Amount of Substance (DP IB Physics)

Amount of Substance

• The mole is one of the seven SI base units

  • It is used to measure the amount of substance, not a mass

• One mole is defined as follows:

  The amount of substance that contains as many elementary entities as the number of atoms in 12 g of carbon-12

• This amount of substance is exactly 6.02214076 × 10²³ elementary entities (i.e. particles, atoms, molecules)

  • At IB level, this number can be rounded to 6.02 × 10²³

• One mole of gas contains a number of particles (atoms or molecules) equal to the Avogadro Constant

• For example, 1 mole of sodium (Na) contains 6.02 × 10²³ atoms of sodium

• The number of atoms can be determined if the number of moles is known by multiplying by N_A.

  • For example: 2.0 mol of argon contains:  2.0 × N_A = 2.0 × 6.02 × 10²³ = 1.20 × 10²⁴ atoms

• The number of moles, n of a substance can be calculated using the equation

• Where:

  • N = number of particles (molecules or atoms, depending on the substance)

  • n = number of moles of gas (mol)

  • N_A = 6.02 × 10²³ mol^–1 (Avogadro constant)

Molar Mass

• One mole of any element is equal to the relative atomic mass of that element in grams

  • E.g. Helium has a relative atomic mass of 4 - this means 1 mole of helium has a mass of 4 g

• If the substance is a compound, add up the relative atomic masses, for example, water (H₂O) is made up of

  • 2 hydrogen atoms (each with atomic mass of 1) and 1 oxygen atom (atomic mass of 16)

  • So, 1 mole of water would have a mass of (2 × 1) + 16 = 18 g

• The molar mass, m of a substance is defined as the mass m of the substance divided by the amount (in moles) of that substance

• The molar mass is calculated as follows:

• Where:

  • m_r = molar mass in g mol^–1

  • m = mass in grams (g)

  • n = number of moles (mol)