3 - amount of substance and the mole

• Mole

  One mole is the amount of a substance that contains the same number of particles (6.02 x 10²³ particles.) as there are atoms in 12 grams of carbon-12.
• What does the amount of a substance or moles refer to


  The amount of a substance and moles refer to anything not just atoms. When you work in moles, it is important to use the formula or unambiguous name of a substance for clarity.

  
  

  E.g. 1 mol of H: 1 mol of hydrogen atoms

  1 mol of H₂: 1 mol of hydrogen molecules
• Molar mass


  The mass, in grams of one mole of a substance and is given the unit g mol⁻¹. E.g. carbons molar mass is 12 (same as Ar)
• Equation linking amount of substance n, mass m, and molar mass M.

  amount n = mass m / molar mass M

  
  

  n = m/M
• Molecules


  Two or more atoms held together by covalent bonds.
• Molecular formula


  The number of atoms of each element in a molecule.
• Empirical formula


  The empirical formula is the simplest whole-number ratio of atoms of each element in a compound.
• Relative molecular mass


  Mr compares the mass of a molecule with the mass of an atom of carbon-12.

  
  

  E.g. Mr(H₂O) = (1.0x2) + 16.0 = 18.0
• Relative formula mass


  Compares the mass of a formula unit with the mass of an atom of carbon-12.
• How would you carry out an experiment to determine the water of crystallisation in hydrated crystals?


  Step 1: Weigh an empty crucible

  
  

  Step 2: Add the hydrated salt into the weighed crucible. Weigh the crucible and hydrated salt.

  
  

  Step 3: Using a pipe-clay triangle, support the crucible containing the hydrated salt on a tripod. Heat the crucible and contents gently for about 1 minute. Then heat strongly for a further 3 minutes.

  
  

  Step 4: Leave the crucible to cool. Then weigh the crucible and anhydrous salt.
• Water of crystallisation


  Water molecules that form part of the crystalline structure of an ionic compound (salt).
• How to convert dm³ to cm³

  Multiply by 1000
• Solute


  The dissolved compound.
• Concentration


  The amount of solute, in moles, dissolved in each 1dm³ (1000cm³) of solution.
• Equation for calculating concentration of a solution


  Concentration = no. of moles / volume (in dm³)

  
  

  c = n / V (in dm³)
• Standard solution


  A solution of known concentration.
• How to prepare a standard solution?


  Dissolve and exact mass of the solute in a solvent and make up the solution to an exact volume.
• How to calculate the mass of a solute required to prepare a standard solution of specific concentration and volume?


  Step 1: Work out the amount, in moles,of solute required (n=cV).

  
  

  Step 2: Work out molar mass of solute.

  
  

  Step 3: Rearrange n=m/M to calculate the mass of solute required.
• Molar gas volume

  Gas volume per mole, units dm^3 mol^-1. The volume of one mole of a gas at a given temperature and pressure.
• What is the volume, at RTP, of 1 mole of gas molecules?


  24.0 dm³ = 24,000 cm³
• How to calculate the volume of a gas at RTP from the number of moles?


  no. of moles x 24
• Equation to convert between the number of moles and gas volume


  amount n (mol) = volume V / molar gas volume Vm.
• What is the molar gas volume at RTP?


  24.0 dm³ mol⁻¹
• What are the assumptions made for the molecules making up an ideal gas?


  1. random motion

  2. elastic collisions

  3. negligible size

  4. no intermolecular forces
• Ideal gas equation


  pV=nRT
• What does p stand for in the idea gas equation?


  pressure
• What does V stand for in the idea gas equation?


  Volume
• What does n stand for in the ideal gas equation?


  amount of gas molecules
• What does R stand for in the ideal gas equation?


  Ideal gas constant
• What does T stand for in the ideal gas equation


  Temperature
• What is pressure measured in in the ideal gas equation?


  Pascals (Pa)
• What is volume measured in in the ideal gas equation?


  m³
• What is the amount of gas molecules measured in in the ideal gas equation?


  Moles (mol)
• What is temperature measured in in the ideal gas equation?


  Kelvin (K)
• °C to K

  + 273
• kPa to Pa


  x 10³
• Stoichiometry


  The molar ratio between the substances taking part in a reaction. I.e. the big numbers used when an equation is balanced.
• Theoretical yield


  The maximum possible amount of a product (determined by the molar ratio of the balanced equation)
• Why might the theoretical yield not be produced?

  1. The reaction may not have gone to completion

  
  

  2. Other reactants (side reactions) may have taken place alongside the main reaction.

  
  

  3. Purification of the product may have resulted in loss of some of the product.
• Actual yield


  The measured amount of a product obtained from a reaction, calculated by using n=m/M (typically lower than the theoretical yield).