News from MicroLab Inc.

Two Featured Lab Applications:

1. An Improved and Affordable Procedure for a Rapid and Accurate Estimate of Absolute Zero

Doug Schumacher and Kiran Kurmi (class of 2011),

Luther
College, Decorah, IA

One of the most common activities in first semester general chemistry lab is the study of ideal gas law. The topics that these investigations cover include determinations of the molecular weight of a gas, studies of Dalton's law of Partial Pressures, gas phase reactions, and the determination of absolute zero using pressure and temperature measurements. While these experiments generally produce acceptable results, they are not without problems and limitations. In particular, either the experiments must be done at atmospheric pressure or one must be able to measure the pressure accurately within the particular setup. In the latter case, the vessel that is used must be rated for the range of pressures that the procedure uses.

One experiment done in the general chemistry course at Luther College extrapolates the temperature-pressure relationship of a gas to determine an experimental value for absolute zero. The setup is very simple: a 25-mL side-arm flask is immersed in a room temperature water bath set on top of a stirring hotplate. This flask is connected to a MicroLab unit (at Luther, we still use our older model 402 units, but of course the newer FS-522 can also be used), which is used to follow the pressure inside the flask and the temperature of the water bath as the temperature of the water bath is slowly increased.

After the data are collected, the students plot temperature (y-axis) vs. pressure (x-axis) and extrapolate back to zero pressure for their value of absolute zero. This method produces results typically within ±20% of the accepted value of absolute zero: not too bad, but actually not so good either. Unfortunately, this setup suffers from a number of issues that caused us to thoroughly examine what we do. The most important of these are (1) most side-arm flasks are not rated for any sort of excess internal pressure and (2) the internal temperature of the flask is assumed, often incorrectly, to be the same as the temperature of the water bath.

During the past semester, we examined several replacement vessels for our absolute zero determination experiment. The most important considerations in our search were that the container must be inert to any reaction we wished to carry out in it, it must tolerate internal pressure, it must be easy to connect to the MicroLab 16-bit 0-2 atm pressure sensor, and (most of all) it must be very affordable. Our candidates ranged from PVC pipe, plastic bottles, copper tubing of various diameters, and glass soda bottles. Each of these items was tested by performing our absolute zero experiment with the container and analyzing the results.

After all of the analysis was done, we settled on a glass 10oz Schweppes™ ginger ale bottle for our experiments. This container met all of our requirements, and it was extremely affordable at $4.50 for 6 re-usable bottles (which include the ginger ale as an added bonus). As you can see in Figure 1, the size of the mouth of the bottle easily accepted a # 3 rubber stopper with a single ¼" hole in it.

**Figure 1. Setup of the ginger ale bottle showing detail of thermistor and pressure connector.**

This allowed us to connect the bottle (filled with air) to the MicroLab pressure sensor and follow the internal pressure of the bottle. Additionally, we drilled a 1/8" hole in the stopper to accept a MicroLab Model 103 thermistor, which allowed us to measure the internal temperature of the bottle rather than the water bath. We obtained a value of absolute zero of -269.38°C (see Figure 2), which is within 1.3% of the accepted value of -273.15°C, with a correlation coefficient >0.998. MicroLab's high resolution, low noise data provided an excellent result considering the large extrapolation from the measured data. We also examined the

Figure 2. The improved accuracy and precision of the results obtained from measuring the temperature and pressure within the container are evident. Also evident is the sensitivity and low noise of the pressure sensor, with a range of 0.07 atm (<60 torr).

effect of water vapor in the bottle by filling the bottle with dry nitrogen, but we did not observe a noticeable effect in the measured value of absolute zero. Most importantly, we found that the experiment itself was very fast. The data shown were collected in 30 minutes, which allows several determinations to be completed in a single lab period.

In short, replacing the side arm flask with a glass soda bottle has been a success. The setup is very affordable, easily replaceable, and gives excellent results. We believe that we have only touched on the potential of this setup for gas-phase experiments.

2. A Simple and Rapid Method to Determine the Moles of CO2 Gas Produced in a Chemical Reaction Using the MicroLab FS522

Tom Kuntzleman and Scott Reese,
Spring Arbor University, Spring Arbor, MI

Reaction stoichiometry is one of the key concepts of any introductory chemistry course. There are lots of ways to investigate stoichiometry, including titrations, precipitations, colorimetry, and gas generation. Titrations and colorimetry are usually introduced in the context of analytical determinations. Most precipitation reactions involve metals and can be expensive - and very time consuming if mass measurement is involved. Our method, which involves the direct measurement of the change in pressure from a gas generated in a reaction, takes advantage of the 16-bit precision of the MicroLab FS522 unit to measure relatively small pressure changes rapidly, accurately, and precisely. We save money on reagents and still get excellent results with low risk from pressure buildup. An additional pedagogical advantage is that we use the Ideal Gas Law, PV=nRT, to convert P, V, T measurements to moles of gas produced.

The reaction studied is the acid-base reaction between acetic acid (vinegar) and sodium hydrogen carbonate (baking soda):

CH₃COOH (aq) +NaHCO₃ (s) à H₂O + CH₃COONa (aq) + CO₂ (g)

P_CO2 and T are found using the MicroLab FS-522 pressure and temperature sensors and MicroLab's software. The volume of the gas produced is measured by finding the volume of water required to fill the flask-stopper-hose assembly used in this experiment. R is the gas constant in units of L atm mol^-1 K^-1. Students compare the number of moles produced in the reaction as calculated by PV = nRT with the number of moles expected based on the stoichiometry of the reaction and mass of NaHCO₃ used. Students complete the calculations by demonstrating that NaHCO₃ is the limiting reagent based on the stoichiometry of the reaction.

Apparatus:

A dry 250 mL flask is fitted with 2-hole rubber stopper. In one hole of the stopper is placed an on/off Luer Lock stopcock, to be fitted later into a 3 mL plastic syringe. A snug-fitting hose is placed into and through the second hole, the other end of which attaches directly to the MicroLab pressure sensor input. This should be tested for leaks to make sure that all seals are air tight. (The Luer Lock fittings, tubing, and syringes are available from MicroLab for your convenience or from various suppliers.)The volume of the gas, which must be measured when the reaction is complete, is the same as the volume of the flask assembly and hose to the pressure sensor connector on the MicroLab unit.

Figure: The plot shows a 7 sec baseline after which the vinegar was injected but not allowed to come in contact with the sodium bicarbonate. The pressure rise between 7-8 sec is due to the vapor pressure change and the ~1% decrease in air volume from adding the solution. At the 10 sec mark, the flask was swirled to initiate the reaction. The entire run took about 1 minute.

The MicroLab FS-522 Role:

To build this experiment in the MicroLab software, three sensors are used: pressure, thermistor temperature, and time. The pressure (torr) sensor is set up on the y-axis as well as in the spreadsheet and in the digital display. Time (sec) is placed on the x-axis. A thermistor is used to get the air temperature, which is a good approximation of the system temperature. Alternatively, the entire flask assembly can be placed in a large beaker of water to maintain a constant temperature.

Safety and disposal:

None of the reagents or products requires special disposal. The usual practice of wearing eye and hand protection should be observed. There is no pressure release on the system, so the reaction should not be scaled up without including a pressure release vent to prevent an unsafe buildup of pressure within the system.

Procedure:

Approximately 1 mmol of NaHCO₃ (1 mmol = 0.0840 grams), weighed precisely, is placed into one side of the bottom of the flask as shown in the photo, and the stopper attached. Next, 3 mL of white vinegar (5% CH₃COOH) are loaded into the syringe and, with the Luer Lock stopcock closed, the syringe is twisted tightly onto the stopcock. The "Start Experiment" button is pressed on the MicroLab screen. Once a baseline pressure is confirmed, the stopcock is opened and the vinegar is injected into side of the flask opposite the NaHCO₃, and the stopcock closed. (We like to tilt the flask to be certain the vinegar ends up on the side of the flask opposite the baking soda). This allows the system to adjust for any pressure change due to the injection and to the vapor pressure of the water in the vinegar solution. A new baseline pressure is confirmed. Next, the flask is gently swirled to allow the reagents to mix completely. Data are collected until the MicroLab pressure vs. time graph levels off (see the Figure). The experiment is stopped, and data are analyzed. P(CO2) is taken as the difference between the maximum, constant final pressure and the baseline pressure after injection of vinegar. The software can be used to get the average temperature. The volume is measured by obtaining the mass of the water needed to fill the empty flask assembly with water.

Sample calculations from an actual trial:

Mass of NaHCO₃ = 0.0840 g = 1.00 mmol

Volume of vinegar added: 3.0 mL (ca. 2.5 mmol)

PV=nRT and so n=PV/RT

P_CO2 = P(final) − P(new baseline after Injection)

= (800.79-735.95) torr/760torr/atm = .0853 atm

V_CO2 = V(flask assembly) = 292.3 mL=.2923 L

R= 0.0821 L*atm/K*mol

T= 23.783 ^oC = 296.933 K

n = (0.0853 atm)(0.2923 L)/(0.082144 L*atm/K*mol)(296.933 K)

= 0.00102 mol CO₂formed

moles CO₂/moles NaHCO₃ = 0.00102/0.00100 = 1.02/1

Conclusions:

This experiment is a safe, easy to perform, and easy to understand application of pressure measurements to get accurate information about a chemical reaction involving a gas. There is time in lab to do multiple runs. Thus there is plenty of time in lab for doing calculations and discussing results in the context of limiting reagents, balanced equations, and sources of error. It is also possible to extend this lab by varying the amount of vinegar or the amount of NaHCO₃ to prepare a plot of P(CO2) vs. mole ratio of NaHCO₃/CH₃COOH to determine reaction stoichiometry. Variations on this procedure can be extended to a whole host of gas-producing reactions. This experiment is a great example of how MicroLab can change how students learn in lab.