Calculations Involving Masses

• Dot and cross, ball and stick models, two- and three-dimensional representations

  Representations and models used to describe atoms and molecules
• Limitations of particular representations and models

  • Main limitation is that they apply really well only to the small class of solids composed of Group 1 and 2 elements with highly electronegative elements such as the halogens
  • In covalent molecular, the dot-cross diagrams don't express the relative attraction of shared electrons due to electronegativity
  • 2D diagrams don't show the 3D arrangement of atoms, and 3D diagrams don't show the share or transfer of electrons
• Metals


  Shiny solids which have high melting points, high density and are good conductors of electricity
• Non-metals


  Have low boiling points and are poor conductors of electricity
• Calculating relative formula mass


  Sum of the relative atomic masses of the atoms in the numbers shown in the formula
• Calculating relative formula mass in a balanced chemical equation

  Sum of Mr of reactants in quantities shown = sum of Mr of products in quantities shown
• Calculating formula from reacting masses

  1. Work out moles of each using moles = mass / molar mass

  2. Work out the ratio of moles

  3. Times the ratio so that you get the smallest whole numbers possible

  4. Find the formula by timesing each element by their number in the ratio
• Empirical formula


  Shows the simplest ratio of the number of atoms of different types of elements in a compound
• Deducing empirical formula from formula of molecule

  1. If there is a common multiple, the empirical formula is the simplest whole number ratio

  2. If there is no common multiple, the empirical formula is the same as the molecular formula
• Deducing molecular formula from empirical formula and relative molecular mass

  1. Find relative molecular mass of the empirical formula

  2. Divide relative molecular mass of compound by that of the empirical formula

  3. Multiply the number of each type of atom in the empirical formula by this number
• Experiment to determine empirical formula of magnesium oxide

  1. Weigh some pure magnesium

  2. Heat magnesium to burning in a crucible to form magnesium oxide

  3. Weigh the mass of the magnesium oxide
• Law of conservation of mass


  No atoms are lost or made during a chemical reaction so the mass of the products = mass of the reactants
• Chemical reactions can be represented by balanced symbol equations
• Precipitation reaction in a closed flask

  Precipitate that forms is insoluble and is a solid, as all the reactants and products remain in the sealed reaction container then it is easy to show that the total mass is unchanged
• Reaction in an open flask that takes in or gives out a gas

  Mass will change from what it was at the start of the reaction as some mass is lost when the gas is given off
• Calculating masses of reactants and products from balanced equations

  1. Find moles of one substance: moles = mass / molar mass

  2. Use balancing numbers to find the moles of desired reactant or product

  3. Mass = moles x molar mass (of the reactant/product) to find mass
• Calculating concentration of solutions in g dm-3

  Concentration (g dm-3) = mass of solute (g) / volume (dm3)
• Calculate masses of reactants and products from balanced equations, given the mass of one substance

  1. Find moles of that one substance: moles = mass / molar mass

  2. Use balancing numbers to find the moles of desired reactant or product

  3. Mass = moles x molar mass(of the reactant/product) to find mass
• Concentration of solutions


  Measured in mass per given volume of solution e.g. grams per dm3 (g/dm3)
• Calculate concentration of a solution

  1. Concentration (g dm-3) = mass of solute (g) ¨ volume (dm3)

  2. To calculate mass of solute in a given volume of a known concentration use: mass = conc x vol i.e. g = g/dm3 x dm3 (think about the units!)
• Mole


  The Avogadro constant number of particles (6.02 x 10^23 atoms, molecules, formulae or ions) of that substance and a mass of 'relative particle mass' g
• Calculate the number of: moles of particles of a substance in a given mass of that substance and vice versa, particles of a substance in a given number of moles of that substance and vice versa and particles of a substance in a given mass of that substance and vice versa

  1. Chemical amounts are measured in moles. The symbol for the unit mole is mol.

  2. The mass of one mole of a substance in grams is numerically equal to its relative formula mass.

  3. One mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance

  4. You can convert between moles and grams by using: moles = mass (g) ¨ relative atomic mass

  5. The number of particles, atoms, molecules or ions in a mole of a given substance is the Avogadro constant: 6.02 x 10^23 per mole.
• Stoichiometry


  The balancing numbers in front of compounds/elements in reaction equations
• Deduce the stoichiometry of a reaction from the masses of the reactants and products

  1. Convert the masses in grams to amounts in moles (moles = mass/Mr)

  2. Convert the numbers of moles to simple whole number ratios