Writing a Chemical Equation from Titration Data
Harold M. State
Allegheny College Meadville, Pennsylvania
Experiment for general chemistry
The quantit.ative significance of chemical equations can be made more real to beginning students by inverting several standard t,itrimetric procedures. St.ndents are provided with solntions for which concentarationsare given in grams of solute per milliliter and are asked to perform the titrations. From the volumes of t,he solutions used and their concentrations, the number of grams and the number of moles of each reactant may be calculated; from the latter the mole ratio of the reactants may be ohtained which then permits the student. to balance the chemical equation which he is given in generalized form. Since most students have some acquaintance with acid-base reactions, t,hey are first directed to titrate a 40 to 45 ml sample (qnant,itat.ively measured, of course) of phosphoric acid solution with sodium hydroxide solntion using bromcresol green as indicator. Both the acid and hydroxide solutions are made approximately 0.1 molar hut are standardized by the usual quantitative procedures,' and their concentrations are given in grams of solute per millilit,er. From his data the student is asked to calculate n, the "moles" of NaOH reacting with one mole of H3P04and to complete the eqnat,ion, nNaOH
+ H81'04
-
+ nH2O
N~.H~-,POI
Sample calculations from n student.'^ report are shown below. NaOH
Concentrations, glml Volumes used, ml
4.08 x lw3 42.5
HsPOd 1.03 X 1 0 P 41.0 = 4,33
Moles NaOH used =
42.5 X 4.08 X 40.0
Moles HsPO, used =
41 . 0 X 1 .O3 X lo-' = 4,31 98.0
lo-5
' WILLARD,H. H., FURMAN, N . H., and BRICKER, C . E., ''Elements of Quantitative Analyeis," 4th ed., D. van Nostrand, Princeton, N. J., 1956, p. 141ff.and p. 184. The phosphoric arid may he standardized again~tthe hydroxide solution. Ihid.,p. 256ff. and p. lG4B.
x I O - ~= 4.31 X 1 0 F
Xole ratio, n = 4.33
In a class of 80 students, 48 obtained results between 0.98 and 1.02 for the mole ratio, while 70 were within + 5% of the accepted valne (1.000). Using about 25 ml of the phosphoric acid solution, the titration is repeated using phenolphthalein as indicator, and the student is again asked to determine n and to balance the equation. In the same class 52 of the results for n lay in the range 1.9G2.04, while 66 were within * 5 % of the accepted value (2.000). These titrations are followed by an iodine-thiosulfate titration which is less familiar to the student and for which he is less likely to think he knows the answer. The equation is presented in ionic form in this case:
+ bI,
aS20r-2
-
cI-
+ S,O,-"
Approximately 0.1 molar thiosulfate and 0.05 molar iodine (K18-) solutions, st,andardized by the customary procedure^,^ are employed in this titration; starch solution is used as indicator. From his data the student is asked to calculate the mole ratio, a/b,and to complete the equation; the values for c, x, y, and n in the equation follow by logical reasoning once the mole ratio of the reactants is established. I n this reaction only 16 of the 80 students obtained a ratio of a/b within 2% of the accepted value of 2.000, but 86% had results within *5yO. The experiment serves t,o emphasize not only that a chemical equation represents specific gram- and moleratios of the reagents but alsothat quantitative measurements are necessary before a chemical equation can be written withcertainty. Many students are genuinely surprised that "n" in the acid-base reaction is not three in both titrations. The experiment also affords an introduction to titrat,ion techniques and to the fact that not all acid-base indicators are "neutralization" indicators. The author will he glad to furnish a copy of the student directions for the experiment t o interest,ed persons.
Volume 39, Number
6, June 1962 / 297
