Experiment 6: Experimental Determination of Ideal Gas Law
Constant
Purpose: To determine the value for the universal gas constant, R, using the gas
generated when hydrochloric acid reacts with magnesium metal.
Prelab work:
 Read the procedure described below.
 Write a balanced equation for the reaction between HCI and Mg.
 Set up the stoichiometric relationship needed to answer question 8.
 Prepare a data table for the 6 values that will be collected.
 Prepare a calculation tables for personal and class results.
Procedure
1. Fill a 250 or 400 mL beaker about half full of tap water and allow it to adjust to room
temperature.
2. Cut a piece of magnesium ribbon between 3.30 and 4.30 cm long. Cut the ends of
the ribbon squarely, and measure it to the nearest 0.01 cm.
3.
Obtain a piece of copper wire about 15 cm long, tie it to the magnesium ribbon which has been folded
to fit inside the gas measuring tube and wrap the wire around the magnesium making a “basket”.
Fig. 1 Magnesium ribbon
in copper wire basket.
4. Prepare a ring stand and clamp to support the gas measuring tube as shown.
5. Slowly pour about 10 mL of 6M HCI into the gas tube.
6. Incline the gas tube and slowly fill it completely with tap water, mixing the water and
acid as little as possible.
7. With the tube completely full of water, insert
the copper while holding the Mg ribbon about 3
or 4 cm into the tube. (Don't let go!) Secure the
copper wire against the side of the tube by
inserting a 1-hole stopper. The stopper should
force water and all air bubbles out of the tube and
should hold the copper wire in place.
8. With your finger over the hole in the stopper
(make sure there is no air in the hole of the
stopper), invert the tube and place the stoppered
end in the beaker half full of water. Clamp the gas
measuring tube in place so that the bottom of the
rubber stopper is not resting on the bottom of the
beaker.
9. After the reaction is completely over and the
products of the reaction have cooled to room
temperature, record the final volume and
temperature of the collected gas. Also record the
height of the water column in the gas measuring
tube (from the water's surface to the top of the
water in the tube).
10. Record the barometric pressure, room
temperature, and the mass of 100 cm of
magnesium ribbon provided during class.
Fig. 2 Eudiometer before and after reaction.
Calculations
1. Assume uniform thickness and width of the Mg ribbon and the mass of one meter to
calculate the mass of magnesium which reacted in your experiment.
2. Calculate the moles of magnesium used and the moles of hydrogen produced.
3. Calculate the partial pressure of the hydrogen gas that was produced in the tube by
correcting the barometric pressure (air pressure in the room) for the partial pressure of
water vapor and the height of the water column that remained in the tube.
Use the equation shown to determine this partial pressure of “dry” hydrogen. The last
component corrects for the height of the water column in the eudiometer (gas
measuring tube). *Of course, all pressures must be in the same units *
pH2 = pair - pH20 - height of water column (in cm) X 760 torr/(33.9 feet X 12 inches/ft X
2.54 cm/ft)
Use Fig. 3 to help understand why the partial pressure caused by water molecules must
be subtracted if the pressure of “dry” hydrogen is desired.
Fig. 3 Illustration of “dry” hydrogen
over mercury.
4. Solve for R using YOUR experimental values for V, n, T and P of the hydrogen gas
you collected.
5. Determine the percent error of your calculated R and also for the average R using
class data.
6. Calculate your volume of hydrogen gas at STP using the Combined Gas Law: (PV/T
= PV/T).
7. Use your data to calculate the molar volume of hydrogen at STP and “room
temperature”.