chapter 3 amount of substance

amount of substance and the mole

  • amount of substance n is used to count the number of particles in a substance
  • this is measured in moles
  • the Avogadro constant is 6.02 x 10^23 mol^-1 the number of particles in each mole of carbon-12
  • one mole of atoms of any element is its relative atomic mass in grams 
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molar mass

  • molar mass is the mass per mole of a substance
  • units of molar mass are g mol^-1

amount n = mass m / molar mass M

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molecular formulae

  • the molecular formula is the number of atoms of each element in a molecule
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empirical formula

  • the simplest whole number ratio of each element in a compound
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relative molecular masses

  • relative molecular mass compares the mass of a molecule with the mass of an atom of carbon-12
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relative formula mass

  • relative formula mass compares the mass of a formula unit with the mass of ana tom of carbon-12
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hydrated salts

  • many coloured salts are hydrated
  • water molecules are part of their crystalline structure
  • the water is known as water of crystallisation
  • to calculate:

calculate the amount in mol of anhydrous salt

calculate the amount in mol of water 

find the smallest whole number ratio of the water and salt

  • how accurate is an experimental formula?

not all the water may have been lost when the water was driven off

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using volume for measuring amount of substance

  • liquids and gases are measured by volume
  • 1dm^3 = 1000cm^3 = 1000ml = 1 litre
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moles and solutions

  • the concentration of a solution is the amount of solute in moles dissolved in each dm^3 of solution
  • a 1mol dm^3 solution contains 1 mol of solute dissolved in every 1 dm^3 of solution
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converting between moles and solution volumes

  • n = c x v (dm^3)

or

  • n = c x ( v (cm^3) / 1000 )
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standard solution

  • A standard solution is a solution of known concentration
  • Are usually prepared by dissolving an exact mass of the solute in a solvent and making up the solution to an exact volume 
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molar gas volume

  • the molar gas volume is the volume per mole of gas molecules at stated temperature and pressure
  • many experiments are carried out at room temperature and pressure
  • RTP is about 20c and 101kPa pressure
  • At RTP 1 mole of gas molecules has a volume of 24.0dm^3 = 2400cm^3
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converting between amounts in moles and gas volume

  • amount n = volume V / Molar gas volume Vm
  • When V is in dm^3 molae gas volume is 24
  • When V is in cm^3 molar gas volume is 24000
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ideal gas equation

  • molecules making up an ideal gas have:

-random motion

-Elastic collisions

-Negligible size

-No intermolecular forces

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pV = nRT

  • P - pressure (pa)
  • V - volume (m^3)
  • N - the number of gas molecules (mol)
  • R - ideal gas constant (8.31 Jmol^-1K^-1)
  • To convert:

- cm^3 to m^3 - x10^-6

-dm^3 to m^3 - x10^-3

- degrees to K - +273

-kPa to Pa - x10^3

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stoichiometry

  • In a balanced equation, the balancing numbers give the ratio of the amount in moles of each substance
  • The ratio is called stoichiometry
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quantities from amounts and equations

  • Work out the amount in moles of whatever you can
  • Use the equation to work out the amount in moles of the unknown chemical
  • Work out any unknown information required
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percentage yield

  • The maximum possible amount of product is the theoretical yield
  • This is difficult to achieve because the reaction may have not gone to completion
  • Other reactions may have taken place alongside the main reaction
  • Purification of the product may result in the loss of some product
  • The actual yield obtained from a reaction is usually lower than the theoretical yield

percentage yield = (actual yield /theoretical yield) x 100

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limiting reagent

  • the reagent that is not in excess will be completely used up first and stop the reaction - this is the limiting reagent
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atom economy

  • measure how well atoms have been utilised
  • reactions with high atom economies:

-produce a large proportion of desired products and few unwanted waste products

-Are important for sustainability as they make the best use of natural resources

atom economy = (sum of molar masses of desired products/sum of molar masses of all products) x 100

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