What is the formula weight and molar mass for alum, KAl(SO4)2• 12 H2O
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.
4. Add
up the masses for all of the elements.
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.
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(SO4)2• 12 H2O = 474 grams alum KAl(SO4)2• 12 H2O