Mole Conversions
Sample Problem - - (moles to grams)
Calculate the mass in grams in 5.00 moles of sulfur, S8 (molecules)
Solution
Since there are 8 atoms of sulfur in each molecule of sulfur (S8), we know there are
8 moles of S (atoms) in each mole of sulfur molecules (S8) and so we multiply the atomic mass of sulfur
(32.07) by 8 to get the molar mass of sulfur (S8).
5.00 moles S8
256.56 g S8
= 1,280 g S8
= 1,282.8 g S8
1 moles S8
Sample Problem - - (moles to atoms [or molecules])
Calculate the number of atoms in 5.00 moles of Lithium
Solution
Since we are going from moles to number of molecules, we use Avogadro’s number
5.00 moles Li
6.022 x 1023 atoms
Li Li
1 moles
= 30.11 x 1023 atoms
= 3.01 x 1024 atoms Li
Sample Problem - - (grams to molecules [or atoms])
Calculate the number of molecules in 5.00 grams of Iodine,I2
Solution
Since we are going from mass to number of molecules, we must first use the molar mass, and then use
Avogadro’s number. Also, since there are 2 atoms of iodine in each molecule, we multiply the atomic mass
of iodine (127) by 2 to get the molar mass of iodine (I2). Memorize the diatomic elements. HNO-halogen
5.00 grams I2
grams
1 moles I2 6.022 x 1023 molecules
1 moles I2
253.8 g I2
molar mass
moles
= 0.119 x 1023
Avogadro’s No.
= 1.19 x 1022 molecules I2
atoms/molecules
You will always have Avogadro’s number available to you so you don’t have to memorize it.
Memorize This:
1. Use Avogadro’s number ONLY when converting to or from atoms (or molecules).
2. Use the Molar Mass when converting between grams and moles.