Chap 3 Stochiometry

• Relative atomic mass


  The average mass of the Isotopes of an element compared to 1 12th of the mass of an atom of 12 carbon
• Relative molecular mass


  The sum of the relative atomic masses
• Relative formula mass


  The total mass of an ionic compound
• Calculating reacting masses in simple proportions

  1. Write the balanced equation

  2. Find the relative atomic mass of each element

  3. Add up the total relative molecular masses of reactants

  4. Add up the total relative molecular masses of products

  5. Ensure the total masses are equal
• The mass of 2 MgO is 2 * 24 + 2 * 16 = 80 g
• The total reacting mass and total product mass in the MgO reaction is 80 g
• Calculating reacting masses in simple proportions

  1. Write the balanced equation

  2. Find the relative atomic mass of each element

  3. Add up the total relative masses of reactants

  4. Add up the total relative masses of products
• Concentration


  How much of something is mixed in a liquid, can be measured in g/dm^3 or mol/dm^3
• Mole


  The unit of amount of substance, 1 mole contains 6.02 x 10^23 particles (atoms, ions or molecules)
• Avogadro's Law


  Equal volumes of different gases at the same temperature and pressure contain the same number of molecules
• Molar gas volume


  The space taken up by 1 mole of any gas at room temperature and pressure (24 dm^3 or 24,000 cm^3)
• Calculating gas volume from moles

  1. Volume of gas (dm^3) = Number of moles x Molar gas volume (24 dm^3/mol)

  2. Volume of gas (cm^3) = Number of moles x Molar gas volume (24,000 cm^3/mol)
• Calculating moles from gas volume

  Number of moles = Volume of gas / Molar gas volume (24 dm^3/mol or 24,000 cm^3/mol)
• 2 moles of oxygen gas has a volume of 48 dm^3 at standard temperature and pressure
• 4.6 moles of gas has a volume of 11,400 cm^3 at standard temperature and pressure
• 672 dm^3 of gas at standard temperature and pressure contains 28 moles
• 3,000 cm^3 of gas at standard temperature and pressure contains 0.125 moles
• The mass of one mole of a substance is the molar mass or capital M
• Molar mass

  For an element, the same as the element's relative atomic mass. For a compound, the relative formula mass or relative molecular mass
• Molar mass is expressed in G per mole
• Amount of substance

  Equal to mass divided by molar mass
• Calculating mass from moles

  Substitute number of moles and molar mass into formula: mass = amount of substance x molar mass
• Calculating moles from mass

  Substitute mass and molar mass into formula: amount of substance = mass / molar mass
• Calculating number of molecules and atoms from moles

  Use Avogadro's constant (1 mole = 6.02 x 10^23 particles) to find number of molecules. Then multiply number of molecules by number of atoms per molecule.
• Calculating mass of product from mass of reactant

  Find moles of reactant

  2. Use mole ratio from balanced equation to find moles of product

  3. Substitute moles of product into mass = moles x molar mass
• Empirical formula


  The simplest whole number ratio of the different atoms or ions in a compound
• Molecular formula


  The actual number and type of different atoms in one molecule
• Organic molecules can have different empirical and molecular formulas but for ionic compounds their formula is always the empirical one
• Calculating empirical formula


  1. List the elements present

  2. Write the masses of each element

  3. Convert to moles using molar masses

  4. Find the mole ratio by dividing by the smallest number of moles

  5. Write the empirical formula with the simplest whole number subscripts
• Calculating molecular formula


  1. Find the factor = relative formula mass of molecular formula / relative formula mass of empirical formula

  2. Multiply the empirical formula by the factor to get the molecular formula
• Yield


  How much product you get from a reaction
• Actual yield


  How much product you actually get from a reaction
• Theoretical yield


  The most product you could get in a perfect chemical reaction
• Pure substance


  A substance that is not mixed with any other substances