edited bv: University of Cincinnati Cincinnati. Ohio 45221
A Problem on Formulas Elaine J. Browne University of Tasmania Hobart, Ausnaiia 7005 This problem can be used to test senior high schdol pupils both for their learning of basic chemical principles (parts (a) and (b)), and their understanding and application of these (parts (c) and (d)). As all information required is supplied, little or no background in organic chemistry is necessary. Question A carboxylic acid contains the functional group
A carboxylic acid was extracted from rhubarb leaves and isolated from water as white crystals (Ad, which analyzed as: C, 19.1; H, 4.8; 0,76.4%. When 1.00 g of (Al) was dried near 100°C the mass of the acid was reduced to 0.71 g, giving the form (Ad. (AJ was reformed on recrystallization of (Ad from water. A solution of 1.00 g of the carboxylic acid (Ad in water was neutralized by 22.2 ml of 1.00 M aqueous sodium hydroxide solution.
Loss of 18 g/E.F.W. of (A1) is consistent with the loss of one molecule of water per E.F. of (A,). :. E.F. of (Az) is CHOz (E.F.W. = 45 g). Note that it is not valid to assume, without doing the calculation. either that it is water that is lost from (A,) or, if so, that exactly one mole of water is lost per E.F.W. of (Ad. (b) As (Az) is derived from (A1) hy loss of water under moderate conditions and (A1) reforms on recrystallization of (Az) from water, this suggests that (A1) contains weakly bound water of crystallization (or hydration). The percentage loss of weight on drying indicates that the empirical formula of (A1) could be writtin CHOTH~O. (c) No, neither empirical formula can be the same as the molecular formula The E.F. of (Ad. . . . CHOI. come8oonds to the whole of the carboaylie acid group
known to be present, and in which the carbon atom necessarily has an unsatisfied valency. The molecular formula of (Ad must therefore he (CH02),,where n is an integer >l. (d) 1.00 g (A2)reacts with 22.2 ml1.0 molar NaOH = 22.2 X 1.011WO moles NaOH. 1male of NaOH reacts with 1000 gl22.2 (Ad = 45.0g (Az),which, from (a), is also theE.F.W. of (Az)It follows, from (4,that n = 2, and that the structural formula of (Ad must be
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(Relative atomic weights: H = 1.C = 1 2 . 0 = 16. Valencies: H = 1, 0 = 2, c = 4.) (a) What are the empirical formulas of (Ad and (Ad? (b) How might the loss of weight of (Al) near 100' be explained? (c) Will the empirical formulas from (a) he the same as the molecular formulas? (d) Suggest a stmctural formula for (Ad. Acceptable Solution
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19.1 4.8 76.4 16 1.59:4.8:4.775 1 1:3.02:3.00 :. Empirical Formula (E.F.) of A1 is CH303. This corresponds to an Empirical Formula Weight (E.F.W.) of 63 g.
(a) Atomic ratios (A1) C:H:O
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Two molecules of water of crystallization per molecule of (Az)are then present in the hydrated crystals (Ad. In conclusion, parts (a) and (b) may be solved by application of rote learning, but conceptual grasp beyond the concrete ooerational level is reouired to answer oart (c) correctlv. Considerable further &sight is needed to de&ce that t6e solution to part (d) must be the symmetrical oxalic acid, and only 10%of grade 12 chemistry studentstested achieved this last step. This problem might thus be suitable both a s an indicator of level of abstract thought attained, and as an aid in selecting - students for admission to honors or advanced placement courses.
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Journal of Chemical Education
