The fact that ‹all solvents› and ‹all solutes› are included in this hypothesis, with the sole limiting condition that the solution must be dilute, is one of great significance and of greatest practical importance, as we may use any suitable solvent for determinations.
When molecular weights are determined in this way, a very large number of compounds give the same molecular weight by the solution method as by the gas method. For instance we have:
Substance. Mol. Wt. Mol. Wt. Solvent. Gas Method. Sol. Method. Chloroform, CHCl_{ 3} 119.5 119.5 Benzene Carbon bisulphide, CS_{ 2} 76 76 Benzene Methyl (wood) alcohol, CH_{ 4}O 32 32 Water Ethyl (ordinary) alcohol, C_{ 2}H_{ 6}O 46 46 Water Ether, C_{ 4}H_{ 10}O 74 74 Acet. Acid
Further, the molecular weight of glucose is found in aqueous solutions to be 180, conforming to the formula C_{ 6}H_{ 12}O_{ 6}, and agreeing with the molecular weight as obtained by a chemical study of compounds derived from glucose.
While there are, then, very many agreements in the molecular weights determined by the solution and by the older methods, it was recognized, at the outset,[58] that there is also a large number of apparently ‹abnormal› cases, in which, in particular, ‹much lower molecular weights› are obtained by the solution methods than by the gas method,—lower even than the weights consistent with the accepted atomic weights of the elements in the compounds in question.[59] For instance, we find 36.5 to be the molecular weight of hydrogen chloride in the gas form, but in ‹aqueous› solution its apparent molecular weight, as determined on the basis of van 't Hoff's hypothesis, is not even a constant; it is found to be less than 36.5 and approaches the limit 18.25, the more dilute the solution, [p038] the lower being the apparent molecular weight.[60] For sodium chloride, the formula weight, corresponding to the formula NaCl, is 58.5. This would also represent its smallest molecular weight in gas form, consistent with the accepted atomic weights for sodium and chlorine. In ‹aqueous› solution, again, the apparent molecular weight of sodium chloride is found to be less than 58.5, and more than 29.25, the value found depending on the concentration of the solution used. For zinc chloride we have, likewise, in aqueous solution values much less than 136 and tending toward the limit 45, whereas the formula weight for ZnCl_{ 2} is 136.