 2: Determine how many of each element you have Look at the formula: RbI2 We have 1 “Rb” atom and 2 “I” atoms
Step 3:Add all the masses together You will need to show this work: Because the units are the same we can add these two numbers together, so… 253.80 g/mol + 85.47 g/mol = 339.27 g/mol 339.27 g/mol is the “molar mass”
Converting from moles of a compound to grams • Example: I need 3.00 mol NaCl for an experiment. How many grams is that? • Step 1: Find the molar mass Molar mass = 22.09g/mol + 35.45g/mol = 57.54 g/mol • Step 2: Use the molar mass like a conversion factor.
Converting from grams of a compound to moles • Example: How many moles are in 10.0 g of Na2SO4? • Step 1: Find the molar mass. • Molar mass = 142.1 g/mol • Step 2: Use the molar mass like a conversion factor. You need “grams” on the bottom of the fraction.
6.3 Formulas of Compounds • Calculate “percent composition” • Just like any other % • Stuff = grams of elements
Calculate “percent composition” • Ex: calculate % of Cu and S in Cu2S • Stuff = grams Cu • (63.55 g/mol Cu)(2 mol Cu) = 127.1g Cu • Total stuff = grams Cu + grams S = 127.1 g + 32.07 g = 159.17 g = 159.2 g
You should be able to… Identify an “empirical formula” and a “molecular formula” Empirical formula – simplest ratio of atoms of each element in a compound (whole #’s only) Molecular formula – actual # of atoms of each element in a compound Molecular H2O2 C3H6 N2O3 C2H6 C3H9 Empirical HO CH2 N2O3 ? ?
Using % composition to determine a formula Law of Definite Composition – Any amount of a pure compound will always have the same ratio of masses for the elements that make up that compound Ex: H2O is always 88.9% O and 11.1% H by mass Only the simplest formula (ratio) can be found… in other words, you can only find empirical formulas
Using % composition to calculate the formula Process is as follows: Calculate % by mass of each element Determine mass of each element Easy if you use 100 g of the chemical Use mass to find the # of moles of each element Find the smallest ratio of the atoms ÷ the number of moles of each element by the element with the smallest # of moles Round to the nearest whole #
Example A molecule is 75% C & 25% H. Calculate the empirical formula. Using 100g total = 75g C and 25 g H Calculate moles of each = Ratio = 6.2C : 25H, simplify by ÷ each by 6.2. Whole number only!! Final ratio ≈ 1C : 4H so CH4 H C 1 4
Find the molecular formula Ex: C3H6O2 is an empirical formula for a chemical. The molar mass of the compound is 148 g/mol. What is the molecular formula of the compound?? Point: The ratio of C:H:O will always be what the empirical formula shows Steps 1. Calculate the empirical formula mass 2. Calculate molar mass/empirical formula mass 3. Multiply your subscripts by that #.
Steps • 1. Calculate the empirical formula mass: C3H6O2 mass = (3)(12.01) + (6)(1.01) + (2)(16.00) = 148.09 g/mol • 2. Calculate (molar mass)/(empirical formula mass) Round to a WHOLE number. • 3. Multiply the subscripts of the empirical formula by that number. C3x2H6x2O2x2 = C6H12O4