C2.1 Purity and Separating Mixtures

• What is the chemical formula of aluminium sulfate?

  Al$v2$(SO$v4$)$v3$
• What does the number 3 in the formula Al$v2$(SO$v4$)$v3$ indicate?


  Three times the number of each atom inside brackets
• What is relative formula mass denoted as?

  Mr
• How is relative formula mass defined?

  Mean mass of a unit compared to 1/12 of C atom
• What is relative atomic mass?

  Mean mass of an atom compared to carbon-12
• How are the masses of subatomic particles described?

  By relative masses compared to a proton
• How are relative atomic masses determined?

  Compared to 1/12 the mass of carbon-12
• Can you find Mr values in the periodic table?

  No, you must calculate them
• How do you calculate Mr values?

  Add together Ar values for all atoms in the formula
• What is the relative atomic mass of carbon-12 defined as?

  12.0 exactly
• What are the units for Ar and Mr values?

  They have no units
• What does it mean if an atom has an Ar below 12.0?

  It has less mass than a carbon-12 atom
• What does it mean if an atom has an Ar above 12.0?

  It has more mass than a carbon-12 atom
• Where can you find the relative atomic masses of elements?

  In the periodic table
• What does a chemical formula indicate?

  How many atoms of each element are present
• What are the steps to calculate the relative formula mass of magnesium hydroxide, Mg(OH)$v2$?


  1. Write down Ar values: Mg=24.3, O=16.0, H=1.0
  2. Determine number of atoms: Mg=1, O=2, H=2
  3. Calculate Mr:

  • Mr = (1 x 24.3) + (2 x 16.0) + (2 x 1.0)
  • Mr = 24.3 + 32.0 + 2.0 = 58.3
• What is the significance of relative atomic mass in chemistry?

  • Allows comparison of atom masses
  • Essential for calculating molar masses
  • Helps in stoichiometric calculations
• What information is provided in the periodic table regarding elements?

  • Relative atomic masses (Ar)
  • Element symbols
  • Atomic numbers
• What does an empirical formula represent?

  It shows the simplest ratio of atoms
• How are some chemical formulae already in their simplest form?

  They cannot be simplified further
• What is the process to calculate the empirical formula?

  1. Find the highest common factor (HCF)
  2. Divide the chemical formula by the HCF
  3. Write down the empirical formula
• What does a balanced chemical equation show?

  The formulae and number of units
• What happens when carbon reacts with oxygen?

  It produces carbon dioxide
• How can you check if Mr values are calculated correctly?

  Compare totals on both sides of the equation
• What do Ar and Mr values represent?

  Atomic and relative molecular masses
• How can you determine the Mr for a substance in a balanced equation?

  Use known Ar and Mr values of other substances
• What is the significance of a balanced chemical equation?

  • Shows conservation of mass
  • Indicates the ratio of reactants to products
  • Helps in calculating relative formula masses
• What does 'pure' mean in everyday life?

  Natural substances not processed or changed
• Why is 'pure brilliant white' paint not pure scientifically?

  It is a mixture of many substances
• What constitutes a pure substance in science?

  Just one element or compound
• What does it mean for mixtures to be impure substances?

  They contain more than one element or compound
• Why is it difficult to obtain pure substances?

  One atom or molecule makes it impure
• What can contaminate ultrapure water?

  Carbon dioxide from the air
• What is air composed of?

  Nitrogen, oxygen, and other substances
• How can mixtures be useful?

  They produce desired properties in materials
• What is an alloy?

  A mixture of a metal with other elements
• Why are most metals used alloys?

  They provide enhanced properties for use
• What is the melting point of a substance?

  The temperature it changes from solid to liquid
• What characterizes the melting point of a pure substance?

  It is a single temperature
• How does the melting point of an impure substance compare to that of a pure substance?

  It is less than that of the pure substance