EXPERIMENT 6: USING THE IDEAL GAS LAW TO DETERMINE PURITY OF A ZINC SAMPLE

Lansing Community College General Chemistry Laboratory I

LEARNING OBJECTIVES

1. Generate and collect hydrogen gas over water by the reaction of zinc metal with HCl (aq).
2. Determine the percent purity of zinc sample combining the ideal gas law with stoichiometry.

INTRODUCTION

An ideal gas follows the ideal gas law at all conditions of P and T. The particles in an ideal gas do not have finite size and volume. The collisions between the ideal gas particles are said to be elastic, they exert no attractive or repulsive forces. Hydrogen gas generated in today’s experiment is, however, a real gas not an ideal gas. Real gases consist of molecules of finite size, which exert forces on each other. A gas will act like an ideal gas if its gas molecules are small, when the pressure is low, and the temperature is high. For our experimental conditions today, we can assume that hydrogen gas adheres to the ideal gas law.

Today’s experiment is an example of a single replacement reaction. It involves the measurement of the volume of hydrogen gas generated from a reaction of zinc with excess hydrochloric acid.

Zn(s) + 2HCl(aq)  → ZnCl2(aq) + H2(g)                                                                                                                                  Equation 1

The hydrogen gas produced will be collected over water in a buret. This results in a gas sample that is a mixture of hydrogen gas and water vapor inside the buret. To determine the pressure of hydrogen gas alone, you will need to use Dalton’s Law of Partial Pressures. In accordance with Dalton's Law, the atmospheric pressure (PT) is equal to the sum of the pressures of hydrogen gas (PH) and the vapor pressure of water (PW):

PT = PH + PW                                                                                                                                                                                                                                                                                                                                                                                     Equation 2

To obtain the vapor pressure of water at a given temperature, refer to Table 1 in page 2.

The volume collected can then be used to calculate the moles of hydrogen produced by using the ideal gas law.

Equation 3

The grams of zinc present in the impure sample can be determined by using the calculated the moles of hydrogen gas produced and the coefficients of the balanced equation (Equation 1).

CALCULATIONS

You will use the ideal gas law to determine the moles of hydrogen gas generated in this experiment.

Equation 4

Moles of hydrogen gas ( ) evolved is calculated by rearranging equation 4.

Equation 5

PH2 = partial pressure of hydrogen gas = PA- PW

PH2 is in atm or mmHg. Use the appropriate R value in equation 5.

PT = atmospheric pressure provided by your instructor

PW = vapor pressure of water at temperature T

V = volume in L

nH2 = moles of hydrogen gas evolved

R = Ideal gas constant, 0.08206

R = Ideal gas constant, 62.36

T = Temperature in Kelvin (°C + 273)

The grams of zinc present in the impure sample can be determined by using the calculated the moles from equation 4.

Gram of Zn reacted = _____ mol H2 x  = _____ g Zn                          Equation 6

Finally determine the percent purity of the zinc sample by dividing the mass of zinc reacted by the mass of the impure sample and multiplying by 100%.

% Zn in the Sample =  x 100%                                                                                                                                                                        Equation 7

Table 1.  Vapor Pressure of Water at different Temperatures

 Temperature (°C) Vapor Pressure (mm Hg) Temperature (°C) Vapor Pressure (mm Hg) 16.0 13.6 24.0 22.4 17.0 14.5 25.0 23.8 18.0 15.5 26.0 25.2 19.0 16.5 27.0 26.7 20.0 17.5 28.0 28.3 21.0 18.6 29.0 30.0 22.0 19.8 30.0 31.8 23.0 21.1 31.0 33.7

EXPERIMENTAL PROCEDURE

(Students should work in pairs for this experiment.)

The apparatus is shown in Figure 6.1. Add distilled water through the funnel until the water level is just below the 0.0 mL mark in the burette. Raise and lower the funnel to help expel any air bubbles. (It is important that no air bubbles remain in the tubing or funnel stem at the beginning of the experiment.) You may have to adjust the height of the funnel relative to the rest of the apparatus. Be very careful during this process so that water does not flow through the rubber tubing used to connect the large test tube to the burette.

Figure 6.1

Tare a large test tube in a 150 mL beaker. Use a spatula to add between 0.100 g - 0.110 g of zinc powder into the large test tube. Record the mass of the zinc in your DATA TABLE. Add about 2.0 mL of 6.0 M HCl into a small test tube (70 mm x 10 mm). Using a forceps, gently slide the small test tube and contents into the larger test tube containing the zinc powder. The small test tube should be in the vertical position (mouth pointing upwards). You should be very careful so that none of the HCl comes in contact with the zinc. If any of the HCl comes in contact with the zinc, you will have to start the experiment all over again. Connect the large test tube to the apparatus while keeping the small test tube containing the HCl in the vertical position. Move the funnel either up or down so that the water in the funnel and the burette are at the same level. This equalizes the pressure of the gases trapped inside the buret with the atmospheric pressure. Record the volume of water in the burette to the nearest 0.01 mL as your initial burette reading in the DATA TABLE. Always read your volume measure from the top down. Measure the temperature of the water in the funnel using a thermometer. Hold the thermometer in the funnel containing water until the temperature is stabilized. Record the temperature to one decimal place in the DATA TABLE. Record the atmospheric pressure (PT) in the DATA TABLE.

Carefully read through the rest of the procedure before continuing.  This portion of the procedure must be carried out by you and your partner in a coordinated manner. If any part of this is unclear, please consult your instructor. The reaction between 6.0 M HCl and zinc occurs rapidly after mixing. One student will be responsible for mixing the zinc and the 6.0 M HCl in the test tubes and the other student will be required to lower the funnel so that the level of water in the funnel is at the same height as the level of water in the burette during the time the hydrogen gas is generated. If you are not sure about what you should do, please consult your instructor before you proceed to the next step.

With one student ready to lower the funnel, the other student should gently incline the large test tube so that a portion of the HCl in the small test tube will run out of the small test tube and come in contact with the zinc powder in the large test tube. Be careful not to allow any of the HCl to flow through the rubber tubing connecting the large test tube and the burette. The student responsible for lowering the funnel must do so, when necessary, after the HCl and zinc have been mixed so that the level of water in the funnel is at the same height as the level of water in the burette. Once the reaction has subsided, gently incline the test tube again so that more HCl comes in contact with the zinc.  After all the hydrogen gas has evolved, the level of water in the burette will remain constant. At this point you should not observe any unreacted zinc. Adjust the height of the funnel so that the level of water in the funnel is at the same height as the level of water in the burette. Record the final burette reading in your DATA TABLE. The volume of hydrogen gas produced is the difference between the final and initial burette readings.

Pour the contents of the large test tube into a labeled waste container in the hood. Remove the small test tube with forceps. Wash the large and small test tubes with soap and water followed by rinsing with distilled water. Place the large test tubes in the tray and the small test tubes in a 400 mL beaker found next to the oven.

Do a complete set of calculations on the data from run 1.  Show the % zinc in the sample to your instructor and then proceed with runs 2 and 3. For each run use a clean, dry set of glass tubes from the side bench. Record your results. Use your data to calculate the percent purity of zinc.

Clean Up & Waste Disposal

1. Pour all the contents of the large test tube into the labeled zinc chloride waste container in the hood.
2. Wash all the test tubes with soap and water followed by rinsing with distilled water. Leave the wet test tubes in a tray next to the oven.
3. Put away the beaker and the thermometer.
4. Clean the bench top with a moist paper towel.

DATS SHEET FOR EXPERIMENT 6 - USING THE IDEAL GAS LAW TO DETERMINE PURITY OF A ZINC SAMPLE

Name   _________________________________

Date   _________________

Lab Partner Name: ______________________________

Record the measurements with the correct units and the correct number of significant figures.

DATA TABLE (3 pts.)

 Quantity Run 1 Run 2 Run 3 Water temperature (T) Atmospheric pressure (PT ) Vapor pressure of water (PW) from Table 1 Partial pressure of H2  (P ) Mass of impure Zn sample used Final burette reading (mL) Initial burette reading (mL) Volume of H2 gas (VH) (mL) Volume of H2 gas (VH) (L)

Observations: Use the numbered steps from your procedure followed by your observations. Include at least three observations for full credit. (2 pts.)

Calculations:

1. Use the ideal gas law (See equation 5.) and data from the table on the previous page to calculate the moles of hydrogen gas. Show the calculation setups for Run 1 with units in place below. Be sure to report your answers to the correct number of significant figures in the appropriate box. (3 pts.)

 Quantity Run 1 Run 2 Run 3 Moles of H2 gas evolved

1. Solve stoichiometry problem (See equation 6.) to determine the grams of zinc reacted. Show the calculation setups for Run 1 with units in place below. Be sure to report your answers to the correct number of significant figures in the appropriate box. (2 pts.)

 Quantity Run 1 Run 2 Run 3 Grams of zinc reacted

1. Calculate the percent purity of the zinc sample (See equation 7.). Show the calculation setups for Run 1 with units in space below. Be sure to report your answers to the correct number of significant figures in the appropriate box. (2 pts.)

 Run 1 Run 2 Run 3 % zinc in the sample

POST LABORATORY FOR EXPERIMENT 6 - USING THE IDEAL GAS LAW TO DETERMINE PURITY OF A ZINC SAMPLE

Name_______________________________                                    Date: ___________________

1. Using the combined gas law, calculate the volume of H2 gas in mL at STP starting with your experimental data in Run 1. (3 pts.)

1. Calculate the average deviation for percent zinc. Show your work in the space below. Use the average deviation value to discuss precision. (In this experiment, an average deviation for percent zinc of 3.0 % is considered good.) (3 pts.)

1. How would the following errors affect your calculated value for % purity of zinc (the calculated value would be higher, lower or unaffected)? Refer to equations 5 - 7.

1. When calculating the moles of hydrogen produced, you forget to subtract the vapor pressure of water from the atmospheric pressure in determining the partial pressure of hydrogen. Explain. (2 pts.)

1. There was a leak in your system during the reaction between the zinc and HCl. Explain.

(2 pts.)