Quantitative chemistry

• Calculating the Relative Formula Mass (Mr)


  1. Know relative atomic masses (Ar)

  2. Multiply Ar values by number of atoms

  3. Add these numbers together
• In chemical reactions, substances may seem to lose or gain mass
• Normally, 1 of the reactants (or products) is a gas, and gas can escape into the surrounding environment
• When wood is burnt

  It releases carbon dioxide and water (and often other gases)
• These gases can escape into the atmosphere
• These gases can't be weighed easily at the end of a reaction
• The mass of the oxide produced will be larger than the mass of the starting metals because oxygen atoms from the atmosphere have been added
• The change in mass

  Will equal the mass of the oxygen atoms which have been added
• During the reaction of zinc and hydrochloric acid, the mass of the flask and reactants is expected to decrease
• This is because the hydrogen gas produced will escape from the flask
• Every chemical measurement involves some uncertainty
• A digital thermometer is able to measure to a precision of 0.1oC
• If a thermometer displayed a reading of 35.4oC, the actual temperature could be anywhere between 35.35oC and 35.45oC
• The larger the uncertainty, the larger the range of results you should expect
• It is important to repeat the measurements from experiments several times, and calculate an average from them
• Chemical equations must be balanced to make sure that there are the same number of atoms of each element before and after a reaction
• Molecular formula


  Shows the actual amount of atoms which make up a molecule
• Empirical formula


  Shows the simplest ratio of atoms which make up a molecule
• Moles


  A measurement of chemical amounts
• Because atoms and molecules are so small, it is inconvenient to talk about individual atoms
• 1 mole of a substance is 6.02x10^23 atoms of that substance
• Avogadro constant


  The number 6.02x10^23
• Relative Formula Mass (Mr)


  Equal to the mass of 1 mole of that substance (in grams)
• For an element, the relative formula mass is the same as the relative atomic mass
• The mass of 1 mole of carbon is 12 g, which is the same as the relative atomic mass of carbon
• Every carbon dioxide molecule has 1 carbon atom and 2 oxygen atoms
• Ar of carbon = 12
• Ar of oxygen = 16
• In many chemical reactions, it is important to use up a reactant completely
• Limiting reactant


  The reactant that is completely used up
• The reaction stops when the limiting reactant is used up
• This is achieved by using an excess (‘more than necessary’) amount of the other reactant