Quantitative Chemistry

• what is the law of conservation of mass

  no atoms are lost or made during a chemical reaction so the mass of the products equals the mass as the reactants
• how to calculate relative formula mass (Mr)

  sum of the relative atomic masses of the atoms shown in the numbers shown in the formula

  e.g. H2O 2+16 = 18
• how do the relative formula masses of the reactants compare to the products

  they are equal
• why could the mass of products be less than the mass of the reactants

  a gas is made which can escape

  e.g. a metal carbonate and acid makes carbon dioxide or during thermal decomposition
• why could the mass of products be more than the mass of the reactants

  when a gas is added to a reactant

  e.g. when magnesium reacts with air oxygen is added
• How do you calculate % element in a compound

  (Ar of Element ÷ Mr of Compound) x 100
• what are chemical amounts measured in (HT only)

  moles, the unit is mol
• what is the mass of one mole (HT only)

  the relative formula mass in grams
• What is the number of particles in one mole called (HT only)

  Avogadro's constant

  6.02 x 10^23
• how do the number of particles in one mole of oygen compare to one mole of water

  they are the same, a mole is a fixed number of particles
• how do you calculate the mass of one atom (HT only)

  mass of one mole ÷ Avogadro's constant
• how can you calculate number of moles

  moles = mass ÷ Mr
• what unit is used for mass in chemistry

  grams, g
• how many moles are there in 500g of NaOH

  Mr of NaOH is 40, moles = 500÷40 = 12.5 moles
• what is the Mr of a compound when 3 moles has a mass of 126g

  Mr = mass÷moles, 126÷3 = 42
• how many moles are reacted in Mg + 2HCl --> MgCl2 + H2 (HT only)

  1 mole of magnesium with two moles of hydrochloric acid
• what is limiting reactant (HT only)

  the reactant that is completely used up (the other is then in excess)
• what does it mean if the reactant is said to be in excess

  more than enough present to allow the limiting reactant to get fully used up
• what is concentration measured in

  grams per decimetre (g/dm3)
• how do you calculate concentration

  mass ÷ volume
• what units do mass and volume have to be in when calculating concentration

  mass must be in g, volume must be in dm3
• how do you convert cm^3 to dm^3

  ÷ by 1000
• how do you convert Kg to g

  x 1000