Amount of Substance

Empirical and Molecular Formulae

Empirical and Molecular Formulae

The empirical formula can be calculated if you know the composition by mass of the compound. It represents the simplest whole number ratio in which the atoms of a compound exist. The molecular formula only applies to molecular substances and can be calculated when the molar mass and empirical formulae of a compound are known. It represents the number of each atom type that makes up one molecule of a substance.

If you know the empirical or the molecular formulae it is also possible to figure out the percentage composition by mass of a substance.

Using Moles

By utilising the four relationships already described and knowing the chemical equation and the amount of one reacting species it is possible to figure out the amount of any substance within a chemical reaction.

For gases:

  • n = PV / RT
  • n = mass / RMM

For solutions:

  • n = CV
  • n = particles / L

The ratios in the equation can be used to find the number of moles of other species.

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Gases

The volume that a gas occupies depends on:

  • Temperature: the hotter the gas the faster the particles are moving and the more space occupied.
  • Pressure: the higher the pressure the more compressed the particles and the less space occupied.
  • Gas amount: the more gas particles the more space occupied.

The volume of gas is not dependent on the type of gas because one mole of any gas, at the same temperature and pressure, has the same volume

Pressure, temperature, volume and gas amount are related through the following equation:

PV = nRT

In which:

  • P = pressure (measured in pascals (Pa) or Nm-2)
  • V = volume (measured in m3)
  • T = absolute temperature (measured in Kelvin (K))
  • R = the molar gas constant (value is 8.31 Jmol-1 K-1)

By using the relationship m = n x mr this equation can be rearranged in order to calculate the density of gases and the RMM of gases.

PV = mRT / mr

So, the mass of one mole in kg:

mr = mRT / PV

The answer should then be converted into grams.

The density of a gas or a mass/volume can be calculated through:

(m/V) = mrP / RT