3-Quantative chemistry

• relative atomic mass of an element - Ar - little no. represent how many

  • number of atoms of the element x relative atomic mass
  • O₂ = 2 x 16 = 32
• relative formula mass of a molecule/compound - Mr

  • sum of the relative atomic masses of all the elements in the compound
  • CO₂ = 12 + 32 = 44
• percentage mass of an element

  • total relative atomic mass of the element divided by the relative formula mass of the compound
  • then times it by 100
  • % mass of C in CO₂= 12/44 x 100 = 27.3
• mass (g) = moles (mol) x Mr
• mole is the unit used to measure the amount of chemical there is,

  the big numbers in front represent the moles, if there is no number it is 1
• • mole is the unit used to measure the amount of chemical there is
  • 1 mole of a substance is the amount of that substance that contains 6.02 x 10²³ particles (Avogadro's constant)
  • the big numbers in front represent the moles, if there is no number it is 1
• calculating mass in reactions

  1. write out the balanced equation
  2. find moles of the known substance (mass = moles x Mr)
  3. find the moles of the unknown substance (molar ratio from balanced equation)(divide by ratio of unknown substance)
  4. find the mass of the unknown substance (mass= moles x Mr)
• conservation of mass states that mass is always conserved in a chemical reaction but may appear to change due to other factors such as:

  • one of the reactants being a gas which increases mass because it could react with the air
  • one of the products being a gas which decreases mass because it could have escaped into the air
  • this could be resolved by doing experiments in sealed containers
• • a limiting reactant is one that will fully react and limit how much product is formed
  • an excess reactant is one that will not fully react and not affect how much product is being formed