Quantitative Chemistry

• 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
• Balanced chemical equation


  • The numbers of atoms of each element involved are the same on both sides of the equation
• Relative formula mass (Mr)


  Sum of the relative atomic masses of the atoms in the numbers shown in the formula
• In a balanced chemical equation, the sum of Mr of reactants in quantities shown = sum of Mr of products in quantities shown
• If a reaction appears to involve a change in mass


  Check if this is due to a reactant or a product as a gas and its mass has not been taken into account
• When a metal reacts with oxygen, the mass of the metal oxide product is greater than the mass of the metal
• Whenever a measurement is made there is always some uncertainty about the result obtained
• Estimation of uncertainty


  Be prepared to make estimations of uncertainty when making measurements
• Mole


  The unit used to measure chemical amounts
• The mass of one mole of a substance in grams is numerically equal to its relative formula mass
• One mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance
• Converting between moles and grams

  Use the triangle or the equation: mass = moles x molar mass
• The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant: 6.02 x 10^23 per mole
• Balanced symbol equation


  Can be interpreted in terms of moles
• The total moles of one element must be the same on both sides of the equation
• Balancing numbers in a symbol equation

  Convert the masses in grams to amounts in moles

  b. Convert the numbers of moles to simple whole number ratios
• Limiting reactant


  The reactant that is used up / not in excess (since it limits the amount of products)
• If a limiting reactant is used, the amount of product produced is restricted to the amount of the excess reactant that reacts with the limiting one
• Concentration of a solution

  Measured in mass per given volume of solution e.g. grams per dm^3 (g/dm^3)
• Calculating mass of solute in a given volume of a known concentration


  Use mass = conc x vol
• A smaller volume or larger mass of solute


  Gives a higher concentration
• A larger volume or smaller mass of solute


  Gives a lower concentration