IB Chemistry Sub-topic S1.4 Notes

The mole

In reactions, substances are often measured in moles (n). This is a measure of the number of particles given by Avogadro’s number (N_A) equal to 6.022 x 10²³ particles/mole. It relates to the number of particles that would make Carbon-12 exactly 12 grams as the standard element. Therefore, just like one million is 1,000,000, one mole is 6.022 x 10²³ particles. 

Note that this describes the number of particles so 1 mole of any diatomic molecule, such as H₂, has 1.2044 x 10²⁴ atoms because every molecule (and therefore particle) has two component atoms.

The mass of an atom can be expressed as molar mass (M), the mass of one mole of a substance in gmol^-1. The most often used one is the relative atomic mass (A_r), the weighted mean molar mass of all naturally occurring isotopes of the element relative to Carbon-12. This is the case due to Avogadro's number.

Calculating molar mass is possible with the equation:

$\textrm{molar mass (M)} = \frac{\textrm{mass (m)}}{\textrm{moles (n)}}$

Formula Mass

The relative atomic mass of each element is then used in further calculations. These often involve calculating the molar mass of a molecule. This is referred to as the relative formula mass (M_r), is the sum of the relative atomic masses of its components. For example, the relative atomic mass of hydrogen is 1.01 gmol^-1 and of carbon is 12.01 gmol^-1 so the relative formula mass of C₂H₆ is 30.08 gmol^-1, as shown below!

Formulas

Lastly, when forming molecules and compounds, it is important to understand how to correctly communicate them. This can be done via three forms of formulas: molecular, empirical, or structural.

1. Molecular formulae state the compound's atomic composition in the exact ratio they appear. For ethane, this is C₂H₆.
2. Empirical formulae state the compound's atomic composition in the most simplified ratio. For ethane, the ratio is simplified to CH₃.
3. Structural formulae show the compound's atomic composition and the physical arrangment of the atoms in space.