Communication pubs.acs.org/jchemeduc
Comparing Mass and Volumetric Titrations in the General Chemistry Laboratory Lauren McMills,* Frazier Nyasulu, and Rebecca Barlag Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, United States ABSTRACT: Students in the general chemistry laboratory are commonly exposed to volumetric titrations as part of their laboratory curriculum. However, the use of mass burets instead of volumetric burets can save time, expense, and reduce errors. In this laboratory exercise, students are asked to standardize a solution of NaOH(aq) using potassium hydrogen phthalate and determine the acetic acid concentration in vinegar using both techniques. Students are then asked to compare the two approaches. When given a choice in a subsequent laboratory exercise, approximately 75% of the students choose to use the mass approach. KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Aqueous Solution Chemistry, Titration/Volumetric Analysis
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lthough the use of mass burets in place of volumetric burets can save time, expense, and reduce errors,1,2 the use of mass burets does not appear to be increasing. To encourage the use of mass burets, a number of mass-based titrations are included in the general chemistry laboratory at this university. Cash3 published an excellent mass-based titration laboratory exercise; however, a volumetric titration was not performed. The advantage of one procedure over the other is best demonstrated by a head-to-head comparison between them. This lab also provides experience in performing dimensional analysis. In the first 3-h lab session, NaOH(aq) is standardized using potassium hydrogen phthalate (KHP), and in a follow up exercise, the standardized NaOH(aq) is used to determine the concentration of acetic acid in vinegar. Each analysis is performed four times and the mean and percent relative standard deviation are calculated. In the second 3-h lab session, a mass buret is used for the same analysis. Students pool their percent relative standard deviations to determine which method has overall better precision.
Figure 1. The mass buret.
determined by measuring the masses of 1.00 mL aliquots a total of four times.
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VOLUMETRIC TITRATION Students make a ∼0.1 M NaOH(aq) solution in a 100 mL volumetric flask and standardize it using KHP and phenolphthalein indicator. The densities of the NaOH(aq) and vinegar are determined by measuring the masses of 1.00 mL aliquots a total of four times; these aliquots are delivered using an autopipettor.
INTERCONVERSION BETWEEN MOLARITY AND % MASS/MASS Students are required to interconvert units. Examples of interconversions are as follows. p M NaOH(aq) with density q g/mL soln converted to % m/ m NaOH(aq):
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%(m/m) NaOH(aq) 39.99 g NaOH p mol NaOH = × L NaOH(aq) 1 mol NaOH
MASS TITRATION Wash and dropper bottles have been proposed for use as mass burets.3−5 The mass buret setup herein is a small plastic container with a short-stem glass dropping pipet (Figure 1). The mass buret is weighed on a milligram balance before and after the titration; the difference is the mass of NaOH(aq) delivered. Students prepare a ∼4% mass/mass solution of NaOH. The densities of the NaOH(aq) and vinegar are © 2012 American Chemical Society and Division of Chemical Education, Inc.
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1 L NaOH(aq) mL NaOH(aq) × × 100% q g NaOH(aq) 1000 mL NaOH(aq)
Published: May 2, 2012 958
dx.doi.org/10.1021/ed2003466 | J. Chem. Educ. 2012, 89, 958−959
Journal of Chemical Education
Communication
(3) Cash, D. Mohawk College; Gravimetric Titration Part 1: A Simple, Fast Alternative To Using a Volumetric Buret. http://www. uclmail.net/users/dn.cash/GravTitr1.doc (accessed Apr 2012). (4) Christian, R. J. Chem. Educ. 1967, 44, 144. (5) Butler, E. A.; Swift, E. H. J. Chem. Educ. 1972, 49, 425−427.
r mL NaOH(aq) of p M NaOH(aq) used to titrate s grams of vinegar converted to % m/m acetic acid (AA) in vinegar (vin): %(m/m) AA(vin) = r mL NaOH(aq) 1 L NaOH(aq) 1000 mL NaOH(aq) p mol NaOH 1 mol AA × × L NaOH(aq) 1 mol NaOH 60.05 g AA 1 × × × 100% s g vin mol AA ×
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STUDENT COMPARISON BETWEEN THE TWO APPROACHES In a postlab review, students compared the two procedures according to (i) ease of solution preparation, (ii) ease and speed of setup, (iii) ease of titrant delivery before the end point, (iv) end point delivery control, (v) ease of reading, and (vi) magnitude of the % relative standard deviation. Students were asked to pick which of the two methods yielded better results. The summary of these evaluations (Table 1) was distributed to all students. In future labs, we plan to add student assessment of the accuracy of the acetic acid in vinegar analysis. Table 1. Summary of Student Comparison of Volume-Based and Mass-Based Titrations Item
Volume-Based Titrations/Number of Studentsa
Ease of solution preparation Ease and speed of setup Ease of titrant delivery before the end point End point delivery control Ease of reading the buret vs scale Smaller % Relative Standard Deviation a
Mass-Based Titrations/ Number of Studentsa
8
92
0 53
100 47
41
59
3
97
15 (1.9%)
85 (0.34%)
The total number of students was 100.
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CONSIDERATIONS AND CONCLUSIONS By making their own solutions, students can see firsthand why it is necessary to standardize NaOH(aq); actual concentrations are lower. This is one of a few lab exercises in which our students rigorously compare and contrast two methods of analysis. A few weeks later, when students perform a redox titration in which KMnO4(aq) is standardized with Fe(NH4)2(SO4)2·6H2O and the standardized KMnO4(aq) used to analyze household H2O2 solutions, students are offered the option of a volumetric-based or a mass-based titration. About 80% of the students choose the mass-based approach.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
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REFERENCES
(1) Hawkes, S. J. Chem. Educ. 2004, 81, 1715. (2) Ramette, R. W. J. Chem. Educ. 2004, 81, 1715. 959
dx.doi.org/10.1021/ed2003466 | J. Chem. Educ. 2012, 89, 958−959
