Experiment 11

Objectives

The Ideal Gas Equation (PV=nRT) is one of the most important equations used by the chemist. The Universal Gas Constant (R) used in this equation is not just for gases. Many other formulas used in chemistry also use this constant (you can view the gas constant in many different units here).  Therefore, a speedy but accurate determination of this gas constant is worthwhile.  In the experiment you will be performing today, you will rearrange the Ideal Gas Equation to solve for R. Once you have determined the value of this gas constant, you will also determine a percent error, based on the actual value of this constant. In addition, to more accurately determine the value for R, you must use vapor pressure as well. Vapor pressure is a value that has been determined for volatile liquids at different temperatures. Since you will be producing hydrogen gas during this experiment, and collecting this gas over a column of water, some of the gas will actually be water vapor in addition to the anticipated collection of hydrogen gas. The total pressure (and the total volume) inside the eudiometer is due to both hydrogen gas and water vapor. By considering water vapor, you can more accurately determine a value for R since the volume of gas produced inside the eudiometer, is more than just hydrogen since some of the gas is water vapor. The known vapor pressure of water at about 20^oC is actually subtracted from the atmospheric pressure determined by ready a barometer.

Procedure

In today's experiment, you will be reacting Mg ribbon with hydrochloric acid (HCl) using a single-replacement reaction shown below. Please note that the moles (n) of hydrogen gas produced is equal to the number of moles (n) of Mg(s) consumed in the reaction.:

Mg(s) + 2 HCl(aq) → H₂ + MgCl₂(aq)

To set up your reaction you will need the following.

• 100-mL Eudiometer
• A one-hole rubber stopper of the size necessary to fit into the Eudiometer
• A piece of copper wire secured to the outside the the rubber stopper, and threaded through the hole in the stopper.  From the narrow part of the stopper, make a small coil of copper wire designed to hold the Mg ribbon used in the experiment.

To perform the reaction, you will first add about 20 mL of 6 M HCl to the Eudiometer. To secure the Eudiometer, use a ring stand with a buret clamp attached. After adding the HCl, carefully, trying not to mix the HCl, add enough DI water to completely fill the Eudiometer. Attach a piece of Mg ribbon to the coiled part of the copper wire that is inserted into the one-hole rubber stopper.

The Mg ribbon you will be using will likely have some black material on it.  This black material is not Mg, but an oxidized product and must be removed prior to doing the experiment. In order to remove this black material, cut a piece of Mg ribbon about 5 inches long.  Take some sand paper and rub the black material off until the Mg ribbon is metallic looking and almost all the black material is removed. This may take a little bit of time but is essential to getting good results. After you have removed the black material, having now a metallic appearing Mg ribbon, carefully weight the Mg and ensure that it weighs less than 0.0800 g. You cannot use more than 0.0800 g Mg ribbon because that will produce too much H₂ gas, and you will not be able to accurately determine its volume.

Carefully insert the rubber stopper (with the clean Mg ribbon attached to the copper wire that has been inserted through the stopper) into the top of the Eudiometer. After firmly attaching the stopper, some of the DI water will come out of the stopper.  This is normal and, in fact, desired to have only liquid in the Eudiometer.

Once you have set up the reaction in the Eudiometer, remove it from the buret clamp, and invert the Eudiometer so the the sealed end is now on top, and the rubber stopper on the bottom. It is helpful to plug the hole in the stopper with your finger as you inverted the Eudiometer into the beaker containing water. After inverting the Eudiometer, quickly place the Eudiometer back into the buret clamp, but with the rubber stopper immersed in a 500-mL beaker containing about 200-300 mL water. Once you have inverted the Eudiometer, the 6 M HCl, which is more dense than DI water, will start to settle to the bottom of the inverted Eudiometer, where the Mg ribbon is located. When the HCl comes in contact with the Mg ribbon, a visible reaction (gas being produced) will be obvious. As gas (hydrogen) is produced, it will displace water in the Eudiometer.  This is referred to as using "downward displacement." As the hydrogen gas is being produced, since it is displacing the water, some water vapor will also be produced. The amount of water vapor is dependent on the temperature of the water.

To calculate the Universal Gas Constant, you must solve for R starting with the Ideal Gas Equation (PV=nRT). The correct solution would be R=PV/nT where the units are as follows:

• Volume must be in Liters (L); convert the mL volume measured in the Eudiometer into L by dividing by 1000.
• Pressure is in torr, which is mm Hg read directly from your barometer (do not convert your barometric pressure into atm, keep it in torr).
• Temperature must be in Kelvin (K), so you will need to convert your Celsius (^oC) temperature into K by adding 273.15 to your ^oC value.
• The number of moles (n) of hydrogen gas produced is equal to the number of moles (n) of Mg used (see your reaction equation shown above).

Once you have each of the values shown above (and included in the table above, you can calculate the Universal Gas Constant.  You should do at least two different Eudiometer experiments to obtain this data. The table above allows you to include data for three experiments, but you will likely only need to do two. Place your calculations for R, based on your experiment data in the table above.

Error Analysis for R

The correct value for R should be 62.363 L.torr/mol.K. How close did you come to this value using your data above? To determine a percent error for you R value, follow the instructions below.

%Error To obtain %Error for a measurement, you need to know the Theoretical and Experimental values for your measurement, or set of calculations.  Use this formula to obtain the %Error, which is always a positve (absolute) value.

Using the formula above, determine your percent error for each of the experimentally determined values for R from this experiment.

This concludes the experiment on Gas Laws.  Hopefully you have learned something about Gas Laws and how to use a Eudiometer. In addition, you should have learned how to read a barometer to get barometric pressure as well as doing a series of calculations to use the Ideal Gas Equation. Doing experiments is what makes chemistry more interesting and practical, since chemistry is an experimental science.

NOTES:

QUESTIONS