What is the formula weight
and molar mass for alum, KAl(SO_{4})_{2}•
12 H_{2}O

**1. Check
the periodic table for the atomic masses for each atom in the formula**

**2. Count
the number of each type of atom in formula. **

**3. Multiply
the number of atoms by the atomic mass for each element.**

element average atomic mass number of atoms in
formula rounded to nearest
one unit for simplicity potassium
k 39.1 amu 1 = 39. amu aluminum 26.98 amu 1 27. amu sulfur 32.07 amu 2 64. amu oxygen 16.00 amu 8 + 12 = 20 320. amu hydrogen 1.008 amu 2 x 12 = 24 24. amu 474. amu This is the
formula weight.**4. Add
up the masses for all of the elements.**

**Molar
mass is 474 grams. This is a mass in grams that is numerically (474) the same
as the formula weight. **

**1 mole
alum KAl(SO _{4})_{2}• 12 H_{2}O = 474 grams alum
KAl(SO_{4})_{2}• 12 H_{2}O**