Moles and Molecules
A molecule is the smallest possible unit of a compound. It consists of two or more atoms chemically
bonded together.
A molecule of hydrogen fluoride, HF, is made up of 1 hydrogen atom bonded to 1 fluorine atom.
Since a hydrogen atom weighs 1.0 amu and a fluorine atom weighs 19.0 amu, 1 molecule of HF
weighs 1.0 amu + 19.0 amu = 20.0 amu.
A molecule of carbon tetrafluoride, CF4 , is made up of 1 carbon atom bonded to 4 fluorine atoms.
Each carbon atom weighs 12.0 amu and each fluorine atom weighs 19.0 amu so 1 CF4 molecule
weighs 1×12.0 amu + 4×19.0 amu = 88.0 amu. This number, 88.0, is called the molecular weight (or
molecular mass) of CF4. It is obtained by adding the atomic weights of all the atoms in the
compound.
The molecular weight of a compound is the sum of all the atomic weights in a molecule. Just like
atomic weights are the relative masses of atoms relative to carbon-12, which is assigned a mass of
12.00000, molecular weights are relative masses of molecules relative to the mass of a carbon-12
atom. For example, the molecular weight of C2H5Cl is 2×12.01 + 5×1.008 + 1×35.45 = 64.51 .
One mole of molecules is 6.022x1023 molecules. One mole of HF is 6.022x1023 molecules of HF.
Since each HF molecule (weighing 20.0 amu) is 20.0 times as heavy as a hydrogen atom ( weighing
1.0 amu), 1 mole of HF molecules must weigh 20.0 times as much as 1 mole of H atoms. Since 1
mole of H atoms weigh 1.0 g, then 1 mole HF molecules must weigh 20.0 g.
So just like for atoms: 1 mole H = 6.022x1023 atoms H = 1.0 g H ( the atomic weight in grams)
1 mole HF = 6.022x1023 molecules HF = 20.0 g HF ( the molecular weight in grams)
Likewise 1 mole CF4 = 6.022x1023 molecules CF4 = 88.0 g CF4
Also 1 mole CF4 = 6.022x1023 molecules CF4 = 6.022x1023 atoms C + 4×6.022x1023 atoms F, since
each molecule of CF4 contains 1 atom of C and 4 atoms of F.
So 1 mole CF4 = 1 mole C + 4 moles F = 12.0 g C + 4×19.0 g F = 88.0 g CF4
Try the following problems:
1. 3 molecules of CF4 contains how may atoms of carbon and how may atoms of fluorine?
2. 3.00 moles CF4 contains how many moles of fluorine and how many grams of fluorine?
3. 3.00 grams CF4 contains how many molecules of CF4 and how many grams of carbon?
4. 3.00x1022 molecules of SnF4 contain how many grams of SnF4 and how many moles of fluorine?
5. 3.00 g IF7 contain how many atoms of I, how many moles of IF7, and how may grams of F?
Answers:
1. 3 atoms C , 12 atoms F 2. 12.0 mol F , 228.0 g F
3. 2.05x1022 molecules CF4 , 0.409 g C 4. 9.70 g SnF4 , 0.199 mol F
5. 6.95x1021 atoms I , 0.0115 mole IF7 , 1.54 g F
