Ideal Gas Law Experiment 11 Guide
What Is the Ideal Gas Law Experiment?
The Ideal Gas Law experiment is a staple in chemistry and physics labs. It verifies the relationship between pressure, volume, temperature, and the amount of gas. Most textbooks refer to it as Experiment 11 because that's where it typically appears in lab manuals.
You work with the equation PV = nRT. The goal is usually to determine the gas constant R or find an unknown property of a gas through controlled measurements.
This guide covers the theory, procedure, calculations, and common pitfalls so you can actually understand what you're doing instead of just plugging numbers into a calculator.
The Equation You Need to Know
Before you touch any equipment, memorize this:
PV = nRT
- P = Pressure (atm, Pa, or mmHg)
- V = Volume (L)
- n = Number of moles
- R = Gas constant (0.0821 L·atm/mol·K or 8.314 J/mol·K)
- T = Temperature (Kelvin)
If any three variables are known, the fourth can be calculated. Most Ideal Gas Law experiments involve solving for one unknown while controlling the others.
Common Versions of the Experiment
Experiment 11A: Finding the Gas Constant R
This is the most common version. You collect a known volume of gas (usually hydrogen or oxygen) over water, measure its pressure and volume, then calculate R and compare it to the accepted value.
Experiment 11B: Determining Molar Mass
You find the molar mass of an unknown volatile liquid by vaporizing it, measuring the gas produced, and applying the Ideal Gas Law.
Experiment 11C: Pressure-Temperature Relationship
You verify that pressure is directly proportional to temperature at constant volume (Gay-Lussac's Law), which is a special case of the Ideal Gas Law.
Materials You'll Need
- Gas collection apparatus (eudiometer or graduated cylinder)
- Water trough or basin
- Thermometer
- Barometer
- Heat source (hot plate or Bunsen burner)
- Reaction flask or test tube
- Rubber tubing
- Stopwatch
- Balance (for mass measurements)
The exact setup depends on which version of the experiment you're performing. Check your specific lab manual for details.
The Procedure: Step by Step
For the Molar Mass Determination (Most Common)
- Weigh a small sample of the unknown liquid (volatile organic compounds work well).
- Place it in a reaction flask and seal it.
- Heat the flask to vaporize the liquid completely.
- The vapor displaces air and collects in a graduated tube.
- Record the volume of collected gas.
- Measure the temperature of the gas.
- Measure the atmospheric pressure with a barometer.
- Subtract water vapor pressure from total pressure to get partial pressure of the gas.
- Calculate the number of moles using the Ideal Gas Law.
- Divide the original mass by moles to get molar mass.
That last step is where most students mess up. The mass you started with is the mass of the vapor, so use it directly.
For Finding the Gas Constant R
- Generate a known gas (hydrogen from Mg + HCl, or oxygen from H2O2 decomposition).
- Collect the gas over water in an inverted graduated cylinder.
- Record the volume of gas collected.
- Measure the water temperature.
- Record atmospheric pressure.
- Subtract water vapor pressure to get dry gas pressure.
- Calculate moles of gas from the known reaction stoichiometry.
- Solve for R using R = PV/nT.
Critical Calculations
Water Vapor Pressure Correction
This trips up almost everyone. Gases collected over water are saturated with water vapor. The partial pressure of water must be subtracted:
P(gas) = P(atm) - P(H₂O)
Use a water vapor pressure table for your measured temperature. This value changes dramatically with temperature.
Temperature Conversion
Always use Kelvin, never Celsius. The Ideal Gas Law breaks with Celsius because it allows negative values.
K = °C + 273.15
Pressure Units
Make sure your units match. If R = 0.0821 L·atm/mol·K, then pressure must be in atm and volume in liters. Convert mmHg to atm: 1 atm = 760 mmHg.
Data Analysis: Comparing Methods
| Method | Materials | Difficulty | Typical Error |
|---|---|---|---|
| Hydrogen generation | Mg ribbon, HCl | Moderate | Incomplete collection, leaks |
| Oxygen generation | H2O2, catalyst | Moderate | Temperature fluctuation |
| Vaporization of liquid | Unknown volatile liquid | High | Loss of vapor, incomplete vaporization |
| CO2 from baking soda | NaHCO3, vinegar | Low | Gas dissolves in water |
Each method has trade-offs. Hydrogen generation is straightforward but requires careful handling of acids. Vaporization methods are more sensitive to experimental errors.
Common Mistakes That Ruin Your Results
- Not subtracting water vapor pressure — this alone can throw off your answer by 5-30% depending on temperature.
- Using Celsius instead of Kelvin — an easy way to get completely wrong numbers.
- Leaks in the apparatus — gas escapes and your measured volume is too low.
- Not waiting for temperature equilibrium — measuring gas temperature before it stabilizes.
- Ignoring significant figures — your calculated R might look precise but be wildly inaccurate.
- Forgetting to correct for meniscus — read the gas volume at the bottom of the curved water surface.
How to Get Started: Practical Checklist
- Read the entire procedure before you enter the lab. Know what each step does.
- Set up your apparatus and check for leaks. Bubbles where they shouldn't be mean problems.
- Record the barometric pressure before you start. It doesn't change much during a lab, but note it.
- Measure temperatures immediately. Gas cools or warms fast.
- Take multiple trials. One measurement is never enough.
- Calculate R or molar mass as you go. Catch errors while you can still repeat the trial.
What Your Results Should Look Like
If you're finding the gas constant R, expect a value close to 0.0821 L·atm/mol·K. A result between 0.07 and 0.09 is reasonable for most lab conditions. Anything outside that range means something went wrong.
For molar mass determinations, compare your calculated value to known values. Acetone is around 58 g/mol. Water is 18 g/mol. Ethanol is 46 g/mol. If you're way off, check your mass measurements and pressure corrections.
Troubleshooting Weird Results
Calculated R is too high: Your pressure measurement might be wrong, or water vapor pressure wasn't subtracted properly. Also check if your volume measurement is accurate.
Calculated R is too low: Gas likely escaped, or your temperature reading is wrong. Leaky tubing is the usual suspect.
Molar mass too high: Some liquid didn't vaporize completely. You measured more mass than actually became gas.
Molar mass too low: Air got into your collection system, or the gas contained water vapor you didn't account for.
Final Notes
The Ideal Gas Law experiment teaches you to think about gas behavior quantitatively. The math is simple. The experimental technique is where most people struggle.
Focus on the corrections: water vapor pressure, temperature in Kelvin, proper unit conversion. Get those right and your results will be accurate. Get them wrong and no amount of careful measurement will save you.