Measuring Substances

To measure chemicals requires different strategies depending on the state of matter.

  • Solids: Weigh out the substance
  • Liquids: Measure volume
  • Solutions: Use the solution concentration to calculate moles

Running Example

Example:

1L Aqueous Solution has 571.6 gm of H2SO4 and has a density of 1.329 g cm-3. Determine the molarity, molality, mole fraction and mass percentage.

Density = Mass/Volume. Therefore Mass = Density * 1L = Density.
Therefore Mass = 1.329 Kg.

Measuring Concentration

Concentration of X is written as [X] -> e.g. NH3 [2]
There are many different ways of expressing concentration

Molarity/Molar Concentration

  • Molarity = Moles per Litre
  • Concentration = Amount of Solute/Volume of Solution (C = n/v)
  • However this changes with temperature, as Volume increases/decreases

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Example

  • H2SO4 has an atomic weight of 98.08 (g).
  • 571.6/98.08 = 5.827
  • 5.827/Litres of Solution * 1L = 5.827 Mol L -1

Molality

  • Molality = Moles per kilogram of solvent
  • Concentration = Amount of Solute/Mass of Solvent

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Example

  • 1.329 Kg - 571.6 g = 757.4 gm of H2O
  • 5.827/0.7574 = 7.69 Mol Kg -1

Molar Fraction

  • Molar Fraction = Not a unit, just what fraction of moles is X
  • Concentration = Amount of Solute/Total Moles

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Example

  • Weight of 757.4/18.016 (mass of H2O) = 42.04 moles of water
  • Mole Fraction = 5.827/(5.827+42.04) = 0.12

Percentage Composition by Mass

  • Percentage Composition = What percentage of the mass is X
  • Concentration = (Mass of Solute/Total Mass) * 100

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Example

  • 571.6 gm/1329 gm = 0.43
  • 0.43 * 100 = 43%