Dilute solutions boiling point

Calculate the following colligative properties of a dilute solution boiling-point elevation, freezing-point depression, and osmotic pressure. 

Selection of solvents. The choice of solvent will naturally depend in the first place upon the solubility relations of the substance. If this is already in solution, for example, as an extract, it is usually evaporated to dryness under reduced pressure and then dissolved in a suitable medium the solution must be dilute since crystallisation in the column must be avoided. The solvents generally employed possess boiling points between 40° and 85°. The most widely used medium is light petroleum (b.p. not above 80°) others are cycZohexane, carbon disulphide, benzene, chloroform, carbon tetrachloride, methylene chloride, ethyl acetate, ethyl alcohol, acetone, ether and acetic acid. 

Dissolve or suspend 0 - 5 g. of the acid in 5 ml. of water in a small conical flask, add a drop or two of phenolphthalein indicator, and then 4-5 per cent, sodium hydroxide solution until the acid is just neutrahsed. Add a few drops of very dilute hydrochloric acid so that the final solution is faintly acid (litmus).f Introduce 0-5 g. of p-bromophenacyl bromide (m.p. 109°) dissolved in 5 ml. of rectified (or methylated) spirit, and heat the mixture under reflux for 1 hour if the mixture is not homogeneous at the boiling point or a solid separates out, add just sufficient alcohol to produce homogeneity. [Di- and tri-basic acids require proportionately larger amounts of the reagent and longer refluxing periods.] Allow the solution to cool, filter the separated crystals at the pump, wash with a little alcohol and then with water. Recrystallise from dilute alcohol dissolve the solid in hot alcohol, add hot water until a turbidity just results, clear the latter with a few drops of alcohol, and allow to cool. Acetone may sometimes be employed for recrystallisation. 

Diethylbarbituric acid. In a dry 250 ml. distilling flask, fitted with a thermometer reaching to within 3-4 cm. of the bottom and a condenser, place 51 g. of clean sodium and add 110 g. (140 ml.) of super-dr ethyl alcohol (Section 11,47,5). When all the sodium has reacted, introduce 20 g. of ethyl diethylmalonate and 7 0 g. of dry imea (dried at 60 for 4 hours). Heat the flask in an oil bath and slowly distil off the ethyl alcohol. As soon as the temperature of the liquid reaches 110-115°, adjust the flame beneath the bath so that the contents of the flask are maintained at this temperature for at least 4 hours. Allow the flask to cool somewhat, add 100 ml. of water and warm until the solid (veronal-sodium) dissolves. Pour the solution into a beaker, and add a further 100 ml. of water but containing 7 0 ml. of concentrated siilplmric acid this will hberate the veronal from the sodium derivative. The veronal usually crystallises out if it does not, add a few more drops of dilute sulphuric acid until the solution is acid to Congo red. Heat the contents of the beaker, with stirring and the addition of more water if necessary, until all the veronal dissolves at the boiling point. Allow the hot solution to cool, filter off the crystals of veronal and diy in the air. The yield is 12 g., m.p. 190°. 

Elevation of the boiling point by dissolved solids. Organic substances dissolved in organic solvents cause a rise in boiling point which is proportional to the concentration of the substance, and the extent of rise in temperature is characteristic of the solvent. The following equation applies for dilute solutions and non-associating substances ... 

After dilution with 200 ml. of benzene, the solution is transferred to a 2-1. separatory funnel containing 800 ml. of ice water and shaken thoroughly. The aqueous layer is separated, acidified to pH 3-4 with 2-3 ml. of concentrated hydrochloric acid, and extracted with three 100-ml. portions of benzene. All the organic layers are then combined and dried over anhydrous sodium sulfate. Filtration and concentration of the solution with a rotary evaporator, followed by exposure to high vacuum for 2-3 hours, affords 17.3-19.3 g. of the crude product (Note 3). Low-boiling impurities are removed by vacuum distillation (Note 4), the residual oil (14-15 g.) is transferred to a 50-ml. flask equipped with a short-path distillation apparatus, and vacuum distillation is continued. A forerun is taken until no rise in boiling point is observed, and then 7.2-8.6 g. (23-27%) of dimethyl nitrosuccinate is collected as a colorless oil, b.p. 85° (0.07 mm.), 1.4441 (Note 5). 

Cyano-17-(2 -tetrahydropyranyloxy)-androst-5-en-3j5-ol Acetate A solution of 10 g of 17-cyanoandrost-5-en-3j5,17-diol 3-acetate in 40 ml of 2,3-dihydropyran is treated at the boiling point with 0.2 ml of phosphorus oxychloride for 1.5 hr. The solution is then diluted with ether, washed with aqueous sodium carbonate, and then water, dried over sodium sulfate and distilled under reduced pressure. The oily residue is crystalhzed from petroleum ether to give 6.7 g of 17a-cyano-17j5-(2 -tetrahydropyranyloxy)-androst-5-en-3) -ol acetate mp 127-130° [a]o —92° (diox). The mother liquor when evaporated to dryness yields an oily residue of 7.1 g which resists crystallization attempts. Subsequent reactions (see below) indicate it to be 17jS-cyano-17a-(2 -tetrahydropyranyloxy)-androst-5-en-3jS-ol acetate. 

Preparation of l9-Norandrost-A-ene-3, l-dionef A solution of 1.1 g of 10y5-cyano-19-norandrost-5-ene-3,17-dione bis-ethylene ketal in a mixture of 15 ml of ethanol and 15 ml of toluene is carefully added to a vigorously stirred suspension of 10 g of sodium in 150 ml of boiling toluene. The addition is regulated to maintain the reaction mixture at the boiling point of the solvent. Another 40 ml of anhydrous ethanol is then added at the same rate. The solution is cooled and the excess of sodium is decomposed by addition of 95% ethanol. The reaction mixture is then diluted with water, the toluene layer separated and the aqueous phase extracted twice with ether. The organic solution is washed with water, dried and evaporated to yield 1 g of an amorphous mixture of the bis-ethylene ketals of 19- norahd-rost-5- and -5(10)-ene-3,17-dione (Note 1). 

The mixture is then slowly heated to the boiling point of toluene and kept boiling for one hour under reflux. After the mixture has been allowed to cool the sodium chloride which precipitates is separated by extraction with water. The solution of toluene is then extracted with dilute hydrochloric acid. From the hydrochloric acid extract the basic substance is separated in the form of an oil by means of caustic soda solution and is introduced into ether. The ethereal solution Is dried with the aid of potassium carbonate and then distilled. 

Pipette a 25 mL or 50 mL aliquot of the clear sample solution into a 250 mL conical flask, add 5 mL concentrated sulphuric acid, 5 mL 85 per cent phosphoric(V) acid, and 1-2 mL of 0.1 M silver nitrate solution, and dilute to about 80 mL. Add 5 g potassium persulphate, swirl the contents of the flask until most of the salt has dissolved, and heat to boiling. Keep at the boiling point for 5-7 minutes. Cool slightly, and add 0.5 g pure potassium periodate. Again heat to boiling and maintain at the boiling point for about 5 minutes. Cool, transfer to a lOOmL graduated flask, and measure the absorbances at 440 nm and 545 nm in 1 cm cells. 

In physical chemistry, we apply the term colligative to those properties that depend upon number of molecules present. The principal colligative properties are boiling point elevation, freezing point depression, vapour pressure lowering, and osmotic pressure. All such methods require extrapolation of experimental data back to infinite dilution. This arises due to the fact that the physical properties of any solute at a reasonable concentration in a solvent are... 

Unlike the saturated hydrocarbons, unsaturated aliphatic hydrocarbons are soluble in concentrated sulphuric acid and exhibit characteristic reactions with dilute potassium permanganate solution and with bromine. Nevertheless, no satisfactory derivatives have yet been developed for these hydrocarbons, and their characterisation must therefore be based upon a determination of their physical properties (boiling point, density and refractive index). The physical properties of a number of selected unsaturated hydrocarbons are collected in Table 111,11. 

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