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 AMU??? WHY? Why, cruel world? WwWWwwhhHHHhhyyYYYYyy???

6.1 Atoms and Moles Avogadro’s Number x Avogadro discovered that there are x atoms in 1 gram of hydrogen. Amedeo Avogadro Count Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e Cerreto

Be able to explain and use the concept of the “mole” 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. Hmmm… I shall call x a “mole”. Yes…that has a nice ring to it.

So How Big is a “MOLE” 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? Don’t be cruel now… Aaaiiie e

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 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.

Atomic mass = grams of 1 mole of this element, Cobalt

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 = g/mol (round to 2 decimals)  “mol” is the abbreviation of “Mole”… I know it’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 = 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 RbI 2 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?  grams/mol Rb I is atomic # 53. How much does it weigh?  g/mol I

Step 2: Determine how many of each element you have Look at the formula: RbI 2 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… g/mol g/mol = g/mol 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 g/mol = 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 Na 2 SO 4 ?  Step 1: Find the molar mass. Molar mass = 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 Cu 2 S Stuff = grams Cu  (63.55 g/mol Cu)(2 mol Cu) = 127.1g Cu Total stuff = grams Cu + grams S = g g = g = 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 H 2 O 2 C 3 H 6 N 2 O 3 C 2 H 6 C 3 H 9 Empirical HO CH 2 N 2 O 3 ?

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: H 2 O 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: 1.Calculate % by mass of each element 2.Determine mass of each element Easy if you use 100 g of the chemical 3.Use mass to find the # of moles of each element 4.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 CH C H

CH Percent75%25% 100 g total75g25g Moles 6.2 mol25 mol Ratio == 14 FormulaCH 4

Find the molecular formula Ex: C 3 H 6 O 2 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: C 3 H 6 O 2 mass = (3)(12.01) + (6)(1.01) + (2)(16.00) = 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. C 3x2 H 6x2 O 2x2 = C 6 H 12 O 4