1 / 29

Chapter 6: Moles, Molar Mass, Percent Composition and Formulas

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?

phiala
Download Presentation

Chapter 6: Moles, Molar Mass, Percent Composition and Formulas

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 6: Moles, Molar Mass, Percent Composition and Formulas From moles to mass and to the moon!

  2. 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???

  3. 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.

  4. 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.

  5. 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?

  6. 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!

  7. 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!

  8. 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

  9. 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:

  10. 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.

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

  12. 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!!!

  13. 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

  14. 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

  15. 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

  16. 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

  17. Step 2: Determine how many of each element you have Look at the formula: RbI2 We have 1 “Rb” atom and 2 “I” atoms

  18. 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”

  19. 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.

  20. 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.

  21. 6.3 Formulas of Compounds • Calculate “percent composition” • Just like any other % • Stuff = grams of elements

  22. 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

  23. 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 ? ?

  24. 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

  25. 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 #

  26. 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

  27. 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 #.

  28. 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

More Related