Included in the presentation is the step-by-step computation of the percentage composition

1. 1. To understand the definition of molar mass
2. To learn to convert between moles and mass
3. To learn to calculate the mass percent of an element
in a compound
Objectives
Molar Mass and Percentage
Composition

2. A. Molar Mass
 A compound is a collection of atoms bound together.
• The molar mass of a compound is obtained by summing
the masses of the component atoms.

3. A. Molar Mass
 For compounds containing ions, the molar mass is
obtained by summing the masses of the component
ions.

4.  Mass in grams of one mole of the substance:
Molar Mass of N = 14.01 g/mol
Molar Mass of H2O = 18.02 g/mol
(2 × 1.008 g) + 16.00 g
Molar Mass of Ba(NO3)2 = 261.35 g/mol
137.33 g + (2 × 14.01 g) + (6 × 16.00 g)

5. A. Molar Mass
 Moles of a compound =
Calculations Using Molar Mass
• Mass of a sample (g) = (moles of sample)(molar mass of
compound)

6. Concept Check
Calculate the number of copper atoms in a
63.55 g sample of copper.

8. Percentage
composition
Indicates the relative amount of each
element present in a compound.

9. Calculating percentage
composition
 Step 1 : Calculate molar mass
 Step 2 : Divide the subtotal for each element’s
mass by the molar mass.
 Step 3: Multiply by 100 to convert to a
percentage.

10. Example 1
 1. Calculate the molar mass of water, H2O.
 Step 1 – H- 2(1.01)= 2.02
 O – 1(16.0)= 16.0
 ________
 18.02

11.  Step 2 2.02/18.02 X 100 = 11.21%
 16.0 /18.02 X 100 = 88.79 %
 Water is composed of 11.21 % hydrogen and
88.79 % oxygen.

12. Example 2
 Calculate the percentage composition of Sodium
chloride , NaCl.

13. Example 3
 Find the percentage composition of hydrogen in
sulfurous acid.

14. Example 4
 Find the percentage composition of Manganese
in Potassium Permanganate.