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Molar Calculations 2

Applied Science: BTEC Level 3 National Extended Diploma, topic 1.1.14

Molar Calculations of Gases

We can also calculate the number of moles of a gas. It is difficult to measure the mass or the concentration of a gas so we must use another equation.

Avogadro's Law

Avogadro's Law

  • The equation is based upon Avogadro's Law:
  • Avogadro's Law says that at the same temperature and pressure, one mole of two different gases will be the same volume.
  • So, regardless of what the gas is, one mole of a gas will always be a particular volume at a specific temperature and pressure.
Room temperature and pressure

Room temperature and pressure

  • At room temperature and pressure, 24 dm3 is the volume taken up by one mole of any gas.
The equation for moles in a gas

The equation for moles in a gas

  • This leads to the equation:
  • Number of moles of a gas = volume of the gas ÷ 24
  • This equation is only true if:
  • The volume is measured in dm3.
  • The gas is at room temperature and pressure.
The equation for volume of a gas

The equation for volume of a gas

  • Another form of this equation gives the equation
  • Volume of a gas = moles of the gas × 24
  • This equation volume calculated:
  • Is in dm3.
  • Is correct at room temperature and pressure.
Converting the equation

Converting the equation

  • A useful conversion for this equation is:
  • 1 m3 = 1,000 dm3
  • Alternatively, we can use the converted equation:
  • Number of moles of a gas = volume of the gas ÷ 24,000
  • Volume of a gas = number of moles of the gas × 24,000
  • This is true for the volume in cm3.

Stoichiometry

The stoichiometry of a chemical equation refers to the big numbers that sit before each component in the equation.

An example

An example

  • In the chemical equation:
  • N2 + 3H2 → 2NH3
  • The stoichiometry refers to the coefficients 1, 3 and 2 that sit before each of the species.
The coefficients

The coefficients

  • We have seen these coefficients many times before.
  • They balance the equations.
  • The coefficients, however, offer more than balancing equations. They are very useful in molar calculations.
Use in molar calculations

Use in molar calculations

  • The coefficients are useful as they represent the ratio of the amount of each substance used or gained in the reaction.
  • We know that the number of moles is a measure of the amount of each substance.
  • If we know the number of moles of one species, we can use the coefficients to determine the number of moles of the other species.
  • It is easiest to consider this using an example.
A calculation example

A calculation example

  • In the reaction:
  • N2 + 3H2 → 2NH3
  • We know there are 0.5 moles of N2
  • Using coefficient ratios, we know there is three times as many moles of H2
  • There are two times as many moles of NH3
  • So:
  • There are 1.5 moles of H2
  • There is 1 mole NH3

Test yourself on Molar Calculations 2

  • At room temperature and pressure, what volume is taken up by one mole of a gas?

  • How do we calculate the number of moles of a gas at room temperature and pressure?

  • What is the equation for the volume of a gas at room temperature and pressure?

  • N2 + 3H2 → 2NH3
    In this reaction, how many times more moles are there of NH3 than N2?

  • Which of the following is true about coefficients in chemical equations?

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