Reacting Volume Calculations
- Reacting volume calculations are commonly associated with the reactions of gases
- All gases occupy the same volume under the same conditions
- At room temperature and pressure (r.t.p.), the molar gas volume of any gas is 24.0 dm3
- Room temperature and pressure are 293 K / 20 oC and 101.3 kPa respectively
- At standard temperature and pressure (s.t.p.), the molar gas volume of any gas is 22.4 dm3
- Standard temperature and pressure are 273 K / 0 oC and 101.3 kPa respectively
- At room temperature and pressure (r.t.p.), the molar gas volume of any gas is 24.0 dm3
- The equation to calculate the number of moles for any volume of gas is:
Number of moles =
Worked example
Molar gas volume calculations
Calculate the number of moles present in 4.5 dm3 of carbon dioxide at:
- Room temperature and pressure
- Standard temperature and pressure
Answers
-
- Number of moles 0.1875 moles
- Number of moles 0.201 moles (to 3 s.f.)
- Volumes of gas can also be calculated using
- Number of moles calculations
- Volume calculations
- This can help determine the size of the equipment that you use in an experiment
Worked example
Calculating gas volumes from moles
Calculate the volume of gas produced when 1.50 g of sodium reacts with water at standard temperature and pressure.
Answer
Step 1: Write the balanced equation for the reaction
-
- 2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
Step 2: Calculate the number of moles of sodium
-
- Number of moles 0.0652 moles (to 3s.f.)
Step 3: Use the stoichiometry of the equation to calculate the number of moles of hydrogen
-
- The stoichiometric ratio of Na : H2 is 2 : 1
- Therefore, the number of moles of hydrogen is 0.0326 moles
Step 4: Calculate the volume of hydrogen gas evolved
-
- Volume of gas = number of moles x molar gas volume
- Volume of gas = 0.0326 x 22.4 = 0.730 dm3 (to 3s.f.)
Worked example
Calculating gas volumes from other volumes
Calculate the total volume of gas produced when 6.50 dm3 of propane combusts completely
Answer
Step 1: Write the balanced equation for the reaction
-
- C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (g)
Step 2: Determine the number of moles of gas produced
-
- One mole of propane produces 3 moles of carbon dioxide and 4 moles of water
- Therefore, one mole of propane produces a total of 7 moles of gas
Step 3: Calculate the volume of gas that is produced
-
- 6.50 dm3 of propane will produce 7 x 6.50 dm3 of gas = 45.5 dm3 gas