C3 - Quantitative Chemistry

• 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.
• Compounds have a relative formula mass
• Relative Formula Mass - Mr
• Percentage mass of an element in a compound:

  (Mass number x Number of elements of that compound) ÷

  Relative formula mass x 100
• The relative formula mass (Mr ) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
• In a balanced chemical equation, the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown.
• Some reactions may appear to involve a change in mass because a reactant or product is a gas and its mass has not been taken into account. For example: when a metal reacts with oxygen the mass of the oxide produced is greater than the mass of the metal or in thermal decompositions of metal carbonates carbon dioxide is produced and escapes into the atmosphere leaving the metal oxide as the only solid product
• If mass decreases it's probably because one of the products is a gas and all the reactants are solids, liquids or aqueous.
• Mass decreases:

  • Before the reaction, all the reactants are contained in the reaction vessel
  • If the vessel isn't enclosed, then the gas can escape from the reaction vessel as it's formed. It's no longer contained in the reaction vessel, so you can't account for its mass - the total mass of the stuff inside the reaction vessel decreases.
• Mass decreases:

  When a metal carbonate thermally decomposes to form a metal oxide and carbon dioxide gas, the mass of the reaction vessel will decrease if it isn't sealed. The mass of the metal oxide and carbon dioxide produced will equal the mass of the metal carbonate that decomposed
• If the mass increases it's because one of the reactants is a gas that's found in air and all the products are solids, liquids or aqueous
• Mass increases:

  • Before the reaction, the gas is floating around in the air. It is contained in the reaction vessel so you can't account for its mass
  • When the gas reacts to form part of the product, it becomes contained inside the reaction vessel - so the total mass of the stuff inside the reaction vessel increases.
• Mass increases:

  When a metal reacts with oxygen in an unsealed container, the mass of the container increases. The mass of the metal oxide produced equals the total mass of the metal and the oxygen that reacted from the air.
• Whenever a measurement is made there is always some uncertainty about the result obtained.
• Chemical amounts are measured in moles
• The symbol for the unit mole is mol.
• 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
• Mole - amount of a substance
• The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant.
• The value of the Avogadro constant is 6.02 x 10^23 per mole.
• Number of moles:

  Mass (of element or compound) ÷ Relative atomic mass (of element or compound)
• A reaction stops when all of one of the reactants is used up. Any other reactants are in excess
• Limiting reactant - Reactant that's used up in a reaction
• The amount of product formed is directly proportional to the amount of limiting reactant.
• More reactant means more reactant particles which means more product particles
• Concentration - measure of how crowded things are
• Lots of reactions take place between substances that are dissolved in a solution.
• The amount of a substance in a certain volume of a solution is called the concentration
• The more solute there is in a given volume, the more concentrated the solution
• Concentration = Mass of solute ÷ volume of solvent
• Moles are used to compare quantities of substances
• When a substance is dissolved in water, it dissociates into its ions