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Mole concept, atomic and molecular weight

This lesson on Mole concept, atomic and molecular weight begins by explaining the mole as a concept that refers to a particular number of particles in exactly the same way that a dozen refers to a quantity of twelve, and why the mole is such a useful way of expressing numbers of atoms and molecules in chemistry. It then introduces the Avogadro number and the formula for converting numbers of particles to moles. Worked examples then show how to apply this mole - number of particles formula.
The relative atomic mass, also referred to as the relative atomic weight scale is then explained. Calculating molecular weight also referred to as formula mass or molar mass from the chemical formulae of substances is then explained. The formula for converting mass into moles is then introduced. Several examples then show how to apply it to calculate number of moles in a mass of an element or compound or to calculate mass when number of moles is known.
A worked example is then used to explain how to calculate the mass in kilograms of one atomic mass unit (amu). Further worked examples also explain the calculation of percentage by mass of elements in a compound, and also the determination of average atomic mass (weight) from isotopic abundances and the masses of the isotopes.

To fully understand all concept explanations and the problem solving worked examples, work through lesson contents in sequence. To only review a specific item go directly to its link below.

Lesson contents

Click the links below to jump direct to individual lesson topics or worked examples

1.Mole concept, atomic and molecular weight - Lesson introduction (1m:49s)

Introduction to this lesson explaining the mole concept, relative atomic mass scale, atomic and molecular weight, and calculations involving the Avogadro number, formula mass (molar mass). and composition of compounds.

2.Moles and number of particles (7m:03s)

Explanation of mole concept, Avogadro number and equation relating number of moles to number of particles.

3.Worked example - Converting moles to number of particles (1m:10s)

Applying moles to number of particles conversion formula to calculate number of oxygen particles in a particular number of moles of oxygen.

4. Worked example - Converting number of particles to moles (1m:17s)

Converting number of air particles to moles using number of particles to number of moles conversion formula..

5.Relative atomic mass (weight) scale (2m:59s)

Explanation of the relative atomic mass scale which is also referred to as the relative atomic weight scale. Examples of expressing formula masses or molecular weights in terms of the relative atomic weight scale.

6.Moles and masses of substances (8m:37s)

Explanation of why formula mass in grams of a substance is one mole of that substance, and the equation relating number of moles to mass of a substance and its formula mass. Definition of atomic mass unit (amu).

7.Worked example - Converting mass to moles (1m:08s)

Calculating number of moles of zinc in 1.38 g of zinc by applying mass to moles conversion formula.

8.Worked example - Converting moles to mass (1m:16s)

Applying the moles to mass conversion formula to calculate the mass of 1.5 mole of carbon dioxide.

9.Worked example - Calculating the mass of one amu in kilo grams (3m:00s)

Using Avogadro number and the definition of atomic mass unit (amu) to calculate mass of one amu in kg.

10.Worked example - Percentage mass of an element in a compound (4m:38s)

Calculating percentage by mass of nitrogen in two compounds to find out which of them is richer in nitrogen.

11.Worked example - Calculating atomic weight from isotopic abundance (4m:43s)

Calculating average atomic weight of boron from isotopic abundance (percentage abundance of each isotope) of boron and the precise atomic mass of each isotope.

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