1. Moles and Calculating Molar Mass

2. The mole is the S.I. unit for the amount of a substance. A mole is the amount of a substance that contains as many particles as there are atoms in exactly 12 g. of carbon-12.

3. The mole is a counting unit, just like a dozen is. We don’t usually order 12 or 24 ears of corn; we order 1 dozen or 2 dozen. Similarly, a chemist may want 1 mole of carbon, or 2 moles of iron.

4. The number of atoms in a mole is known as Avogadro’s number. Avagadro’s number, 6.022 X 10 23, is the exact number of atoms in exactly 1 mole of a pure substance.

5. The mass of 1 mole of a pure substance is the molar mass of that substance. Molar mass is written in units of g/mol.

6. For example, 4 g. of Helium, 6.94 g. of Lithium, and 200.59 g. of Mercury all contain a mole of atoms. 1 mole of sulfur 1 mole of Potassium dichromate 1 mole of graphite

7. Calculating Molar Mass Molar Mass (also referred to as formula weight)- is the mass of one mole of any substance. The amount is equal to the molecular mass of the substance BUT the unit is grams.

8. To calculate the molar mass, multiply the number of atoms of each element by that elements atomic mass and add them together. Example: NaCl 2 Na (23 g/mol) + Cl 2 (2)(35.4 g/mol) = 93.8 g/mol

9. Example Problem 1: MgCl 2 Mg (24.3 g/mol) + Cl 2 (2)(35.4 g/mol) = 95.1 g/mol

10. Example Problem 2 What is the molar mass of barium nitrate, Ba(NO 3 ) 2

11. Example Problem 3 What is the molar mass of Al 2 S 3 ?