weight

The product of the force of gravity, defined internationally as 9.80665 m/s², times the mass of the body. [A.S. gewiht]

- apothecaries w. an obsolescent system of weights based upon the w. of a grain of wheat. Has been used for centuries in weighing medicines and precious metals (Troy measure). Some drugs which have been available for long periods are still often designated as grains ( e.g., 5 grains of aspirin, 1/2 grain of codeine, 1/100 grain nitroglycerin). This w. system has been largely superseded by the metric system (based on grams). One grain is the equivalent of 64.8 milligrams. One scruple contains 20 grains; one dram contains 60 grains; one apothecary ounce contains 8 drams (480 grains); one apothecary pound contains 12 ounces (5760 grains).

- atomic w. (at. wt., AW) the mass in grams of 1 mol (6.02 × 10²³ atoms) of an atomic species; the mass of an atom of a chemical element in relation to the mass of an atom of carbon-12 (¹²C), which is set equal to 12.000, thus a ratio and therefore dimensionless (although the actual mass, numerically the same, is sometimes expressed in daltons); not necessarily the w. of any individual atom of an element, since most elements are made up of several isotopes of different masses; e.g., the atomic w. of chlorine is 35.4527, because it is composed of ³⁵Cl and ³⁷Cl in proportions that give an average of 35.4527. SEE ALSO: molecular w..

- birth w. in humans, the first w. of an infant obtained within less than the first 60 completed minutes after birth; a full-size infant is one weighing 2500 g or more; a low birth w. is less than 2500 g.; very low birth w. is less than 1500 g.; and extremely low is less than 1000 g.

- combining w. SYN: gram equivalent.

- dry w. the w. of material remaining after removing the water ( E.G., after heating above 100°C).

- equivalent w. SYN: gram equivalent.

- gram-atomic w. atomic w. expressed in grams. Cf.:mole.

- gram-molecular w. molecular w. expressed in grams. Cf.:mole.

- molecular w. (mol wt, MW) the sum of the atomic weights of all the atoms constituting a molecule; the mass of a molecule relative to the mass of a standard atom, now ¹²C (taken as 12.000). Relative molecular mass (M_r) is the mass relative to the dalton and has no units. SEE ALSO: atomic w.. SYN: molecular mass, molecular w. ratio, relative molecular mass.

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weight 'wāt n

1) the amount that a thing weighs

2) a unit of weight or mass

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(wāt) 1. heaviness; the degree to which a body is drawn toward the earth by gravity. See also Appendix 7. Abbreviated wt. 2. in statistics, the process of assigning greater importance to some observations than to others, or a mathematical factor used to apply such a process.