SC9

• Relative Atomic Mass - replace all symbols in a formula with their relative atomic mass then add them all together (H2=2 as hydrogen has a relative atomic mass of 1)
• Empirical Formula - the simplest whole number ratio of atoms in a compound
• Empirical Formula can be calculated using:

  1. Percentage/Mass divided by relative atomic mass of all elements
  2. Divide the values by the lowest one
  3. Write the formula (round or multiply to whole numbers)
• To deduce empirical formula from molecular formula, divide all atom ratios by their highest common factor
• Calculating Molecular Formula:

  1. Find the mass of the empirical formula
  2. Work out the ratio of the empirical and molecular formula masses
  3. Multiply the empirical value by that value
• Determine Empirical Formula Practical:

  1. Record mass of metal piece after sanding it down - increase reactivity
  2. Place in a crucible without the lid and record total mass
  3. Heat metal strongly using a bunsen burner and remember to lift the lid to allow oxygen in - react completely
  4. Continue heating until mass remains constant
  5. Calculate mass of oxygen that reacted using difference in mass
  6. Calculate empirical formula

  Safety concerns include handling hot equipment/products - allow to cool before handling and wear goggles to protect eyes
• In a closed system, the total mass of the reactants are equal to the total mass of the products as atoms cannot be created or destroyed in a chemical reaction
• In an open system, gas produced may make it seem like mass is lost but it is actually in the gas that had been released by the reaction so mass is conserved
• Reacting Masses:

  1. Divide the mass by relative atomic mass for moles (known element)
  2. Ratio the moles corresponding to the balanced reaction (known: unknown)
  3. Multiply the moles by the unknown element relative atomic mass
• Calculating Concentrations:

  • convert cm3 to dm3 by dividing cm3 by 1000
  • g dm-3 is mass divided by volume
• Avogadro's Constant - the number of particles in one mole of a substance (6.02 x 10^23)
• Limiting Reactant - the reactant that is used up first in a chemical reaction

  The product formed is directly proportional to the limiting reactant as there will be more reactant particles and therefore more product particles
• Stoichiometry:

  1. Calculate moles of reactants
  2. Convert values into whole numbers by multiplication
  3. Write one formula with the total number of each atom