Calculate Volumes of Gases (Edexcel IGCSE Chemistry (Modular))

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Calculate Volumes of Gases

Avogadro's Law

  • Avogadro’s Law states that at the same conditions of temperature and pressure, equal amounts of gases occupy the same volume of space

  • At room temperature and pressure, the volume occupied by one mole of any gas was found to be 24 dm3 or 24,000 cm3

  • This is known as the molar gas volume at RTP

  • RTP stands for “room temperature and pressure” and the conditions are 20 ºC and 1 atmosphere (atm)

  • From the molar gas volume the following formula triangle can be derived:

Molar Volume dm3 (decimetre) Formula Triangle, IGCSE & GCSE Chemistry revision notes

Formula triangle showing the relationship between moles of gas, volume in dm3 and the molar volume

  • If the volume is given in cm3 instead of dm3, then divide by 24,000 instead of 24:

Molar Volume cm3 Formula Triangle, IGCSE & GCSE Chemistry revision notes

Formula triangle showing the relationship between moles of gas, volume in cmand the molar volume

  • The formula can be used to calculate the number of moles of gases from a given volume or vice versa

    Simply cover the one you want and the triangle tells you what to do

To find the volume

Volume = Moles x Molar Volume

Examples of Converting Moles into Volumes Table

Name of Gas

Amount of Gas

Volume of Gas

Hydrogen

3 mol

(3 x 24) = 72 dm3

Carbon Dioxide

0.25 mol

(0.25 x 24) = 6 dm3

Oxygen

5.4 mol

(5.4 x 24,000) = 129,600 cm3

Ammonia

0.02 mol

(0.02 x 24) = 0.48 dm3

To find the moles

Moles = Volume ÷ Molar Volume

Examples of Converting Volumes into Moles Table

Name of Gas

Volume of Gas

Amount of Gas

Methane

225.6 dm3

(225.6 ÷ 24) = 9.4 mol

Carbon Monoxide

7.2 dm3

(7.2 ÷ 24) = 0.3 mol

Sulfur Dioxide

960 dm3

(960 ÷ 24) = 40 mol

Oxygen

1200 cm3

(1200 ÷ 24,000) = 0.05 mol

Using mass to calculate the volume of a gas 

  • You may be asked to calculate the volume of a gas from a given amount stated in grams instead of moles

  • To answer these type of questions you must first convert grams to moles and then calculate the volume.

Worked Example

What is the volume of 154 g of nitrogen gas at RTP?

Answer:

  • Step 1: Calculate the moles of nitrogen:

    • Mr (N2) = 2 x 14 = 28

    • Moles of N2 = 154 over 28 = 5.5 mol

  • Step 2: Calculate the molar volume of nitrogen:

    • Volume = moles x 24

    • Volume = 5.5 x 24 = 132 dm3 

  • A second style of gas calculation involves calculating the volumes of gaseous reactants and products from a balanced equation and a given volume of a gaseous reactant or product

  • These problems are straightforward as you are applying Avogadro's Law, so the moles ( and coefficients) in equations are in the same ratio as the gas volumes

Worked Example

The complete combustion of propane gives carbon dioxide and water vapour as the products.

C3H(g) + 5O2 (g) → 3CO(g) + 4H2O (g)

Determine the volume of oxygen needed to react with 150 cmof propane and the total volume of the gaseous products

Answer:

  • The balanced equation shows that 5 moles of oxygen are needed to completely react with 1 mole of propane

  • Therefore, the volume of oxygen needed would be = 5 moles x 150 cm750 cm3

  • The total number of moles of gaseous products is = 3 + 4 = 7 moles

  • The total volume of gaseous products would be = 7 moles x 150 cm1050 cm3

Examiner Tip

Make sure you use the correct units as asked by the question when working through reacting gas volume questions.

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