Chapter 4: Relative Atomic Mass & the Mole

These ‘chapter’ posts are not complete summaries or sets of notes. They are simply collections of supplementary resources that I recommend using in the classroom in addition to the textbook and any other assignments on the course. Most of the resources here are videos, but I will also occasionally put worksheets, quizzes, books, apps and games here as well.

4.1: Masses of particles

Watch the following introductory videos on relative atomic mass.

You no longer need to know the workings of a mass spectrometer because it’s been deleted from the VCE Chemistry study design. However, many schools still use older worksheets, which ask you to label the function of each part of the machine. Ask your teacher whether the mass spectrometer machine will be examined in your school’s Year 11 examination.

Mass spectrometer
Source: Piercenet.com

Even though knowledge of the mass spectrometer is not required, understanding how it works can help students to interpret the kinds of graphs that it produces. This video explains how the mass spectrometer works (beyond VCE level).

Here are some sample questions for you to try.

4.2: The mole

We have several ‘number-words’ in English. Examples include ‘pair’, which means ‘2’; ‘dozen’, which means ’12’, and ‘ream’, which means ‘500’. A mole is a very large number-word with a value of 6.02×1023.

The mole is also called ‘Avogadro’s number’.

Most important formula here is n = N ÷ NA

  • n is the number of moles in your sample;
  • N is the number of particles (either molecules or atoms);
  • NA is Avogadro’s number (6.02×1023).

Click here for the lesson that accompanies the above video.

4.3: Molar mass

Relative molar mass is the mass of a molecule, formula, isotope or atom compared to the molar mass of carbon-12, which has a molar mass of exactly 12.

The following are all forms of relative molar mass and have no units:

  • relative molecular mass (used for molecules)
  • relative formula mass (used for repeating structures such as crystals)
  • relative isotopic mass (used for single isotopes)
  • relative atomic mass (used for single elements that contain mixtures of isotopes)

Most important formula here is n = m ÷ M

  • n is the number of moles of particles in your sample;
  • m is the mass of the sample in grams;
  • M is the relative molar mass of your sample.

4.4: Percentage composition

Use the m-n-ratio method to find the percentage composition of any particular compound. A demonstration is shown below.

Read: Heinemann Chemistry 1 Chapter 4

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