C2.1.3

• Amount of Substance ( n ) is a measure of the number of particles in a substance and is measured in moles
• 1 mol contains 6.02x10^23 particles
• The Avogadro constant ( Nₐ ) is 6.02x10^23 mol-1 and is the number of atoms per mole of the carbon-12 isotope
• A mole is the amount of any substance containing the same amount of particles as there are carbon atoms in 12 grams of the carbon-12 isotope ( 6.02x10^23 particles )
• Molar mass ( M ) is the mass per mole of a substance with the units gmol-1 it is the same as relative molecular mass ( Mr )
• the Molar gas volume ( Vm ) is the volume occupied by one mole of gas molecules at a stated temperature and pressure and is measured in dm3mol-1
• At RTP ( room temperature and pressure ) the molar gas volume is 24 dm3mol-1
• The Empirical formula is the simplest whole number ratio of atoms of each element in a compound
• To calculate the empirical formula:

  1. calculate the moles of each element using moles =( mass or percentage abundance ) / mr
  2. Divide by the smallest moles calculated to find the ratio of elements
  3. Write the formula using the ratio
• To calculate the molecular formula:

  1. find the empirical formula
  2. divide the mr of the compound by the mr of the empirical formula to find the multiplier
  3. multiply the empirical formula by the multiplier (eg CH2O x 6 = C6H12O6)
• An anhydrous compound is a compound that does not contain water molecules
• An anhydrous compound is a compound that does not contain water molecules
• A hydrated compound is a crystalline compound containing water molecules
• A hydrated compound is a crystalline compound containing water molecules
• The water of crystallisation is the water molecules that are bonded into a crystalline structure of a compound
• The water of crystallisation is the water molecules that are bonded into a crystalline structure of a compound
• What is a?

  hydrated copper sulfate

  
• What is b?

  anhydrous copper sulfate

  
• What is c?

  Water

  
• what is the general formula for a hydrated salt ?

  salt ( big dot ) x H2O
• To find the formula of a hydrated salt experimentally:

  1. Find the mass of the hydrated salt
  2. Heat the hydrated salt allowing water to evaporate until you get a constant mass ( weigh , heat , repeat ) Ensure that further decomposition does not occur if heated too long
  3. Find the mass of the anhydrous salt
  4. Find the mass of the water
  5. Calculate the empirical formula by finding the moles of anhydrous salt and water and dividing by the smallest moles calculated
  6. put into a hydrated salt formula
• How do you find the mass of a hydrated salt when finding the formula of a hydrated salt experimentally?

  by subtracting the crucible mass from the combined crucible and salt mass
• How do you find the mass of an anhydrous salt when calculating the formula of a hydrated salt experimentally?

  by weighing the crucible and the anhydrous salt and then subtracting the mass of the crucible
• How do you find the mass of water when calculating the formula of a hydrated salt experimentally?

  Subtract the mass of anhydrous salt from the mass of the hydrated salt to get the mass of the water 
• How long should you heat a hydrated salt for when trying to work out its formula experimentally?

  Until you get a constant mass
• Why should you not heat a hydrated salt for too long when trying to work out its formula experimentally?

  Further decomposition may occur if heated too long
• List the apparatus used when working out the formula of a hydrated salt experimentally:

  • crucible
  • tripod
  • bunsen burner
  • weighing scale
• When finding the empirical formula of a hydrated salt what compounds do you calculate the moles of?

  • The salt
  • water
• The molar mass is the same as the relative molecular mass
• Moles of solid equations:

  • Moles = mass ( g ) / Molar mass ( gmol-1 )
  • moles = mass ( g ) / Mr
• To find no. of molecules in a substance:

  • no. molecules = moles x 6.02x10^23
• To find no. of atoms in a substance:

  • no. atoms = moles x 6.02x10^23 x atoms in the formula (eg. CH4 = 5 atoms )
• Moles equation for gases:

  • moles of gas at RTP = volume ( dm3 ) / 24 ( dm3mol-1 )
  • pV = nRT
• How do you convert cm3 to dm3 ?

  Divide by 1000
• Moles equation for solutions:

  Moles in solution = concentration ( moldm-3 / gdm-3 ) x volume ( dm3 )
• Convert dm3 to cm3

  times by 1000
• How do you convert Mol dm-3 to g dm-3

  Using moles = mass / mr equation
• What is the ideal gas equation?

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

  • pressure
  • measured in Pa
• What does the v stand for in the ideal gas equation?

  • volume
  • measured in m3