Quantitative chemistry

• Conversation of mass 


  -no atoms are lost or made during a chemical reaction so the mass of the products equals the mass

  of the reactants
• So in a balanced equation the mass is the same on both sides
• Relative formula mass (Mr)


  -the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
• Percentage by mass 


  Calcium in calcium chloride

  CaCl2

  Mr = 111 Ar of Ca = 40

  40/111 x 100 = 36%
• Mas changes when gas is involved

  Coper carbonate --> copper oxide + carbon dioxide

  If we wigged the reactants and the products you would see a reduction of the copper carbonate when it turned into copper oxide

  This is because CO2 is a gas and it is given off to the atmosphere
• Examples for when mas increase

  When a metal and oxygen react

  Mg + O2 --> MgO

  Increase in mass at the end because oxygen from the air reacts with the magnesium but we can see it
• Moles


  The mass of one mole of a substance in grams is numerically equal to its relative formula mass. Avogadro constant is 6.02 × 1023 per mole.
• The mass of one mole of a substance in grams is numerically equal to its relative formula mass.

  E.g Oxygen Ar=16 so one mole of O2 has a mass of 16
• In one mole of carbon (C) the number of atoms is the same as the number of molecules in one mole of carbon dioxide (COz).
• What is the mass of 1.5 moles of nitrogen atoms (N)? RAM N=14

  1 mole = 14g because that's it's RAM

  so 1.5 moles: 1.5 x 14 = 21g
• Equation of moles 


  Moles = mass / Mr or Ar
• What is the mass of 1.2 moles of phosphorus P4? Ar = 31

  P4 = 31 x 4 = 124

  Mass = Moles x Ar

  1.2 x 124 = 148.8g
• When balancing equations you can work out mols of each substance to balance it
• Calculate the number of moles of atoms in one mole of calcium hydroxide? Ca(OH)2


  1 atom of oxygen, 2 atoms oxygen and 2 atoms hydrogen

  -5 atoms in total so 5 moles of atoms
• Calculate the number of atoms in one mole of sodium oxide? Na2O

  1 mole = 6.02 x 10^23

  Number of atoms = 6.02 x 10^23 x 3

  =1.806 x 10^24
• What mass of magnesium is chloride is made from 142g of chlorine? Ar: Mg=24 Cl=35.5

  Mg + Cl2 --> MgCl2

  24 + (35.5 x 2) = 95 - Ar of MgCl2

  Cl2 = 35.5 x 2 = 71 - Ar of Cl

  mass/Ar 142/71 = mass/95

  Rearrange equation

  142/71 = 2 2x95=190g

  answer = 190g
• In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used. The reactant that is completely used up is called the limiting reactant because it limits the amount of products.
• 2Mg + O2 --> 2MgO

  0.3mol 0.2mol

  Which one is in excess ?

  2 moles of Magnesium 1 mole of oxygen

  2:1 - 3:1.5

  because oxygen should be half of magnesium

  oxygen is in excess because we only need 1.5 mols but have 2

  Magnesium is the limiting reaction
• Many chemical reactions take place in solutions. The concentration of a solution can be measured in mass per given volume of solution, eg grams per dm^ (g/dm^3).
• Mass of 1 mole is Mr in g

  Work out mass of CO2

  E.g 1 mole of CO2

  Ar of Carbon = 12 Ar of oxygen= 16 (x2) = 32 + 12 = 44

  Mass of 1 Mole of CO2 = 44g
• 1dm^3= 1000cm^3