chem calculations

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  • Created by: reannew
  • Created on: 12-03-16 10:45

the mole

there are many calculations using the mole ; 

  • number of particles = moles x Avogadro's constant (6.02x10^23)
  • number of moles = mass / Mr
  • number of moles = concentration (moldm^-3) x volume (dm^3)
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the ideal gas equation

pV = nRT 

p - pressure (pa)

V - volume (m^3)

n - moles 

R - gas constant (8.31 jk^-1mol^-1)

T - temperature (k - +273 degrees)

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masses from equations

  • 1 - write out balanced equation 
  • 2- find the Mr of given compound 
  • 3- from the given mass, find moles by mass/Mr
  • 4- use the ratios in the equation and the calculated moles to find the moles of the required compound
  • 5- find the Mr of the require compound
  • 6- use the Mr and moles to find the mass by mol / Mr
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empirical formula by % mass

  • 1 - match each element to it's % and find its Mr
  • 2- find the moles of each element by % / Mr
  • 3 - divide each of the moles by the smallest number of moles you have calculated
  • 4- this is the ratio of elements, use this to write the formula
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empirical formula by experimental data

  • 1- find the moles of each element by mass / Mr 
  • 2- find the ratio of moles by dividing all the mol values by the smallest mol value you have calculated.
  • 3- use this to write your emprical formula. 
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concentrations from titrations

  • 1- write a balanced equation for the compounds you need 
  • 2 - work out the moles of the compound given using conc. x vol. 
  • 3- use the mole ratios from the equation to work out the moles of the required compound
  • 4- use conc. = mol / vol to find concentration. 
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% yield

  • 1 - find the theoretical yield by: 
    • a - use mass / Mr to find moles of the given product
    • b - use mole ratios from the balanced equation to find moles of required compound
    • c - find  the mass by moles x Mr 
  • 2 - substitute this into this equation: 
    • % yield = actual yield / theoretical yield
    • you will be given the actual yield 
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atom economy

% atom economy = Mr of desired product / sum of Mrs of all reactants x 100 

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enthalpy changes

  • 1 - find the bonds broken, use the balanced equation. add their mean bond enthalpy values. 
  • 2- do the same for bonds made 
  • 3- minus the value for bonds made from the value of bonds broken to give total change.
    • enthalpy change  = total energy absorbed - total energy released. 
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