I have seen the density of lecithin given from 1.03 to 1.08 grams per cubic centimeter. Since water is 1.0 grams per centimeter, this means that lecithin will slowly sink in water.

But perhaps you are interested in the volumetric equivalents. Purebulk.com sells one kilogram of lecithin powder and states that its volume is 2800 cc. This comes to 22.3 pounds per cubic foot. A more practical measure is about 3 pounds per gallon. A gallon of water weighs about 8 1/3 pounds.

Purebulk.com provides a table of volumetric equivalents. One tablespoon weighs 7468 milligrams. In more familiar terms, a tablespoon weighs slightly more than 1/4 ounce of weight. You would get 61 tablespoons per pound.