Quantitive Chemistry

• what is the conservation of mass

  no atoms are lost or made during a chemical reaction

  • therefore mass of products = mass of reactants
• what is the relative formula mass

  • the relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of the atoms in the numbers shown in the formula
  • the Ar is the no. under the element letter

  eg. the Mr of CaCO3

  • Ar of Ca = 40
  • Ar of C = 12
  • Ar of O = 16 (16x3 for this equation)

  Mr of CaCO3 = 40+12+(16x3)

  = 100
• why would some reactions contain a change in mass

  if a reactant or product is a gas its mass may have not been taken into account
• when a metal reacts with oxygen (2Mg + O2 -> 2MgO) , is the mass of the oxide or metal bigger


  the mass of the oxide is greater than the mass of the metal
• what products are there after the thermal decompositions of metal carbonates

  CaCoO3 -> CaO + CO2

  • carbon dioxide is produce but escapes into the atmosphere
  • this leaves metal oxide as the only solid product
• how can uncertainty be calculated

  by calculating the mean of a range of a set of measurements, you can see how far apart all the measurements are and set an uncertainty

  • measurements : 20.10 , 20.20 , 20 , 20.05 , 20.25
  • mean : (20.10+20.20+20+20.05+20.15)/5 = 20.10
  • uncertainty = ±0.10 (all readings are within ±0.10 of mean)
• what are moles

  • the measure for chemical amounts
  • 1 mol of a substance = the same no. of the stated particle or atoms
  • the mass of a mole in a substance in grams = its relative formula mass

  e.g. in a mole of carbon, the no. of atoms = the no. of atoms in a mole of carbon dioxide
• what is the avogadros constant

  • the no. of atoms or molecules or ions in a mole of a substance
  • avagadros constant = 6.02 x 10^23 moles
• moles =

  mass / Mr
• how many atoms are in one mole of copper + sodium + carbon dioxide

  1 mole of copper atoms = 6.02 x 10^23

  1 mole of sodium ions = 6.02 x 10^23

  1 mole of carbon dioxide molecules = 6.02 x 10^23
• number of particles =


  number of moles x avvogadros constant
• how many atoms of tin are there in a 6g sample of tin metal

  1. calculate the number of moles

  • moles = mass / tin Ar
  • = 6 / 119
  • = 0.0504 mol

  2. calculate the number of atoms

  • no. of atoms = moles x avogadro
  • = 0.0504 x 6.02 x10^23
  • = 3.04 x 10^22
• calculate the mass of 1 atom of sodium

  mass of 1 atom = mass of 1 mole (Ar) / avogadros

  = 23/6.02 x 10^23

  = 3.82 x 10^-23g
• how can chemical equations be intrpreted in terms of moles

  Mg + 2HCI -> MgCI2 + H2

  • shows 1 mole of magnesium reacts with 2 moles of hydrochloric acid
  • this produces 1 mole of magnesium chloride and 1 mole of hydrogen gas
• general method for reacting mass questions

  1. work out the no. of moles for the substance for which the mass has been given (no. moles = mass/Mr)
  2. use the ratio of moles in a balanced equation to work out moles on the other side
  3. work out the mass of the second substance (mass = moles x Mr)
• calculate the mass of carbon dioxide produced from heating 5.5g of sodium hydrogencarbonate

  
  

  2NaHCO3 -> Na2CO3 + CO2 + H2O

  
  

  1.) work out moles of sodium hydrogencarbonate

  moles = mass / Mr

  = 5.5 / 84

  = 0.0655 mol

  
  

  2.) use balanced equation to give moles of CO2

  2 moles NaHCO3 : 1 mole CO2

  so 0.06555 moles HNO3 : 0.0328 moles CO2

  
  

  3.) work out mass of CO2

  mass = moles x Mr

  = 0.0328 x 44

  = 1.44g
• 8.01 g of copper reacts with sulfur to form 12.03 g of copper sulphide (CuS)

  calculate the moles of Cu, S and CuS + show the balanced equation

  1.) use conservation of mass to work out mass of missing substance

  • mass of sulfur = copper sulfide mass - copper mass
  • mass of sulfur = 4.02g

  2.) calculate moles for each chemical

  • moles of chemical = mass/Ar
  • moles of all substance = 0.125

  3.) divided each mole amount by smallest no. of moles to get whole number ratio (normally do for each different substance)

  • 0.125/0.125 = 1

  4.) use moles to write full equation

  1Cu + 1S -> 1 CuS
• what are the two typer of reactants in most chemical reactions

  1. in excess

  • ensures that all of the other reactant is used
  • not all will react

  2. limiting reactant

  • limits the amount of product
  • the no. of moles of limiting reactant determine the no. of moles of product formed
  • all will react
• general method for limiting reactant questions

  1. work out the number of moles of the substance for each reactant (no. of moles = mass / Mr)
  2. use the ratios of moles in the balanced equation to work out which reactant is the limiting
  3. use moles ratios in the balanced equation to convert moles of the limiting reactant to the moles of a product
  4. worm out the mass of the product (mass = moles x Mr)
• 5g of magnesium react with 6g of oxygen to make magnesium oxide - what is the limiting reactant + calculate the mass of magnesium oxide that will be formed?

  1.) calculate no. of moles for each reactant

  mg moles = mass / Ar -> 5/24 = 0.208mol

  o2 moles = mass / Mr -> 6/32 = 0.188 mol

  2.)use the mole ratio of a balanced equation to find reactants

  2mg : 1o2

  0.208 mg moles + 0.104 o2 moles

  • but we have 0.188 o2 moles so o2 is excess + mg is limiting

  3.)use the mole ratio of a balanced equation to convert limiting reactant -> product moles
• concentration (in mold/dm^3)

  moles / volume (in dm^3)
• concentration (in g/dm^3)

  mass / volume (in dm^3)
• how can you convert cm^3 into dm^3

  cm^3/1000 = dm^3
• how can you g/dm^3 into mol/dm^3

  divide g/dm^3 by Mr
• why is it not always possible to obtain the calculated amount of a product despite atoms not being gained or lost

  • reaction may not got completion as it is reversible
  • some product may be lost when it is separated from the reaction mixture
  • some reactants may react in ways different to the expected reaction
• what is the percentage yield

  • the amount of a product obtained is known as the yield
  • the % yield is the yield compared to maximum theoretical amount
• % yield =

  ( mass of product actually / maximum theoretical mass of product ) x 100
• what is atom economy

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

  • needed for sustainable development + economic reasons to use reactions with high atom economies
• % atom economy

  (relative formula mass of desired products / sum of relative formula masses of all reactants) x 100
• Calculate the % atom economy for the reaction where Fe is the desired product ,assuming the reaction goes to completion


  Fe2O3 + 3CO -> 2Fe + 3 CO2

  % atom economy = Mr of desired products / sum of Mr of all reactants x100

  = 2 x 56 / (2x56 + 3x16) + 3(12+16) x 100

  = 45.9%
• simple method for titration

  • alkali in burette
  • acid in conical flask -> measure with a 25cm^3 pipette
  • few drops of indicator
  • add alkali to acid until colour changes
  • swirl conical flask
  • add alkali dropwise towards the end
  • not final burette reading
  • repeat until 2 readings are within 0.1cm^3
• when do equal amounts in moles of gases occupy the same volume

  • same temp
  • same pressure
• what is the volume of any one mole of gas at room temp + pressure

  24dm^3

  • 20 degrees
  • 1 atmospheric pressure
• gas volume (dm^3) =

  number of moles x 24
• calculate the volume in dm3 at room temperature and pressure of 50.0g of carbon dioxide gas

  1.) convert mass to moles

  moles = mass / Mr

  moles = 50 / (12 + 16 x 2)

  moles = 1.136 mol

  
  

  2.) convert moles to gas volume

  gas vol = moles x 24

  gas vol = 1.136 x 24

  gas vol = 27.3dm^3
• what are three equations for conversion to moles

  • mass / Mr
  • concentration x volume
  • gas volume / 24
• what are the steps for any quantative question

  1.) convert any given quantity into moles by

  • mass / Mr
  • concentration x volume
  • gas volume / 24

  2.)use a balanced equation to convert moles of the first substance to the moles of the second substance

  3.) convert moles of second substance to quantity question needs

  • mass = moles x Mr
  • gas volume = moles x 24
  • concentration = moles / volume