C3- Quantitative chemistry

• What is the relative formula mass (Mr)?

  It is the sum of all of the relative atomic masses in a compound added together
• How can you calculate the percentage mass of an element in a compound?


  % mass = Ar x number of atoms of that element

  ------------------------------------ X 100

  Mr of the compound
• What is the Avogadro constant?

  6.02 X 10^23
• What is the equation to figure out the number of moles?

  mass (in grams)

  Number of moles= ----------------

  Mr

  (Mr mole lives underground)
• What happens to mass in a chemical reaction?


  Mass is always conserved:

  • No atoms are created or destroyed: this means there are the same number and types of atoms on each side of a reaction equation
  • The total Mr of the reactants=total Mr of products
• Where might you observe a change of mass?

  In an unsealed reaction vessel during a reaction
• Why might you observe an increase of mass in a reaction?


  If the mass increases, it's probably because one of the reactants is a gas found in air, and all of the products are solids, liquids and/or aqueous:

  • Before the reaction, the gas is floating around in the air. It's there, but it's not contained in the 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
• What happens when metals react with oxygen in an unsealed container?


  The mass of the container increase. The mass of the metal oxide produced equals the total mass of the metal and the oxygen that reacted from the air
• Why might you observe a decrease of mass?


  If the mass decreases, it's probably because one of the products is a gas and all the reactants are solids, liquids and/or aqueous:

  • Before the reactions, all the reactants are contained in the reaction vessel
  • If the vessel isn't enclosed, the gas can escape from the reaction as it is 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
• What happens when a metal carbonate forms a metal oxide and carbon dioxide in an unsealed container? 


  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. But in reality, the mass of the metal oxide and the carbon dioxide produced= the mass of the metal carbonate that decomposed
• How can equations be balanced using reacting masses with the moles equation?


  • Divide the mass of each substance by its relative formula mass to find the number of moles
  • Divide the number of moles of each substance by the smallest number of moles in the reaction
  • If any of the numbers are not whole, multiply all the numbers by the same amount so that they all become whole numbers
  • Write the balanced symbol equation for the reaction by putting these number in front of the chemical formulas
• What is a limiting reactant?

  A reactant that causes the entire reaction to stop by getting used up
• What happens when a limiting reactant is used up?

  • The reaction stops when the limiting reactant is used up. Any other reactants are in excess
  • The amount of product formed is directly proportional to the amount of limiting reactant. This is because if you add more reactant, then there will be more reactant particles to take part in the reaction, which means more product particles
• What is concentration?


  The amount of a substance in a given volume or area of a solution. The more solute in a given volume, the more concentrated the solution is
• What is the formula to find concentration?


  mass of solute (g)

  Concentration (g/dm^3)= ---------------------

  Volume of solvent (dm^3)