PAG1: Mass, Gas Volume and Moles

A quantitative reaction connects a measured mass with an amount of gas through a balanced equation. One possible investigation reacts magnesium with excess dilute acid and collects hydrogen. Mg + 2H⁺ → Mg²⁺ + H₂ gives one mole of hydrogen per mole of magnesium. From measured gas volume and a molar volume appropriate to the temperature and pressure, calculate moles of gas, then the metal’s molar mass from mass divided by moles.

Another possible context finds the carbonate content of a mixture from its carbon dioxide production. Check the reaction ratio and whether the remaining material also reacts to give gas. PAG1 includes mass and gas-volume skills; a mass-only hydration experiment does not by itself provide gas-volume evidence. These contexts illustrate the skill group rather than prescribing one universal experiment.

Use the approved surface preparation for magnesium, weigh the sample accurately and record balance readings with their resolution. Oxide contamination changes what the mass represents. Use excess acid so the weighed sample determines the theoretical gas amount. Choose quantities that fit the gas syringe capacity; estimate the final volume before starting. Hydrogen is flammable, so keep ignition sources away and follow the laboratory procedure.

Check for leaks and a freely moving syringe plunger. Start the reaction using the approved arrangement that minimises gas escape before collection begins. A reaction vessel connected to a syringe must retain an unobstructed pressure-release path through that apparatus. If collecting over water, account for gas solubility and, when the accuracy requires it, water vapour and pressure differences. Allow the collected gas to reach the measurement temperature before using the corresponding molar volume.

Convert cm³ to dm³ before dividing by a molar volume in dm³ mol⁻¹. In a magnesium molar-mass calculation, losing gas makes the inferred amount too small. With the measured mass unchanged, M = m/n then becomes too large. In a carbonate-percentage calculation, losing carbon dioxide instead makes the inferred carbonate amount and percentage too small. The direction depends on the calculated quantity, not merely on the phrase “gas loss”.

Repeat independent samples to assess scatter and compare instrument contributions using percentage uncertainties. A larger sample reduces the relative balance contribution but must remain within gas capacity and safe quantities. Repeated runs cannot correct a persistent leak. A specific improvement is a checked collection connection or a start arrangement that avoids losing early gas, with its assumptions stated.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.