Quick answer: The ideal gas law, PV = nRT, links pressure, volume, amount and absolute temperature of a gas. This AP Chemistry lab on ideal gas law gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Magnesium dissolves in acid and releases hydrogen. If you collect the gas over water, measure its volume, temperature and pressure, and know the mass of metal, you can calculate the gas constant R from PV = nRT. This lab tests whether your measurements agree with the accepted value of R.
Level: AP Chemistry · Curriculum: AP Chem Unit 3
Aim
To measure the volume of hydrogen released by magnesium and acid, and to use it to find the gas constant R.
Equipment
- magnesium ribbon (about 0.025 g, 0.050 g and 0.075 g pieces)
- balance reading to 0.001 g
- 1.0 mol/L hydrochloric acid, 20 mL for each run
- 50 mL measuring cylinder, trough, delivery tube and bung
- thermometer and barometer reading (or weather-station pressure)
Safety
Wear goggles. Hydrochloric acid at 1.0 mol/L is an irritant: wash splashes with water. Hydrogen is flammable, so no flames. Magnesium ribbon should be dry and clean.
Method
- Weigh a piece of magnesium ribbon and record its mass.
- Fill the measuring cylinder with water, invert it into the trough without letting air in.
- Put 20 mL of acid into the flask, add the magnesium and quickly fit the bung.
- Wait until the metal disappears and bubbles stop, then read the gas volume with the water level inside and outside equal.
- Record the water temperature and the air pressure.
- Repeat with the other two masses.
Results
Sample data. Air pressure 100.80 kPa, water temperature 21.0 °C (294.15 K), vapour pressure of water at 21.0 °C = 2.49 kPa.
| Trial | Mass of Mg (g) | Volume of H₂ (mL) |
|---|---|---|
| 1 | 0.0250 | 25.6 |
| 2 | 0.0502 | 51.6 |
| 3 | 0.0751 | 76.2 |
Analysis questions
Question 1 Write the equation
Write a balanced equation for the reaction and state the mole ratio Mg : H₂.Question 2 Amount of hydrogen
Calculate n(H₂) for trial 1 (M(Mg) = 24.305 g/mol).Question 3 Pressure of hydrogen
Calculate the partial pressure of dry hydrogen.Question 4 Calculate R
For trial 1, calculate R = PV/(nT) in L·kPa/(mol·K).Question 5 Mean value
Trials 2 and 3 give R = 8.35 and 8.24. Calculate the mean R and the percentage difference from 8.314.Question 6 Graph
Describe the graph of volume against mass and what its gradient gives.Question 7 Sources of error
State two errors and the direction in which each affects R.Question 8 Improvement
Suggest an improvement that reduces the uncertainty.
