Determining the volume of one mole of hydrogen gas | 14–16 years

In this activity, learners measure the volume of hydrogen gas produced by the reaction of a known mass of magnesium with excess hydrochloric acid. They use the results to determine the volume of one mole of hydrogen gas at room temperature and pressure. Finally, they evaluate the accuracy of their results by comparing their experimental value to the true value.

If it is not possible for learners to carry out the experiment, demonstrate the experiment and get learners to analyse and evaluate the results. The experiment takes approximately 40 minutes.

Learning objectives

1. Measure the volume of hydrogen gas produced in a metal-acid reaction.
2. Analyse the results to determing the volume of one mole of a gas.
3. Evaluate the accuracy of the result obtained.

Scaffolding

Scaffolded and unscaffolded versions of the student worksheet are available. In the scaffolded version, steps guide learners through the calculation to determine the volume of one mole of hydrogen gas.

Teaching notes

• Demonstrate the method beforehand.
• Warn learners not to fold the magnesium ribbon, but to puch it into the burette so it is retained under its own tension.
• Demonstrate the inversion: rest the end of the burette on the tip of the beaker and swing the tap end round and upward to a vertical position. It is not necessary to put a finger over the open end of the burette.
• It is important that the liquid level starts on the graduated scale of the burette. If the level is not on the scale, opening the tap momentarily allows the liquid to drop onto the scale.

Technician notes

Equipment

Apparatus (per group)

• Safety glasses
• Burette
• Burette stand
• Beaker, 250 cm³
• Funnel
• Balance (minimum 2 d.p.)
• Emery paper

Chemicals

• Hydrochloric acid, 2.0 mol dm^–3
• Magnesium ribbon 3.5 cm length
  DANGER: flammable
• Distilled water

Method

1. Clean a piece of magnesium ribbon by gently rubbing it with emery paper and measure its mass accurately. It should have a mass between 0.02 and 0.04 g. If not, adjust the amount used.
2. Measure 25 cm³ of 2.0mol dm^-3 hydrochloric acid into the burette. Slightly tilt the burette and carefully add 25 cm³ of water on top of the acid by running the water down the inside of the burette. Try not to disturb the acid layer.
3. Push the magnesium into the end of the burette so it will stay in position with its own tension.
4. Add 50 cm³ of water to a 250 cm³ beaker.
5. Quickly invert the burette into the water. If this is done quickly and carefully, very little is lost. Clamp the burette vertically.
6. It is important that the liquid level in the burette starts on the graduated scale. If it is not on the scale; momentarily open the tap.  This allows the level to drop.
7. Take the burette reading (Remember: the burette is upside down.)
8. Observe the magnesium reacting as the acid diffuses downwards. Wait until all the magnesium has reacted.
9. Record the new volume on the burette (Remember: the burette is upside down). 

Answers

Answers to the evaluation and follow-up questions can be found in the teacher notes. Answers to the practice calculation questions can be found in the teacher notes and lesson slides.