quantitive chemistry - topic 3

• Avogadro constant

  The number of atoms, molecules or ions in a mole of a given substance
• Concentration

  The amount of substance (e.g. the mass) in a certain volume of a solution
• Conservation of mass: The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants
• Limiting reactant

  The reactant that is completely used up since it limits the amount of products formed
• Mole

  Chemical amounts are measured in moles. The mole is the unit for amount of substance. The symbol for the unit mole is mol
• Relative formula mass

  The sum of the relative atomic masses of the atoms in the numbers shown in the formula. It is numerically equal to the mass of one mole of a substance in grams
• Thermal decomposition

  The reaction that occurs when heat is applied to a compound causing it to break down into its different chemical constituents
• Uncertainty: All measurements have a degree of uncertainty regardless of precision and accuracy. Uncertainty can be due to the limitations of the measuring equipment or due to the skill of the experimenter carrying out the measurements
• 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
• In a chemical equation, it is balanced in terms of the numbers of atoms of each element involved on both sides of the equation- there must be exactly the same number on each side
• Relative formula mass (Mr) of a compound

  Sum of the relative atomic masses of the atoms in the numbers shown in the formula
• In a balanced chemical equation: sum of Mr of reactants in quantities shown = sum of Mr of products in quantities shown (essentially no mass is lost during a reaction)
• If a reaction appears to involve a change in mass

  Check to see if this is due to a reactant or a product as a gas and its mass has not been taken into account (e.g. because the gas has been released into the atmosphere)
• Whenever a measurement is made there is always some uncertainty about the result obtained
• Be prepared to make estimations of uncertainty
• Balancing numbers in a symbol equation

  1. Convert the masses in grams to amounts in moles

  2. 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)
• In a chemical reaction with 2 reactants you will often use one in excess to ensure that all of the other reactant is used
• 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 dm3 (g/dm3)
• Calculating mass of solute in a given volume of a known concentration

  mass = conc x vol
• Smaller volume or larger mass of solute

  Gives a higher concentration
• Larger volume or smaller mass of solute

  Gives a lower concentration