MadSci Network: Medicine

Re: What is the percentage of salt in a human?

Date: Thu Mar 22 10:57:40 2001
Posted By: Chris Kaiser, Process Engineer, Anvil Corporation
Area of science: Medicine
ID: 976137504.Me

Hi, Frieda.

Salt is a word that can refer to a lot of different things. Any time we have a compound that consists of positively-charged ions (cations) and negatively-charged ions (anions) that are bonded together ionically (that is, without sharing electrons), that compound can be considered a type of salt. The ions that make up the salt can be single atoms or made up from more than one atom. Some examples of salt are calcium chloride (CaCl2), potassium bromide (KBr), potassium nitrate (KNO3), and sodium chloride (NaCl, or common table salt).

I'm going to assume that you meant table salt when you asked how much salt is in the human body. Here too we run into problems, though. Because table salt is made up of chloride anions and sodium cations stuck together with ionic bonds, and because ionic bonds are so weak, when salt is dissolved in water, the salt dissociates. This means that the salt doesn't exist in the water as sodium chloride anymore, but that it is now sodium ions and chloride ions dissolved in the water. A human body contains a lot of water, so there isn't any free salt in the body, just a bunch of sodium and chlorine floating around.

All right, we're ready to answer your question now. A human body contains approximately 0.15 percent by weight chlorine and 0.15 percent by weight sodium. (This site has a table showing elemental composition in the human body. It agrees for the most part with what I could find in the Pocket Ref, a really outstanding pocket-sized source of all kinds of useful information.) This means that a 50-kilogram human will contain 0.075 kilograms, or 75 grams, of each of the elements in table salt.

So, assuming we could remove all the sodium and chlorine from this person's body (and we wouldn't want to do that, since they are both needed for proper functioning of the body's systems), how much table salt could we make? Well, this uses a concept called stoichiometry, which is the term used to describe the relationship between the weights of the reactants and the products of a reaction. In this case, we want to combine sodium (of which we have 75 grams) with chlorine (of which we also have 75 grams), and we know that each atom of sodium will combine with a single atom of chlorine to form a single molecule of sodium chloride. Look on a periodic table for the atomic mass of these two elements, and you will find that sodium has an atomic weight of about 22.99 and chlorine has an atomic weight of 35.45. Therefore, for every 35.45 grams of chlorine we react, we can only use 22.99 grams of sodium. The calculation for the amount of sodium used is:

(75 grams chlorine)*(22.99 grams sodium / 35.45 grams chlorine) =

48.64 grams sodium

Since we used 75 grams of chlorine and 48.64 grams of sodium, we made 123.64 grams of table salt, sodium chloride. If this were regular fine table salt, it could be formed into a cube approximately 4.7 centimeters on a side. It is also about 7 tablespoons.

I apologize if this is a bit long-winded. Just remember that salt is a word which refers to a lot of different compounds, that the constituents of common table salt exist as separate ions in the human body, and that if we could remove them and reconstitute them into table salt, they would make up about 7 tablespoons of salt for a 50-kilogram person. Hope that helps.

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