The Ideal Gas Law Lab
Purpose: To calculate the ideal gas constant, R, by collecting a known quantity of hydrogen gas over water
at a known temperature and pressure.
Background Information:
The ideal gas law is a mathematical relationship between all four of the gas law variables: Pressure,
Temperature, Volume and moles (n). Using the ideal gas law, you do not need to compare two different sets
of conditions. Instead, if you know three of the four variables, you can solve for the fourth at those
conditions. The equation is as follows:
PV = nRT
where P is pressure, V is volume, n is moles, T is temperature, and R is the Universal Gas Constant. This
constant holds true for any ideal gas (one that closely follows the assumptions of Kinetic Molecular
Theory), and thus it is called the ideal gas law. In this lab, you will be collecting hydrogen gas, which
luckily behaves much like an ideal gas.
You will be collecting the hydrogen gas from a chemical reaction between magnesium metal and
hydrochloric acid. You will then allow it to bubble through water into a gas collection tube. Be VERY
CAREFUL with the gas collection tubes because they cost $70 to replace, and you will be billed if you
break one. By calculating the number of moles of hydrogen gas produced (using stoichiometry) and by
measuring the temperature, pressure and volume, you will actually attempt to calculate the universal gas
constant, R.
This lab requires the knowledge of vapor pressure. Unfortunately, simply because there is water
involved, there will be water vapor present in the hydrogen gas you collect during this lab. Fortunately for
us, the vapor pressure of water is well known, and we can look it up in a table and simply subtract it from
the total pressure to find the actual pressure of the hydrogen gas (on its own).
Procedure:
1.
Obtain a piece of magnesium ribbon from the instructor (approximately 2 cm long) and
measure its length to the nearest 0.01 cm. Record the length.
2.
Roll the length of magnesium ribbon into a tight coil and place it on a copper clip, as shown
in Figure 1 below.
Mg Ribbon
(coiled)
Copper Clip
Figure 1
3.
The next step requires the use of 6M hydrochloric acid (very dangerous). Practice steps 4
through 6 several times before adding the acid and copper clip. Carefully pour about 10 mL of 6M
hydrochloric acid into the gas collecting tube.
4.
While holding the gas collecting tube at about a 45º angle, slowly pour water from the small
beaker down the side of the tube, being careful to layer the water over the acid so they do not mix. Add
additional water until the tube is completely filled. You should be able to see water above the opening of
the tube.
5.
Clip the copper clip (with Mg coil) on the end of the collecting tube. Make sure that the Mg
is inside the collecting tube.
6.
Place you finger securely over the mouth of the collecting tube. Invert tube into the
graduated cylinder, making sure no air gets into the tube. Observe as the acid descends the tube.
7.
When the Mg has disappeared entirely and the reaction is complete, adjust the level of the
gas collecting tube so that the levels of the liquid in the graduated cylinder and the tube are the same. Read
the volume of the gas inside the tube to the nearest 0.01 mL.
8.
Record the room temperature and the barometric pressure.
Data: Make a data table that will have a place for EACH measurement. Don’t forget units!
Results:
SHOW ALL OF YOUR WORK IN YOUR LAB WRITEUP!
1.
Determine the mass of Mg used by using the conversion factor given in class.
2.
Write the balanced equation for the reaction between magnesium and hydrochloric acid.
3.
Determine the moles of H2 gas produced in the reaction using stoichiometry. What is the
limiting reactant?
4.
Subtract the vapor pressure of water at your recorded temperature from the barometric
pressure in the room, and convert this to atmospheres. This is the pressure of the dry H2 gas.
5.
Algebraically rearrange the ideal gas law to solve for R.
6.
Plug in your values of the moles of H2 (from #3), the pressure of the dry H2 gas (#4, in
atmospheres), the temperature (in Kelvin), and the volume (in Liters). Solve for the value of R.
7.
Given that the accepted value of R is 0.0821 L*atm / mol*K, calculate your percent error.
Conclusion – RERUN and make sure you talk about the related chemistry!