quantitative chemistry

• conversion of mass and balanced chemical equations


  law of conversion mass: no atoms are lost or made during a chemical reaction, so mass of the products = mass of the reactants.

  • in a chemical equation it is balanced in the terms of numbers of atoms of each element involved on each side. must be exactly the same number on each side.
• relative formula mass


  relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

  • e.g. for HCL: Mr= 1 + 35.5 = 36.6

  no mass is lost during a reaction
• mass changed when a reactant/product is a gas 


  if a reaction appears to involve a change in mass check if its because of a reactant or product is a mass and hasn't been taken into account.

  • e.g. when a metal reacts with oxygen: mass of metal oxide product > mass of metal
• chemical measurements 


  whenever a measurement is made, there is always some uncertainty about the result obtained.
• moles


  chemical amounts are measured in moles. the symbol for the unit mole is mol.

  • the mass of one mole of a substance in grams is equal to its relative formula mass
  • one mole of a substance contains the same number of the stated particles, atoms, molecules, or ions.
  • you can convert between moles and grams by using the equation mass = moles x molar mass. (grams mr mole)

  
• amounts of substances in equations


  masses of reactants and products can be calculated from balanced symbol equations

  chemical equations can be interpreted in terms of moles

  • total moles of one element must be the same on both sides of the equation
• using moles to balance equations


  balancing numbers in a symbol equation can be calculated from the masses of reactants and products:

  • convert masses in grams to amounts in moles
  • convert number of moles to simple whole number ratios
• percentage yield 


  percentage yield = amount of product produced/maximum amount of product possible x 100
• percentage yield 


  not always possible to obtain the calculated amount of a product for 3 reasons:

  • reaction may not complete as it is reversible
  • some of the product may be lost when it is separated from the reaction mixture
  • some of the reactions may react differently than expected
• to calculate the theoretical mass of a product from a given mass of reactant and balancing equation for the reaction:


  • Calculate mol. of reactant by using mol. = mass / molar mass
  • Use balancing numbers to find mol. of product (e.g. 2HCl + Mg -> MgCl2 if you have 2 mol. of HCl, you would divide by 2 to get 1 mol. of MgCl2 .)
  • Calculate theoretical mass of a product by then using mass = mol. x molar mass
• atom economy


  a measure of the amount of starting materials that end up as useful products

  important for sustainable development and for economic reasons to use reactions with high atom economy.

  =(Mr of desired product from reaction . sum of Mr of all reactants) x 100
• using concentrations of solutions in mol/dm3


  • concentration of a solution can be measured in mol. per given volume of solution
  • mass of a solute and the volume of a solution are related to the concentration of the solution through the equation, moles = concentration x volume.

  
• use of amount of substance in relation to volumes of gases 


  equal amounts of gases occupy the same volume under the same conditions of temperature and pressure

  • volume on 1 mol. of any gas at RTP is 24dm3

  volume of gas (dm3) at RTP = moles x 24