Chemical Quantities Introduction Chapter 7 in our textbook Micro World Macro World atoms & molecules grams Atomic mass is the mass of an atom in atomic mass units (amu) By definition: 1 atom 12C “weighs” 12 amu On this scale H = 1.00794 amu O = 15.9994 amu Natural chlorine is: 75.78% 35Cl (34.968853 amu) 24.22% 37Cl (36.965903 amu) Chlorine Gas Average atomic mass of chlorine: (75.78 x 34.968853) + (24.22 x 36.965903) 100 = 35.45 amu Average atomic mass (35.45) Suppose you made a dot with your pencil like the one below, how many carbon atoms would be in the dot? The answer is: Approximately 4 000 000 000 000 000 000 That’s four billion, billion carbon atoms. So how do we handle such large numbers? Paper is sold by the ream. Eggs are sold Pencils are by the dozen. sold by the gross. Shoes are sold by the pair. Beverages are sold by the case. What about atoms? For really large numbers of items we use the MOLE. The mole (mol) is the amount of a substance that contains as many elementary entities as there are atoms in exactly 12.00 grams of 12C 1 mol = 6.022 x 1023 The value, 6.02 x 1023, is called Avogadro’s number in honor of Amedeo Avogadro. In 1811, Avogadro proposed that equal volumes of gas when at the same pressure and temperature have equal numbers of atoms or molecules regardless of the nature of the gas. Avogadro’s Principle Pressure = Pressure Volume = Volume Temperature = Temperature Number of particles = Number of particles The French physicist Jean Perrin in 1909 proposed naming the constant in honor of Avogadro. So just how big is a mole anyway? Let’s do a thought experiment. Suppose you had $6.02 x 1023 ! Can you spend it? What if you start spending the day you are born and you live to be 100? $ 6.02 x 1023 100 yrs = $ 6.02 x 1021 per year $ 6.02 x 1021 365 days = $ 1.65 x 1019 per day $ 1.65 x 1019 24 hrs = $ 6.87 x 1017 per hour $ 6.87 x 1017 60 min = $ 1.14 x 1016 per minute $ 1.14 x 1016 60 sec = $1.91 x 1014 per second Current world population = 6,800,451,819* $1.91 x 1014 6800451819 = $ 28,070 per person *as of Dec 1, 2009 according to the US Census Bureau eggs Molar mass is the mass of 1 mole of shoes in grams marbles atoms 1 mole 12C atoms = 6.022 x 1023 atoms = 12.00 g 1 12C atom = 12.00 amu 1 mole 12C atoms = 12.00 g 12C 1 mole chlorine atoms = 35.45 g of Cl For any element atomic mass (amu) = molar mass (grams) One Mole of: C S Hg Cu Fe Molecular mass (or molecular weight) is the sum of the atomic masses (in amu) in a molecule. 1S 32.07 amu 2O + 2 x 16.00 amu SO2 SO2 64.07 amu For any molecule molecular mass (amu) = molar mass (grams/mol) 1 molecule SO2 = 64.07 amu 1 mole SO2 = 64.07 g SO2 Formula mass is the sum of the atomic masses (in amu) in a formula unit of an ionic compound. 1Na 22.99 amu NaCl 1Cl + 35.45 amu NaCl 58.44 amu For any ionic compound formula mass (amu) = molar mass (grams/mol) 1 formula unit NaCl = 58.44 amu 1 mole NaCl = 58.44 g NaCl Do You Understand How to Find Molar Mass? What is the molar mass of calcium phosphate ? 1 formula unit of Ca3(PO 4) 2 3 Ca 3 x 40.078 Round to 3 decimal 2 P 2 x 30.97376 places in answer 8 O + 8 x 15.9994 310.177 grams/mole Molar Mass Includes both Molecular Mass Formula Mass (Molecules) (Salts) Both labeled with units of: grams/mole Or g/mol for short.