Weight of Substances in Solution

Prior to the work of Eaoult, who developed (1882-1885) the cryo-scopic method for determining molecular weights of dissolved substances, and to van t Hoff s formulation (1886-1888) of the solution laws, no method was available for quantitatively determining the molecular weights of substances in solution. The vapor density method obviously could not be applied to any but very low polymers. No means was at hand for determining the state of polymerization even in instances where polymerization was suspected. 

The Determination of Molecular Weight by the Freezing-Point Method. The freezing-point method is a very useful way of determining the molecular weights of substances in solution Camphor, with its very large constant, is of particular value for the study of organic substances. 

It follows from this, that a molar solution of sulphuric acid will contain 98.074 grams of sulphuric acid in 1 litre of solution, or 49.037 grams in 500 mL of solution. Similarly, a 0.1 M solution will contain 9.8074 grams of sulphuric acid in 1 litre of solution, and a 0.01 M solution will have 0.980 74 gram in the same volume. So that the concentration of any solution can be expressed in terms of the molar concentration so long as the weight of substance in any specified volume is known. 

The mole is a unit of mass and is expressed as the molecular weight of substance in grams (g). A molar solution will contain one mole solute per liter of solution. A molal solution contains one mole solute per 1000 g of solvent. 

Where M is the molecular weight of the solute, Dt is the elevation of boiling point in °C, c is the concentration of solute in grams for lOOOgm of solvent, and K is the Ebullioscopic Constant (molecular elevation of the boiling point) for the solvent. K is a fixed property (constant) for the particular solvent. This has been very useful for the determination of the molecular weights of organic substances in solution. 

Such data can provide a calibration curve and allow the constants (E) and F ) in equation (20) to be determined. The value of the molecular weight of an unknown solute can then be obtained from its (H) value by reading the value directly from the curve or by calculation using the predetermined constants (E) and (F ) in equation (20). It should be pointed out that an error of up to 30% may not appear to be very useful but, in fact, such precision can be extremely valuable in the preliminary examination of many biochemical substances where only very small quantities of material are available. It is also an ideal method for molecular weight determination before more accurate, labor-intensive and time-consuming methods are considered. 

Corollary 2.—The relative lowering of vapour pressure is proportional to the concentration c = n/ (n0 + n) at all temperatures, so that if equimolecular amounts of different substances are dissolved in equal weights of solvent, the solutions all have the same vapour pressure, independently of the natureof the solute. 

Directions No Calculators may be used with Part B. Write the formulas to show the reactants and the products for any FIVE of the laboratory situations described below. Answers to more than five choices will not be graded. In all cases a reaction occurs. Assume that solutions are aqueous unless otherwise indicated. Represent substances in solution as ions if the substances are extensively ionized. Omit formulas for any ions or molecules that are unchanged by the reaction. You need not balance the equations. The Section II score weighting for this question is 15%. 

In the pharmaceutical industry, medicines are standardized to a particular weight of active pharmaceutical per unit (for example, mg of substance per tablet), so the weight of a substance in a tablet, or volume of medicine, is of more interest than the number of moles. For most pharmaceutical preparations, then, we use the specific absorbance, A(l%, 1 cm), which is the absorbance (logio fo/f) of a 1% w/v solution (i.e. 1 g of substance in 100 cm solvent) in a 1 cm path length cell, in place of the molar absorptivity, e. This is the absolute method of substance identification, and the absorbance. A, can be related to T(l%, 1 cm) by equation (2.4) ... 

Sulphuric acid is a good solvent for most solids, many organic substances in particular dissolving in it readily 7 the molecular weights of substances dissolved in pure sulphuric acid are generally lower than the commonly accepted values, probably due to the formation of dissociable additive compounds between solute and solvent. 

The accuracy of the results of the refractometric determination diminishes, of course, with increase in the non-saccharine substances contained in the liquid. With very impure products this source of error may be obviated to some extent by diluting, not with water, but with a known weight of pure sugar solution in which the water has been determined with the refractometer. Thus, if a weight p of the substance is mixed with... 

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