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A suitable reaction produces a known amount of gas from a measured amount of solid. For magnesium with excess dilute acid, Mg + 2H⁺ → Mg²⁺ + H₂ gives a 1:1 metal-to-hydrogen ratio. Use the known magnesium molar mass to calculate n(Mg), infer n(H₂), then calculate experimental gas molar volume Vm = V/n. Here molar mass is an input and molar volume is the unknown.
Video by Cognito. Foundation recap: the written lesson develops the A-level detail.
Viewing focus: Determine molar volume from a measured mass and gas volume
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: If some gas escapes before collection, what happens to calculated Vm = V/n?
It is too low. The measured V is reduced while n from the weighed metal is unchanged.
After watching 2: Why record temperature and pressure?
Molar volume depends on these conditions. A meaningful reference comparison uses appropriate conditions.
What you'll be able to do
A suitable reaction produces a known amount of gas from a measured amount of solid. For magnesium with excess dilute acid, Mg + 2H⁺ → Mg²⁺ + H₂ gives a 1:1 metal-to-hydrogen ratio. Use the known magnesium molar mass to calculate n(Mg), infer n(H₂), then calculate experimental gas molar volume Vm = V/n. Here molar mass is an input and molar volume is the unknown.
The often-used value near 24 dm³ mol⁻¹ describes an approximation for room conditions, not a universal constant. Record temperature and pressure and compare with a reference for those conditions. A gas collected hot occupies a different volume from the same amount after cooling. If the gas contains water vapour, the total collected pressure is not all due to dry hydrogen.
Use the approved magnesium surface preparation, then measure mass by a suitable balance procedure. Oxide contamination changes the relationship between weighed mass and hydrogen-forming metal. Ensure acid is in excess. Estimate gas volume in advance so the sample fits the syringe capacity. Check the collection path for leaks and the syringe for free movement.
Start the reaction using the laboratory arrangement that minimises gas lost before collection begins. Allow the gas to reach the intended measurement temperature and record the final volume after reaction has finished. An apparent plateau can also arise from an apparatus fault or insufficient acid, so relate it to the reaction design. Hydrogen is flammable: use the approved dilute reagents and keep ignition sources away.
Losing hydrogen lowers V while the weighed metal and inferred n remain unchanged, so experimental Vm is too low. This differs from an experiment using gas volume to infer an unknown metal molar mass: there a smaller inferred n would make m/n too high. Write the equation before predicting the error direction.
Compare relative mass and volume uncertainties and use enough sample to reduce the balance contribution without exceeding capacity. For conservative worst-case uncertainty in a quotient, add the percentage contributions when the stated measurement model calls for it. Repeat independent samples to assess scatter. Repetition cannot compensate for a persistent leak or a wrong molar mass entered in the calculation.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: The mass uncertainty is ±0.0002 g and gas-volume uncertainty ±0.5 cm³. Estimate a conservative percentage uncertainty for the quotient.
Approximately ±1.5%; this estimate does not include a systematic gas leak.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.