The Ideal Gas ConstantName
HrDate
Introduction
The ideal gas law is represented by the formula PV = nRT, where R is the ideal gas constant. In this laboratory investigation you will experimentally determine the value of R. To do this, you must first determine the values of the other variables in the ideal gas equation. You will generate and collect a sample of hydrogen gas and determine its volume, temperature, pressure, and the number of moles produced under laboratory conditions.
The hydrogen gas is generated in a graduated cylinder from the reaction between magnesium and hydrochloric acid. Hydrochloric acid is in excess in the reaction so that the moles of hydrogen gas produced may be determined from the moles of magnesium that react.
Pre Lab Questions
1. Write the balanced equation for the reaction that occurs between magnesium and hydrochloric acid.
Problem
What is the value of the ideal gas constant?
Procedure
1. Fill a large beaker with approximately room temperature water.
2. Obtain a strip of magnesium that is less than 1.0 cm long. Measure the mass of the magnesium and record this mass in the Data Table.
3. Use the micropipet to dispense approximately 3 mL of 3.0 M HCl into the bottom of an empty 10-mL graduated cylinder. Insert the micropipet into the center of the cylinder to avoid getting acid on the sides of the cylinder. CAUTION: Hydrochloric acid is corrosive. Avoid spills and splashes. If you do spill some, rinse the affected area with water and immediately inform your teacher.
4. Fill the rest of the graduated cylinder with tap water using the micropipet. Drip the water down the inside surface of the cylinder to prevent mixing the acid with the water. The water should overfill the cylinder top slightly to form a smooth curved surface.
5. Roll or fold the magnesium and place it carefully onto the surface of the water in the cylinder. It should float. Quickly cover the cylinder with a square of plastic wrap, stretch the plastic tight, and secure it with a rubber band. Make sure that there are no air bubbles trapped under the plastic wrap. See figure at right..
6.With a pin, poke a tiny hole in the plastic over the cylinder mouth. Holding the cylinder by the base (to avoid heating it), immediately invert it in the beaker of water. Hold the inverted cylinder vertically with its mouth submerged. Do not block the pin hole. See Figure 28-2.
7. Holding your finger over the hole in the plastic wrap, quickly' invert the cylinder into the beaker of water. When the top of the cylinder is underwater you may remove your finger. Rest the cylinder in the beaker.
8. Notice the appearance of the acid solution inside the cylinder. Record any indication of a chemical reaction.
9. When the Mg ribbon is no longer reacting, tap the side of the cylinder to release any trapped bubbles.
10. Let the cylinder sit for 5 minutes. Using the thermometer, read and record the temperature in the beaker.
11. Determine and record the atmospheric pressure in the lab. Determine the water vapor pressure from a reference table.
12. Lift the graduated cylinder slightly until the levels of water inside and outside the cylinder are the same.
13. Read and record the volume of gas in the cylinder. Remember that you are reading an inverted cylinder.
14. After reading the volume of gas, remove the cylinder from the beaker and dispose of the contents of the beaker by pouring it down the drain.
15. Clean up your work area and wash your hands before leaving the laboratory.
Data:
Trial 1 / Trial 2mass of Mg strip (g)
volume of hydrogen collected (mL)
temperature of water in basin (°C)
atmospheric pressure (mmHg)
atmospheric pressure (atm)
water vapor pressure (mmHg)
H2 gas pressure
moles Mg (mol)
moles H2 (mol)
R value
Average R value
Calculations-After doing calculations, enter data in data table. Show all work and units.
1. Calculate the number of moles of Mg that reacted.
2. Determine the value of n. Use the balanced equation and the number of moles of Mg that reacted to calculate the moles of H2 produced.
3. Determine the value of P. Calculate the pressure of the H2 gas collected by subtracting the water vapor pressure from the atmospheric pressure. Convert your pressure units from mm Hg to atmospheres.
4. Determine the value of V. Calculate the volume of gas collected in liters. Remember that you must read the bottom of the meniscus, but that the scale is inverted. Then convert the volume units from mL to L.
5. Determine the value of T. Convert the temperature units of the gas collected from °C to kelvins.
6. Using the pressure, volume, temperature, and moles of H2, calculate the value of the gas constant where R = PV/nT. Include all units in your answer. Average the two trials.
Post Lab Questions
1. Why is it necessary to subtract the value for water vapor pressure from atmospheric pressure to determine the pressure of the H2 gas?
2. What evidence of a chemical reaction did you observe?
3. Using the accepted value for the ideal gas constant, determine the percent error of the value you calculated. Then explain the possible sources of experimental error in this investigation.
4. What is the importance of your choice of units in expressing the value of the ideal gas constant?
6. Convert the pressure of dry H2 gas to kilopascals and calculate the value of R in kPa-L/mol-K.
7. If all other conditions remained the same, how would the value of R change if your investigation made use of a gas other than hydrogen? Explain.