3.1.2 - Amount of Substance

• Relative Atomic Mass: The average mass of one atom of an element relative to 1/12th the mass of one atom of C-12
• Relative Formula Mass: (A1 x M1) + (A2 x M2) / Total Abundance
• Ideal Gas Equation:

  
  

  PV = nRT
• Empirical formula = The simplest ratio of atoms of each element in a compound
• The mole is the amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon-12
• Alkali is usually in the conical flask in titration
• What is the main safety precaution in titration? Wear eye protection and gloves (acids and alkalis are corrosive)
• Why do we use a conical flask over a regular beaker? It is easier to swirl the mixture without spilling the contents
• Water does not affect the titration as it doesn't change the number of moles of each reactant
• How many titrations should we carry out? (at least) 3
• What are the colour changes for phenolphthalein and methyl orange?

  
  

  Phenolphthalein [pink (alkaline) to colourless (acid)]

  
  

  methyl orange [yellow (alkali) to red (alkali)]
• Using phenolphthalein vs methyl orange:

  • Use Phenolphthalein if NaOH is used
  • Use Methyl Orange if HCl is used
• How can you reduce uncertainties in titration?

  1. Replacing measuring cylinders with pipettes / burettes which have lower apparatus uncertainty
  2. Increasing the volume and concentration of the substance in the conical flask to increase the titre volume
• White tiles are used during titrations because you can more easily see the colour changes and end points
• Empirical formula: The simplest whole number ratio of atoms of each element in a compound
• Theoretical yield: The mass of product that is calculated and should have formed in a reaction. This is calculated using masses of reactants and a balanced equation.
• How do you calculate percentage yield? Percentage Yield = Actual Yield / Theoretical Yield x 100
• Percentage yield indicates how wasteful a process is, as it determines how much of the product is lost
• Atom economy: A way determining how wasteful a reaction is. Atom economy is a measure of the proportion of reactant atoms that become part of the desired product, rather than by-products
• Reasons why high atom economy is beneficial:

  • High atom economy is desirable for industrial processes.
  • They are better for the environment as they produce less waste
  • More efficient use of raw material means high atom economy is sustainable
• How can we remove water of crystallisation in compounds? by heating, they can be removed as water vapour
• Errors involved in using a gas syringe:

  • gas escapes before bung inserted
  • syringe sticks
  • some gases (e.g. CO2) are soluble in water so true amount isn't measured
• Calculating volumes of gases reacting:

  • equal volumes of gases under same conditions will have equal moles
  • so the, ratio of volumes = ratio of moles
• Steps to work out what reactant is in excess:

  1. Calculate amount of each reactant in mols
  2. Use balanced equation to work out which is in excess 
  3. use the limiting reactant for calculations
• Avogadro constant: Number of particles/atoms/ions in one mole of a substance
• Equation that links mass of 1 mol, mass of 1 atom and Avogadro constant:

  
  

  Mass of 1 mol = mass of 1 atom/molecule X Avogadro constant
• Percentage yield: The % of a product produced by a reaction, compared to a theoretical maximum
• Calculate percentage yield: Mass of useful product ÷ expected mass of useful product
• What can the percentage yield of a practical be used to investigate? Efficiency of practical techniques and whether reactions proceed as estimated
• Atom economy: % of amount of reactants made into a certain (useful) product
• What can the atom economy of a reaction be used to investigate? Efficiency of using a specific reaction to produce a product
• How do you convert between K and C temperatures?

  °C to K + 273

  K to °C - 273
• What is the equation that links moles, mass and Mr? Moles = mass / Mr