Quantitative analysis for carbon and hydrogen was originally carried out using a technique and apparatus (see figure) originally developed in 1831 by the famous chemist Justus Liebig. A carefully weighed sample of organic compound (C) is placed in a combustion tube (A) and vaporized by heating in a furnace (B). The vapours are swept by a stream of oxygen through a heated copper oxide packing (D) and through another furnace (E), which ensures the quantitative oxidation of carbon and hydrogen to carbon dioxide and water. The water vapour is absorbed in a weighed tube (F) containing magnesium perchlorate and the carbon dioxide in another weighed tube (G) containing asbestos impregnated with sodium hydroxide.
A pure liquid sample containing only carbon, hydrogen and oxygen is placed in a 0.57148 g platinum boat, which on reweighing weights 0.61227 g.The sample is ignited and the previously weighed absorption tubes are reweighed.The mass of the water absorption tube has increased from 6.47002 g to 6.50359 g, and the mass of the carbon dioxide tube has increased from 5.46311 g to 5.54466 g.
(a) Calculate the mass composition of the compound.
(b) Give the empirical formula of the compound.To estimate the molar mass of the compound,1.0045 g was gasified. The volume, measured at a temperature of 350 K and a pressure of 35.0 kPa, was 0.95 dm3.
(c) Give the molar mass and the molecular formula of the compound.
(d) Draw possible structures corresponding to the molecular formula excluding cyclic structures, stereo isomers, peroxides and unsaturated compounds.There are about 15 possibilities.Give 10 of them.
When the compound is heated with a sodium hydroxide solution, two products are formed. Fractional distillation of the reaction mixture yields one of the substances.The other substance is purified by distillation after acidification and appears to be an acid.
(e) What structures are possible for compound C?
0.1005 g of the acid are dissolved in water and titrated with a sodium hydroxide solution with a concentration of 0.1000 mol dm-3. The indicator changes colour on addition of 16.75 cm3 of hydroxide solution.
(f) What was the original substance C?
(a) Mass percentage composition: 54.56 % C; 9.21 % H; 36.23 % O
(b) Empirical formula: C2H4O
(c) Molar mass: 88 g mol-1 Molecular formula: C4H8O2
(d) Possible structures:
1. CH3-CH2-CH2-COOH
2. CH3-CH(CH3)-COOH
3. CH3-O-CO-CH2-CH3
4. CH3-CH2-O-CO-CH3
5. CH3-CH2-CH2-O-CO-H
6. CH3-CH(CH3)-O-CO-H
7. CH3-CH2-CH(OH)-CHO
8. CH3-CH(OH)-CH2-CHO
9. CH2(OH)-CH2-CH2-CHO
10. CH3-C(OH)(CH3)-CHO
11. CH2(OH)-CH(CH3)-CHO
12. CH3-O-CH2-CH2-CHO
13. CH3-CH2-O-CH2-CHO
14. CH3-O-CH(CH3)-CHO
15. CH3-CH2-CO-CH2-OH
16. CH3-CH(OH)-CO-CH3
17. CH2(OH)-CH2-CO-CH3
18. CH3-O-CH2-CO-CH3
(e) The possible structures are 3, 4, 5, 6.
(f) The structure of the compound C is CH3-CH2-O-CO-CH3.