Implementing an Inexpensive and Accurate Introductory Gas Density

Activity pubs.acs.org/jchemeduc

Implementing an Inexpensive and Accurate Introductory Gas Density Activity with High School Students W. Patrick Cunningham,* Christopher Joseph, Samantha Morey, Ana Santos Romo, Cullen Shope, Jonathan Strang, and Kevin Yang Science Department, Claudia Taylor Johnson High School, San Antonio, Texas 78259, United States

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 27, 2015 | http://pubs.acs.org Publication Date (Web): July 29, 2015 | doi: 10.1021/acs.jchemed.5b00277

S Supporting Information *

ABSTRACT: A simplified activity examined gas density while employing cost-efficient syringes in place of traditional glass bulbs. The exercise measured the density of methane, with very good accuracy and precision, in both first-year high school and AP chemistry settings. The participating students were tasked with finding the density of a gas. The discovery activity facilitated their understanding of the basis of the table of atomic masses. This activity should provide instructors of pre-AP and AP chemistry classes with an acceptably precise and accurate single-lab-period investigation that functions either within the gas law unit or introduction to atomic theory and atomic masses. KEYWORDS: First-Year Undergraduate/General, Inquiry-Based/Discovery Learning, Laboratory Instruction, Physical Properties, Gases, High School/Introductory Chemistry methane, equipment imprecision, and environment fluctuations.

he empirical study of gas density is important to first-year college chemistry and AP chemistry students.1 Understanding the calculations behind relative gas densities was historically the gateway to tables of atomic and molecular masses.2 Gas density measurement is an essential part of basic chemical education, but it is a challenging property to discover accurately in the lab. Determining relatively small densities with satisfactory precision and accuracy can be a challenging task with unpredictable results.3 Whether as a companion piece within the study of gas properties or atmospheric composition,4 or as a standalone activity, determining gas density involves measurement techniques that must be mastered early in the study of chemistry.5 Traditionally, these measurements are made using expensive, thick-walled glass flasks and vacuum lines or pumps. Although plastic syringes are an inexpensive alternative promoted in a Flinn Scientific activity,6 gas density measurements using them can be both inaccurate and imprecise; the method often yields data sets with large standard deviations and percent errors. Poor laboratory results can make the conceptual and experiential scaffold on which students build their understanding of first-year chemistry weaker than it should be. This problem is not insuperable, however.

T

■

Common Problems/Solutions

A sample group of control students tested the original protocol during their study of gas density and inquiry into atomic masses. The lab was used to supplement instruction and solidify the student’s foundation of atomic masses. When results are accurate and easily replicated by students, it facilitates student understanding of the topic, as measured by student surveys taken after they used the improved protocols. However, the poor accuracy and precision of the unamended protocol can cause the activity to undercut learning from the lecture setting. To optimize both the accuracy and precision of the original protocol, several sources of error were isolated and corrected. Syringe size is chief among data skewing variables. The use of small syringe containers decreases the amount of gas that a student is able to measure. This, combined with the already low density of gas, decreases the accuracy of measurement because the mass of the contained gas approaches the design uncertainty of the commonly used centigram balance. After comparing data from two syringe sizes, 60 and 140 mL, the 140 mL syringe is found to give the best accuracy. Given the small mass of the enclosed sample, particularly for low-density gases, the uncertainty of a centigram balance is close to the actual mass. Subsequently, students used both milligram and centigram balances for accuracy and precision when measuring gas density. A calibrated milligram balance placed on a heavy stone surface optimizes measurements due to the added mass sensitivity. The milligram balance adds one significant digit to the recorded data. Another important factor is gas choice. Following tests of several gases, helium was found to have a mass too close to the balance uncertainty to obtain accurate results, even when using

GAS DENSITY WITH SYRINGES

General Procedure

The original procedure, introduced in his workshops by Irwin Talesnick and popularized further in Flinn Scientific videos (see Supporting Information), substitutes small, inexpensive plastic syringes for glass bulbs. In such an approach, students measure the mass of the gas in a syringe, then determine the mass of the gas alone by subtracting the measured mass of the evacuated syringe. They then perform density calculations after recording the volume of the sample.5 Although simple in theory and operation, this method can be fraught with uncertainty owing to the low density of commonly studied gases like oxygen and © XXXX American Chemical Society and Division of Chemical Education, Inc.

A

DOI: 10.1021/acs.jchemed.5b00277 J. Chem. Educ. XXXX, XXX, XXX−XXX


Journal of Chemical Education

Activity

when expanding an empty syringe against atmospheric pressure. (See Supporting Information.) Keep at hand the MSDS documents for all chemicals.

the milligram instrument. Four common gases are found to be optimal for this lab: natural gas, for its low cost and availability from lab bibs; oxygen, for its almost universal availability in college lab settings; carbon dioxide, for its low cost and ready availability from gas supply houses or in-house synthesis; and propane, for its availability/mobility from local stores. All gases, however, are best secured with an accompanying composition analysis. This permits the instructor to post accepted values for the molar mass of each gas and students to compute a percent deviation. Similarly, fluctuations in temperature and pressure in addition to vibrations of the surface on which the balance rests can cause significant discrepancies in results.7 To resolve this issue, students were supplied with a draft-free chamber around the balance using a clear plastic bin and a granite slab under the balance to reduce environmental vibrations. (See illustration in the Supporting Information.) Temperatures and ambient pressures were measured during the investigation and found to be constant. The problem of underpressurizing or overpressurizing the gases in the syringe was minimized by making sure the syringe was briefly open to the atmosphere after introduction of each gas. After the preparatory work described above, a sample group of 50 students, divided into groups of two and three, performed multiple trials of the revised protocol using natural gas. This activity design had two main pedagogical goals: (i) to help students learn the basic concepts of gas density by capturing and measuring gases and (ii) to enable students to discover the variations in accuracy and precision that the use of different measuring instruments can yield. Each trial alternated between different syringe sizes (60 or 140 mL) and balance sensitivities (centigram or milligram). Throughout this single-blind investigation, participants were timed, averaging 21 min per group. (See the Supporting Information.) Each group was supervised by experienced members of the research team to ensure safety and to clarify procedures if necessary, though the purpose of the activity was withheld from the student investigators. In accordance with research student trials, the first year/AP student data concluded that the optimized activity includes the use of the 140 mL syringe and the milligram balance. (See the Supporting Information.) A review of student reports indicated widespread basic comprehension of the improved precision due to the more sensitive balances and the larger number of significant digits attained with that combination. By measuring the mass, volume, temperature, and pressure of each gas sample, students can calculate gas densities, average deviation, and percent error. After analyzing the students’ results, an improved protocol was written and tested, and it is given in the Supporting Information. Accepted values given in the Supporting Information are for natural gas, a mixture almost entirely composed of methane. Densities were supplied by the local public utility.8

■

CONCLUSION Due to persistent shortfalls in public education spending, chemistry departments may turn to inexpensive syringes as modes of measuring gas density. However, without a specific, standardized procedure resolving the accuracy and precision issues that arise, students may become less engaged or frustrated in the classroom setting. The results of this activity indicate that the milligram balance on stone in a draft free chamber is more accurate and precise than the same students find with the centigram balance. Student-produced data indicated that the use of a 140 mL syringe produced results closest to the accepted values and gives one more significant digit than a syringe less than 100 mL in volume. Therefore, the protocol calls for a 140 mL syringe with a milligram balance mounted on a stone platform within a draft-free chamber measuring one or more of the four recommended gases. This modern version of a traditional gas density lab can be applied as a traditional or inquiry investigation to support lessons on this topic in both high school AP courses and introductory college level courses.

■

ASSOCIATED CONTENT

S Supporting Information *

Notes for instructors with complete activity procedures, diagrams and photographs of the set-ups, and experimental data; handout with procedures; and worksheet for students. The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.5b00277.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. W. Patrick Cunningham teaches chemistry at Claudia Taylor Johnson High School, San Antonio, Texas; he also advises the ACS chemistry club and leads the high school scientific research and design course whose students were involved in this work.

■

REFERENCES

(1) Zumdahl, S. S.; Zumdahl, S. A. Chemistry, AP ed., Vol. 9; Brooks Cole: Belmont, MA, 2014; pp 44−70, 207−208. (2) Zumdahl, S. S.; Zumdahl, S. A. Chemistry, AP ed., Vol. 9; Brooks Cole: Belmont, MA, 2014; pp 44−50. (3) Brown, K. E.; Mickelos, A.; Carver, J. S.; Hunter, W. J. F. A Teaching Plan for Introducing Gas Properties. Chemical Educator 2004, 9, 220−223. (4) Williams, D. R. Earth Fact Sheet. http://nssdc.gsfc.nasa.gov/ planetary/factsheet/earthfact.html (accessed May 2015). (5) Roe, R.; Davenport, D. A. Goals: Why Do We Teach the Gas Laws? J. Chem. Educ. 1985, 62 (6), 505−506. (6) Becker, B. Density of Gases, Flinn Scientific video. https:// elearning.flinnsci.com/ViewProduct.aspx?product=EL9056 (accessed May 2015). (7) Quinlan, F. Experiment 7: The Ideal Gas Law and Density. http://www.napavalley.edu/people/fquinlan/Documents/ Chem%20120/Experiment%207%20-

■

HAZARDS Wear safety goggles, aprons, and gloves throughout this activity. Tape any glass flasks used to produce gases to guard against possible shattering. If contact with chemicals used in generating gases occurs, rinse the affected area with cool water for several minutes, and notify an instructor immediately. Conduct gas collection inside a fume hood. Keep natural gas away from open flames, sparks, and high temperatures. Exercise caution when drawing gases from high-pressure tanks. Care must be taken B


Journal of Chemical Education

Activity


%20The%20Ideal%20Gas%20Law%20and%20Densities.pdf (accessed May 2015). (8) Thrailkill, D. Certificate of Analysis. CPS Energy, San Antonio, Texas. https://www.cpsenergy.com/en.html (accessed May 2015). Email correspondence, 4 June 2014.

C


Implementing an Inexpensive and Accurate Introductory Gas Density

Recommend Documents