Amount of Substance

Unit 1 - Amount of Substance: the mole, equations and calculations, titrations, formulas, Yeild and atom economy.

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The Mole

A mole is just a very large number of particles.

  • Amount of substance measured in moles (mol) given with the symbol n.
  • One mole is roughly 6 x 10 to the 23 particles (Avagadro's constant).
  • Dosen't matter what the particles are.

Molar mass:

  • Molar mass, M = the mass of one mole of something.
  • Molar mass is just the same as the relative molecular mass (or formula mass) just remember "g mol-1".
  • Number of moles = mass of substance/molar mass

Concentraition of solution:

  • Concentraition of solution aka how many moles dissolved in 1 dm3 of solution measured in mol dm-3.
  • nunber of moles = concentraition x volume (in cm3) / 1000
  • or: number of moles = concentraition x volume (in dm3)
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  • Same volume under same conditions.
  • If temperature and pressure stay the same then gas always has the same volume.
  • At room teperature and pressure (r.t.p) its 24dm3
  • Number of moles = volume in dm3/24
  • or: number of moles = volume in cm3/24000

Ideal Gas equation:

  • Allows you to find number of moles in certain volume at any temperature and pressure.
  • pV=nRT
    p=pressure (Pa)
    V=volume (m3)
    n=number of moles
    R=8.31 J K-1 mol-1
    T=temperature (K)
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Equations and Calculations

Balanced equations:

  • Equal numbers of each atom on both sides.
  • Only add more atoms by adding whole compounds. Do this by adding numbers infront of a compound.
    1. Work out how many are on each side.
    2. Balance accordingly.

Ionic balanced equations:

  • Charges must balance, only include reacting particles.
    1. Work out how many on each side.
    2. Balance accordingly.
    3. Balance charges accordingly.

Balanced equation, working out masses

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Need to be done accurately:

  • Titration allow you to find exactly how much acid is needed to neutralise an alkali.
  • Measure out alkali using pipette an put in flask with indicator eg. phenolphthalein.
  • Rough titration to dicern end point (where acid exactly neutralises alkali) adding acid using burette - regualrly swirl.
  • Now accurate titration, run acid within 2 cm of end point then drop slowly.
  • Record amount used to neutralise, repeat till accurate answer reached:
    - Pipette
    - Burette
    - Acid
    - Alkali
    - Scale
    - Idicator

Used to calculate concentraitions from titrations.
number of moles = Concentraition x volume (cm3) / 1000

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Calculate Volumes:

  • Usually used for planning experiments.
  • Number of moles = concentraition x volume (cm3) / 1000 (rearrange).
    1. Balance equation and decide what you want to know.
    2. Now work out amount of moles you have.
    3. Now work out volume with concentraition and number of moles.
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Formulas, Yeild and Atom Economy

Empirical and molecular formula:

  • They are ratios.
  • Empirical formula give smallest whole number ratios of atoms in compound.
  • Molecular formula gives actual numbers of atoms in molecule.
  • Molecular formula made up of whole numbers of empirical units.

Empirical formulas calculated from percentage composition:

  • Worked out from percentages of different elements

Percentage yeild:

  • Never 100%.
  • Theoretical yeild, mass of product that should be formed. Assumed no chemical lost. Uses masses of reactants and balanced equation to caluculate theoretical yeild.
  • Any reaction actal yeild always less.
  • percentage yeild = actual yeild/theoretical yeild x 100
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Atom economy measures efficiency of reaction:

  • Efficiency measured by percentage yeild.
  • But percentage yeild dosen't say how wasteful reaction is. Even reactions with 100 % yeild can be wasteful if desired products wind up in by products.
  • Atom economy measures proportions of reactant atoms.
  • Calculated using:
    % atom economy = mass of desired product/total mass of reactants x 100.
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Bilal Ibrar

thanks a lot!!!!!!!!!!!!

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