Molar Volume of Hydrogen

Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★☆ Some safety precautions required to perform safely

Categories: Chemical Quantities and Calculations, Chemical Reactions

Alternative titles: Magnesium and Acid Gas Collection

Summary

Magnesium ribbon reacts with an acid to produce hydrogen gas, which is collected and measured. The measured volume can then be used to calculate the molar volume of a gas.

Procedure

Cut a short length of clean magnesium ribbon.
Fill a trough or large beaker with water and invert a gas collection tube (graduated cylinder) full of water into it.
Add dilute hydrochloric acid (or sulfuric acid) to a small flask or test tube.
Place a pre-weighed amount of magnesium ribbon into the flask, and quickly connect the flask to the gas collection tube with delivery tubing.
Allow the reaction to proceed as hydrogen gas is generated and bubbles collect in the inverted tube.
When the reaction is complete, record the volume of hydrogen collected.
Compare the measured volume with the theoretical volume calculated using the reaction equation and Avogadro’s law.

Links

AP Chem 3.4-Molar Volume of a Gas Lab: Tips and Calculations - Crowdedbeaker:

Determination of the molar Volume of a Gas at STP - North Carolina School of Science and Mathematics:

Variations

Compare results at different temperatures to show the effect of conditions on molar volume.

Safety Precautions

Wear safety glasses and gloves.
Handle acids with care; use only dilute solutions.
Ensure apparatus is set up securely to avoid spills and breakages.
Do not ignite the collected hydrogen; keep away from open flames or sparks.
Dispose of acidic waste properly by diluting and neutralizing before disposal.

Questions to Consider

What is the balanced equation for the reaction between magnesium and hydrochloric acid? (Mg + 2HCl → MgCl₂ + H₂)
How does the measured gas volume compare with the expected molar volume of a gas at room temperature and pressure (~24 dm³/mol)?
Why is it important to collect the gas over water? (It allows capture of the gas without significant loss to the air.)
What sources of error might cause the measured volume to differ from the theoretical value? (Gas loss during setup, incomplete reaction, temperature/pressure variations, or solubility of hydrogen in water.)