Moles COUNTING BY WEIGHING

Moles (is abbreviated: mol)  It is an amount, defined as the number of carbon atoms in exactly 12 grams of carbon-12.  1 mole = 6.02 x of the representative particles.  Treat it like a very large dozen  6.02 x is called: Avogadro’s number. 2

Practice problems (round to 3 sig. figs.)  How many molecules of CO 2 are in 4.56 moles of CO 2 ?  How many moles of water is 5.87 x molecules?  How many atoms of carbon are in 1.23 moles of C 6 H 12 O 6 ?  How many moles is 7.78 x formula units of MgCl 2 ? 3

Measuring Moles  Remember relative atomic mass? - The amu was one twelfth the mass of a carbon-12 atom.  Since the mole is the number of atoms in 12 grams of carbon-12, the decimal number on the periodic table is also the mass of 1 mole of those atoms in grams. 4

Gram Atomic Mass (gam)  Equals the mass of 1 mole of an element in grams (from periodic table)  grams of C has the same number of pieces as grams of H and grams of iron.  We can write this as: g C = 1 mole C (this is also the molar mass)  We can count things by weighing them. 5

Examples  How much would 2.34 moles of carbon weigh?  How many moles of magnesium is g of Mg?  How many atoms of lithium is 1.00 g of Li?  How much would 3.45 x atoms of U weigh? 6

What about compounds?  in 1 mole of H 2 O molecules there are two moles of H atoms and 1 mole of O atoms (think of a compound as a molar ratio)  To find the mass of one mole of a compound determine the number of moles of the elements present Multiply the number times their mass (from the periodic table) add them up for the total mass 7

Calculating Formula Mass Calculate the formula mass of magnesium carbonate, MgCO g + 12 g + 3 x (16.00 g) = 84.3 g Thus, 84.3 grams is the formula mass for MgCO 3. 8

Molar Mass  Molar mass is the generic term for the mass of one mole of any substance (expressed in grams/mol)  The same as: 1) Gram Molecular Mass (for molecules) 2) Gram Formula Mass (ionic compounds) 3) Gram Atomic Mass (for elements) molar mass is just a much broader term than these other specific masses 9

Since Molar Mass is…  The number of grams in 1 mole of atoms, ions, or molecules,  We can make conversion factors from these. - To change between grams of a compound and moles of a compound. 10