Chapter 3 - Amounts of Substance

• Significant Figures: The number of digits in a value, often a measurement, that contribute to the degree of accuracy of the value
• Relative Molecular Mass: The sum of the protons and neutrons across the entire molecule
• Relative Formula Mass: The sum of the protons and neutrons across the ionic formula of a salt
• Amount of Substance: The number of particles in a substance. This has the unit mole. 1 mole is the amount of substance that contains 6.02 x 10^23 particles.
• Avogadro Constant: The number of particles in each mole of carbon 12 - 6.02 x10^23. For atoms 1 mole of a substance = Relative atomic mass. For molecules 1 mole of a substance = Relative molecular mass
• Molecular formula: The actual number of each atom of each element within a molecule
• Empirical formula: The simplest whole number ratio of atoms in a molecule
• Water of crystallisation: Water molecules that are bonded into a crystalline structure of an ionic compound
• Sources of inaccuracy

  • Some water may remain locked in deep within the structure

  • Structure maybe undergo thermal decomposition during heating
• Moles and gases:

  • particles should be the same size
  • More spaced out at higher temperatures
  • So 1 mole of a gas will occupy more space at a higher temperature
• Molar Volumes

  • 1 mole of any gas at the same temperature and pressure occupies the same volume – it does not depend on the substance’s relative molecular mass.

  • 1 mole at 0 °C and 1 atm occupies 22.4 dm3

  • 1 mole at 25 °C and 1 atm occupies 24.0 dm3

  • This can be monoatomic atoms like helium or molecules like carbon dioxide
• Molar volumes

  • 24 dm3 =1 mole = Relative atomic mass so helium would be 4 g

  • 24 dm3 =1 mole = Relative molecular mass so ammonia would be 17 g
• Ideal Gas Equation

  • pV = nRT
  • p is the pressure measured in pascals (Pa)
  • V is the volume in m3
  • n is the number of moles of the gas
  • R is the molar gas constant and has a value of 8.31 Jmol-1K-1
  • T is the absolute temperature, measured in Kelvin (K)
• Stoichiometrics: The ratio, of the amount in moles, of each substance in a chemical reaction
• Percentage Yields

  It is difficult to achieve a reaction that has a percentage yield of 100%

  • The reaction may not have gone to completion
  • Side reactions
  • Purification of the product may have result in loss of product
• How to calculate percentage yields

  Step 1: Calculate the number of mols of the reactant not in excess, this is the limiting reagent

  Step 2: Calculate the theoretical number of mols of product. (This is the theoretical yield)

  Step 3: Calculate the actual number of mols of product. (This is the actual yield)
• Atom Economy Definition

  A measure of how well atoms have been utilised

  Reactions with high atom economy:

  • Produce a large proportion of the desired products and few unwanted waste products
  • Are important for sustainability as they make the best use of natural resources