Unit 8: The Mole

Formula weight vs. Molecular weight  Formula weight – sum of the atomic weights of each atom in its chemical formula  The formula weight of sulfuric acid (H 2 SO 4 ) is 98.1 amu  If the chemical formula is that of a molecule, then the formula weight is also called the molecular weight  The molecular weight of glucose (C 6 H 12 O 6 ) is amu

Percentage Composition from Formulas  To find the percentage of each element that is in a compound, the following equation is used: % element = (number of atoms of that element)(atomic weight of element) formula weight of compound x 100

The Mole (the real mole, not the chemistry one

The Mole  The term, mole, is used to express the number of atoms, ions, or molecules (mol)  A mole is the amount of matter that contains as many objects (atoms, molecules, or whatever objects we are considering) as the number of atoms in exactly 12 g of 12 C  From experiments, scientists have determined that number to be x 10 23

Avogadro’s Number  The number, x is called Avogadro’s number  This number too large to imagine  Spreading 6.02 x marbles over the entire surface of the Earth would produce a layer about 3 miles thick  If you lay 6.02 x pennies side by side in a straight line, they would encircle the Earth 300 trillion (3 x ) times

Molar Mass  What is molar mass?  Molar mass is the amount of the mass (in grams) of 1 mol of a certain element  Cl has a molar mass of 35.5 g/mol  NaCl has a molar mass of 58.5 g/mol  Molar mass is always numerically equal to the formula weight

Mole Liters of a gas particle Mass (grams) 22.4 L 1 mol 22.4 L 6.02 x particle 1 mol 6.02 x particle 1 mol MM 1 mol

Empirical Formula  Shows the relative number of atoms of each element in a formula  Steps to determine empirical formula: 1. If given % of element, change the % to g 2. Determine the moles of each element (divide by the MM) 3. Divide by the lowest mole value

Empirical Formula Problem  Ascorbic acid (vitamin C) contains 40.92% C, 4.58% H, and 54.50% O by mass. What is the empirical formula of ascorbic acid?

Molecular Formula  Molecular formula can be obtained only if you are given the molecular weight  The subscripts in the molecular formula of a substance are always a whole number multiple of the corresponding subscripts in its empirical formula Whole-number multiple = molecular weight empirical formula weight

Molecular Formula Problem  What is the molecular formula of ethylene glycol, substance used in automobile antifreeze, which is composed of 38.7% C, 9.7%H, and 51.6% O by mass?