Re: Is there a DIFFERENCE between a molal and a normal solution?

Hi Don -

"Normal saline" in medicalese translates to ~155mM NaCl. 155mM = 0.9% NaCl per weight of water (9 grams NaCl per total kilogram of solution, e.g.). As you indicate this is only a special way of describing molality. "Half-normal saline" = 0.45%, and "twice normal saline" = 1.8%

Keep in mind that "normal saline" is not the same as "normality." However, the medical usage of "normal" with regards to saline solutions seems to deviate from how Merriam-Webster defines a normal solution, unless definition (b) is perhaps applied:

of a solution: having a concentration of one gram equivalent of solute per liter b : containing neither basic hydroxyl nor acid hydrogen (normal silver phosphate) c : not associated (normal molecules) d : having a straight-chain structure (normal pentane) (normal butyl alcohol)

Hope this helps..

Lynn Bry, Moderator MadSci Network

{Admin supplement:

Anyone who learnt any chemistry after about 1970 has probably never heard of a "gram equivalent weight" nor of the "normality" of a solution. But this is what Webster's definition a. is referring to. These terms were extensively used 50 years ago and before. In modern terms, the gram equivalent weight of a substance is equal to the molar mass divided by the valency. A 1 normal solution of hydrochloric acid is exactly the same as a 1 molar solution of hydrochloric acid, but a 1 normal solution of sulfuric acid is the same as a 0.5 molar solution of sulfuric acid (because the 'valency' of sulfuric acid is 2). The problem with normality as a measure of concentration was that the normality of the same solution could vary depending on the context of what you wanted to do with it. For example, dilute nitric acid has a 'valency' of 1 as an acid, but a 'valency' of 3 as an oxidant. So a solution of nitric acid that was 1 normal for the purpose of neutralizing with caustic soda was 3 normal for the purpose of dissolving up copper foil. Sometime in the 1960s, the International Union of Pure and Appplied Chemistry wisely decided that normality was a concept that chemists would be better off without, and that thenceforth concentrations would be expressed in terms of molarity.

The bottom line as far as your question is concerned is that there are two possibilities. If the reference is to sodium chloride as 'normal saline' in a medical context, it means 155 millimolar; if, on the other hand it is a very old-fashioned chemical reference, then it means 1 molar, because the 'valency' of sodium chloride in any context is 1.

John Christie (one of the few MadSci-entists who is old enough to remember!) }