Unit 3 Lesson More Moles

mole – The amount of a substance that contains the same number of particles as the number of atoms in 12 g of carbon-12.

The mole is a counting unit, like a dozen.

Avogadro’s # = 6.022 x 10²³

1 mole of carbon-12 = 6.022 x 1023 atoms of carbon-12 = 12 g of carbon-12

The mass (in grams) of one mole of an element.

So the molar

mass of carbon-12 is 12 g.

(If it were a compound, instead of just an element, you simply add

up the masses of all the atoms in the formula.)

To find the molar mass of a compound, simply add up the masses of all the atoms in the formula, and give the total mass (in grams).

For example, the molar mass of H2O = 18.0 g.

To find the percentage composition of an element in a compound, first find the molar mass of the compound.

Then divide the element mass by the compound mass.

(Think of it this

way: What percentage of the total mass of the compound comes from that specific element?)