Section 6.2

Molar Mass and Percent Composition

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

Section 6.2

Molar Mass and Percent Composition

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.

Section 6.2

Molar Mass and Percent Composition

A. Molar Mass

• For compounds containing ions the molar mass is obtained by summing the masses of the component ions.

Section 6.2

Molar Mass and Percent Composition

A. Molar Mass

• Moles of a compound =

Calculations Using Molar Mass

mass of the sample (g)gmolar mass of the compound ( )

mol

• Mass of a sample (g) = (moles of sample)(molar mass of compound)

Section 6.2

Molar Mass and Percent Composition

Objective:1. Calculate the percent composition of a given

compound given the formula or experimental data.

Section 6.2

Molar Mass and Percent Composition

B. Percent Composition of Compounds

*Remember The law of constant composition• Any given compound always contains the same

elements in the same proportions by mass

For example:Carbon dioxide (CO2) contains:

1- carbon atom2 – oxygen atoms

• Always a 2:1 ratio of oxygen:carbon

Section 6.2

Molar Mass and Percent Composition

B. Percent Composition of Compounds

• Compounds always have the same # and type of each atoms and also have the same proportions of each element by mass

• Percentage Composition – mass of each element in a compound relative to the total mass of the compound.

mass of a given element in 1 mol of compound 100%mass of 1 mol of compound

Section 6.2

Molar Mass and Percent Composition

For example: The Molar mass of carbon dioxide is…C 12.011 g/molO 15.9994 g/molO 15.9994 g/mol 44.010 g/mol

What is the percentage of carbon in CO2?

What is the percentage of oxygen in CO2?

Section 6.2

Molar Mass and Percent Composition

•Finding Percentage Composition from experimental data

A sample of 2.45 g of aluminum oxide decomposes into 1.30 g of aluminum and 1.15 g of oxygen. What is the percentage composition of each element in the compound?

Section 6.2

Molar Mass and Percent Composition

B. Percent Composition of Compounds

• Percent composition consists of the mass percent of each element in a compound:

mass of a given element in 1 mol of compound 100%mass of 1 mol of compound

Mass percent =