Doubling of plasma sodium concentration be a reasonable approximation of plasma osmolality?

No. The doubling of sodium accounts for the negative ions associated with sodium and the exclusion of potassium approximately allows for the incomplete dissociation of sodium chloride. Osmolarity is the calculation of the osmolar convergence of plasma and is corresponding to the quantity of particles per liter of arrangement; it is communicated as mmol/L. This is what is utilized when a figured esteem is inferred.