(ko) Quantitative chem

• No atoms can be created or made during a chemical reaction, so the mass of the reactants will equal the mass of the product
• Reaction equation


  Can be shown as a word or symbol equation
• Reaction equation


  • Magnesium + oxygen magnesium oxide
  • Mg + O2 MgO

  
• Symbol equations should also be balanced; they should have the same number of atoms on each side
• Conservation of mass


  Total Mr on the left-hand side is the same as the Mr on the right hand side
• Relative Formula Mass (Mr)


  The sum of all of the relative atomic masses (Ar) of the atoms in the formula
• Calculating percentage mass of an element in a compound


  Percentage mass of an element in a compound = (mass of element / total mass of compound) x 100
• Concentration


  The amount of a substance in a specific volume of a solution. The more substance that is dissolved, then the more concentrated the solution is
• Calculating concentration


  1. Concentration = mass ÷ volume

  2. Mass = concentration x volume
• Units


  • Concentration: g/dm3
  • Mass: g
  • Volume: dm3
• Mole


  Measurement of the amount of a substance
• Avogadro constant


  The number of atoms, molecules or ions in one mole of a given substance (6.02x1023)
• One mole of substance contains the same number of atoms, particles, ions as one mole of any substance
• The number of atoms, molecules or ions in a mole of a given substance is Avogadros constant (6.02x10^23)
• Limiting reactant


  The reactant in a reaction that determines the amount of products formed. Any other reagents are all in excess and will not react
• Limiting reactant


  • The reactant that is all used up is called the limiting reactant, the reactant that is left over is described as being in excess
• Calculating moles of reactants and products


  1. Mg + 2HCl MgCl2 + H2

  2. 6 moles of HCl needed for 1 mole of Mg

  3. 5 moles of MgCl2 produced from 10 moles of HCl

  4. 4 moles of Mg needed to fully react with 8 moles of HCl
• Calculating maximum mass of hydrogen produced

  1. 12g of magnesium reacts completely with excess hydrochloric acid to form magnesium chloride and hydrogen

  2. Mg + 2HCl MgCl2 + H2

  3. Calculate the maximum mass of hydrogen that can be produced (Ar of Mg = 24, Mr of H2 = 2)

  
• Percentage yield


  The theoretical yield is the maximum mass that can be made during a chemical reaction. The percentage yield is the actual mass produced divided by the theoretical yield x 100
• Atom economy


  A measure of the amount of starting material (reactants) that end up as useful products
• Calculating atom economy


  Atom economy = (Mr of desired product / sum of Mr of all reactants) x 100
• Molar gas volume


  The volume of one mole of any gas at room temperature and pressure (20°C and 1 atmosphere pressure) is 24dm3 (24 000cm3)
• Calculating gas volume from moles

  1. Volume = number of moles x molar volume

  2. Determining moles from gas volume: volume = number of moles x molar volume
• Calculating mass of product from gas volume


  1. 2Na + 2H2O 2NaOH + H2

  2. Calculate number of moles of sodium (Ar 23): 3.5/23 = 0.15 mol

  3. Determine the number of moles of hydrogen: 0.15 mol of sodium produces 0.075mol of hydrogen

  4. Determine the volume of hydrogen: volume = 0.075 x 24 = 1.8 dm3

  5. Calculate the mass of sodium chloride (Mr 58.5) that can be produced from 685 cm3 of chlorine: 0.029 moles of chlorine would produce 0.058 moles of sodium chloride, mass = 0.058 x 58.5 = 3.393g
• Titration method


  Measure 25cm³ sodium hydroxide solution and add phenolphthalein

  1. Fill burette with sulfuric acid to 0cm³ line
  2. Slowly add acid to sodium hydroxide until indicator turns colourless
  3. Record total volume of acid added
  4. Repeat twice more
  5. Use results to calculate concentration of solution or volume required to neutralise
• Calculating concentration from titration results

  concentration = number of moles ÷ volume

  To convert between mol/dm3 and g/dm3: multiply by Mr