Name: _________________________________________________
Molar Masses
The molar mass of a substance is the mass of one mole (6.022 × 1023 units) of the substance. The mass in grams
of one mole of an element’s atoms is equal to the numerical value of that element’s atomic weight.
These values for each element can be found on the periodic table.
To find the molar mass of a substance, multiply the number of moles of each element (indicated by the subscript
in the formula) by the molar mass of the element. Then add the masses of the various elements.
Example 1: What is the molar mass of iron(III) oxide, Fe2O3?
(1 mole Fe2O3 = 2 moles Fe + 3 moles O)
2 mol Fe =
2 × 55.85 g
=
111.7 g Fe
3 mol O =
3 × 16.00 g
=
48.00 g O
Molar Mass of Fe2O3 = (111.7 g Fe + 48.00 g O) = 159.7 g Fe2O3
Example 2: What is the molar mass of magnesium hydroxide, Mg(OH)2?
1 mol Mg =
1 × 24.31 g =
24.31 g Mg
2 mol O =
2 × 16.00 g =
32.00 g O
2 mol H =
2 × 1.008 g =
2.016 g H
Molar Mass of Mg(OH)2 = (24.31 g + 32.00 g + 2.016 g) = 58.3 g Mg(OH)2
Determine the molar mass of each substance. Round each answer to the nearest tenths place. Show your work,
including the original numbers you are using from the periodic table in your calculations.
1. fluorine gas: F2
2. copper (II) chloride: CuCl2
3. aluminum oxide: Al2O3
4. potassium hydroxide: KOH
Cont. on back side…
Name: _________________________________________________
5. sodium bicarbonate: NaHCO3
6. glucose: C6H12O6
7. calcium phosphate: Ca3(PO4)2
8. ammonium sulfate: (NH4)2SO4
9. malachite: Cu2CO3(OH)2
10. magnesium phosphate: Mg3(PO4)2