Molar Mass

• Compare the weight of 12 apples and 12 grapes. The apples would weigh considerable more. It is the same for elements. A mole of carbon is considerable lighter than one mole of copper.
• Scientists use the mole to relate to the atomic weight on the periodic table. If we say that Hydrogen has an atomic weight of 1.01 g, what does this mean? How much Hydrogen?
• The mass of one mole of any pure substance is called its molar mass.
• For example:
• Carbon has an atomic weight of 12.01 g. This means that 1 mole of Carbon atoms (or 6.022 x 10²³ atoms of Carbon) have a mass of 12.01 g.
• Mercury has an atomic weight of 200.59 g. This means that 1 mole of Mercury has a mass of 200.59 g.  
• Example problem: Calculate the mass of 0.5000 mol of Helium.
• 0.5000 mol He x 4.003 g He/1 mol He = 2.002 g
• Example problem: How many copper atoms are in a 4.00 g sample of pure copper wire.
• First, we need to find the number of moles.
• 4.00 g Cu x 1 mol Cu/63.55 g Cu = 0.0629 mol Cu
• Second, using Avogadro’s number, we can find the number of atoms.
• 0.0629 mol Cu x 6.0022 x 10²³ atoms Cu/1 mol Cu = 3.79 x 12²² atoms Cu