Lab # 3: Gases Percent Yield of Hydrogen Gas from Magnesium and Hydrochloric Acid

Chemistry 108 Lab #3

Name______Lab # 3: Gases Percent Yield of Hydrogen Gas from Magnesium and Hydrochloric Acid

Introduction For chemical reactions involving gases, gas volume measurements provide a convenient means of determining stoichiometric relationships. A gaseous product is collected in a long, thin graduated glass tube, called a eudiometer, by displacement of a liquid, usually water. Magnesium reacts with hydrochloric acid, producing hydrogen gas:

Mg (s) + 2HCl (aq) ® MgCl2 (aq) + H2 (g)

Note: for every mole of Mg (s) that is reacted, one mole of H2(g) is produced. If we know the mass of Mg(s) we can convert to moles of Mg(s). Then, since we get 1 mole of H2(g) for every mole of Mg(s), we can predict how many moles of H2(g) could be made (theoretical yield). We use an excess of HCl so that we would react all the Mg(s) before we used all of the HCl. When the magnesium reacts with the acid, the evolved hydrogen gas is collected by water displacement and its volume is measured. The temperature of the gas is taken to be the same as the temperature of the water it is in contact with because, given a sufficient amount of time, the two will reach thermal equilibrium. The level of water in the eudiometer is adjusted so that it is equal to the level of water outside the eudiometer. This insures that the pressure in the eudiometer is equal to the prevailing atmospheric pressure. Because the hydrogen gas was collected above water, and water has a significant vapor pressure, to get the pressure of pure hydrogen (dry hydrogen), we must subtract the vapor pressure of water. The pressure of the dry hydrogen gas (PH2) is calculated from Dalton's Law of Partial Pressures:

Ptotal = PH2 + PH2O so PH2 = Ptotal - PH2O

where Ptotal (the pressure in the eudiometer) is atmospheric pressure, and PH2O (the water vapor pressure) is the pressure exerted by water vapor that has evaporated into the eudiometer. We will get the vapor pressure of water from the table below of vapor pressure vs. temperature.

Chemistry 108 Lab #3

The number of moles of hydrogen gas collected can then be calculated from the ideal gas law:

(n= # moles H2) n = PV (Use PH2 here, not Ptotal) RT

This will give you the experimental # moles of hydrogen gas collected. The theoretical # of moles of H2(g) can be calculated by converting the mass of Mg to moles Mg, and understanding that we get 1 mole of H2 from every mole of Mg(s). From the theoretical yield and the experimental yield, one can calculate the percent yield:

Percent Yield = experimental # moles H2 ´ 100% theoretical #mole H2 PROCEDURE:

1. PUT ON RUBBER GLOVES. Fill the largest beaker in your drawer (400 ml or a 600mL beaker) about 2/3 full of water and allow it to sit on the base of a ring stand so that the temperature of the water may adjust to room temperature. Place a buret clamp on the ring stand well above the beaker.

2. Obtain a 4-5 cm length of magnesium ribbon from the back counter of the lab room. Weigh and record the mass of the magnesium ribbon in your data table. Your magnesium should have a mass no larger than 0.0450 g. Roll the magnesium ribbon into a loose coil. Obtain a piece of thread 25 cm in length, and tie it to one end of the magnesium ribbon in such a way that all the loops of coil are tied together (see Figure 1).

3. Get a eudiometer. Always carry a eudiometer in a vertical position. The eudiometer will contain water. Empty out the water into your sink and temporarily attach it to the buret clamp, open end up.

Figure 1

4. Using a glass funnel, add about 10 mL of hydrochloric acid to your eudiometer. Next, add enough water to the eudiometer to fill it completely. Reattach the eudiometer to the buret clamp, open end up (see Figure 2).

5. Obtain a one-hole rubber stopper from the back counter. Take your magnesium coil and lower it into the water of the eudiometer to a depth of about 5 cm. Have the thread attached to the coil hang over the lip and out of the eudiometer. Insert the one- hole rubber stopper into the eudiometer so the thread is held firmly against the edge, and when water squirts out of the hole in the stopper, cover the hole firmly with your thumb (see Figure 1).

2 Figure 2 Chemistry 108 Lab #3

6. Taking care that no air enters, remove the eudiometer from the buret clamp, invert it, and place its open end underwater in the beaker. Re-clamp the eudiometer to the buret clamp so that the bottom of the eudiometer is about 1 cm below the surface of the water in the beaker. The acid will flow down the eudiometer and react with the magnesium.

7. When the magnesium has disappeared entirely and the reaction has stopped, tap the tube with your finger to dislodge any bubbles you see attached to the side of the eudiometer. Measure the temperature of the water in your beaker; this will be the temperature of the hydrogen gas in the eudiometer. Record this value, to the nearest 0.1oC, in your data table. Because your thermometer reads to a tenth of a degree Celsius, add 273.2 when converting to Kelvin.

8. Place your finger over the hole in the stopper and remove the eudiometer from the beaker. Lower the eudiometer into the leveling tank and remove your finger. Raise or lower the eudiometer until the water level inside the eudiometer is the same as the water level in the leveling tank. Read the volume of gas in the eudiometer and record it in your data table. Record the volume to the nearest 0.01 mL.

Figure 3. Inverted eudiometer illustrating gas 9. Your instructor will write the barometer for today’s atmospheric collection and water displacement. pressure on the board. Record this value in your data table. The water vapor pressure can be found in the table on page 2. Record this value for the vapor pressure of water in your data table.

10. When finished with the experiment, clean the eudiometer with deionized water, dry the outside, fill it with deionized water, and return it.

Chemistry 108 Lab #3

DATA TABLE:

Mass of Magnesium Metal = g (must be less than 0.0450 g)

Volume of Gas = mL

Temperature of Gas = Temperature of the water = = °C

Atmospheric Pressure = torr

Water Vapor Pressure (at the above temperature, see table on last page) = torr

CALCULATIONS:

1. Theoretical (calculated) yield of H2 gas (# moles H2). a) Convert mass Mg to #moles Mg

b) Convert # moles Mg reacted to moles of H2 that could be produced (1 mole H2 is produced for every 1 mole Mg reacted- this comes from the balanced chemical equation):

Mg (s) + 2HCl (aq) ® MgCl2 (aq) + H2 (g)

# moles H2 = ______(theoretical yield of H2)

2. Experimental yield of H2 gas (# moles H2). a) Determine pressure of dry H2 (PH2) by subtracting the vapor pressure of water from the total pressure,

Ptotal = PH2 + PH2O so PH2 = Ptotal - PH2O

where Ptotal (the pressure in the eudiometer) is atmospheric pressure, and PH2O (the water vapor pressure) is the pressure exerted by water vapor that has evaporated into the eudiometer. Use the table provided on the last page to find the vapor pressure of water as a function of temperature.

PH2 = ______torr b) Convert this pressure from torr to atm (760.0 torr = 1atm)

PH2 = ______atm 4

Chemistry 108 Lab #3

c) Use the Ideal Gas Equation to calculate the number of moles (n) of H2 that you produced in your experiment (experimental yield). Make sure to use the correct units so that they match the units in the gas constant (R).

3. Calculate the Percent Yield.

% Yield = experimental # moles H2 ´ 100% theoretical #mole H2

% Yield =______

4. What are the possible sources of error in this experiment?