Quantitative Chemistry

• What is the law of conservation of mass?

  States that no atoms are lost of made during a chemical reaction so the mass of the products equals the mass of the reactants
• Define relative atomic mass and relative formula mass:

  RAM: average mass of atoms. An element taking into account masses and abundance of its isotopes, relative to 12C

  RFM: sum of the relative atomic mass of all atoms in the formula
• What is Avogadro’s constant?

  The number of atoms, molecules or ions in a mole of a given substance

  6.02x10 power 23
• What is the formula that links mass, molecular mass and moles together?

  Mass = Mr x Moles
• Nitrogen And hydrogen for ammonia shown by the following equation

  N2(g) + 3H2(g) <-> 2NH3(g)

  Calcualte the mass of nitrogen needed to form 6.8 tonnes of ammonia

  RAM (Ar): H=1, N= 14

  Step 1: Work out n.o of moles of ammonia (Mr= 17)

  6800000/17= 400000 moles of ammonia

  Step 2: Use balanced equation and n.o of moles of ammonia to work out n.o of moles of nitrogen

  -> Ratio of nitrogen to ammonia is 1:2

  Therefore the n.o of moles of nitrogen is

  400000/2= 200000

  Step 3: Work out mass of nitrogen Mr of N2 is 28

  200000 x 28 = 5600000 = 5.6 tonnes
• Hydrogen peroxide decomposes in water to from water and oxygen. How many grams of oxygen gas will be given off from 40.8 g of hydrogen peroxide?

  Step 1: Write balanced equation (Mr of H2O2 = 34)

  Step 2: N.o of moles in 40.8g : 40.8/34= 1.2 moles

  Ratio in balanced equation of H2O2:O2 = 2:1

  Step 3: Therefore n.o of moles of O2 = 0.6 moles

  Step 4: mass of oxygen = 0.6 x 32 (Mr of O2) =19.2
• Two formulae that link concentration mole/mass and volume together:

  Concentration (g per dm3) = mass(g) / volume (dm3)

  Concentration (mol per dm3) = nr of moles/volume (dm3)
• What is atom economy?

  A measure of the amount of staring material that end up as useful products

  It is a ratio of the relative formula mass of desired product to the sum of relative formula masses of reactants