Units of Mass and Weight

As previously discussed, the SI unit of mass is the kilogram while the British unit of mass is the slug. The SI unit of force is the newton (N), given by Newton s second law. One newton of force results from accelerating a mass of 1 kg by 1 m/s2. 

An object having a mass of 1 kg would thus have a weight of 9.8 N. 

The British unit of force is the pound (lb), once again given by Newton s second law  

The weight of a 1 slug mass on the Earth s surface is therefore 32.2 lb. 

You probably learned to convert pounds into kilograms by dividing pounds by 2.2. This is technically incorrect, because you are converting pounds (a weight quantity) into kilograms (a mass quantity), although it does give the numerically correct answer, at least when the acceleration on the object is 9.8 m/s2. The formally correct solution to this question is  

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Gram A unit of mass and weight in the metric system, defined as one thousandth of a kilogram. (A kilogram is about 2.2 pounds.)... 

Metric ton (tonne) — A unit of mass and weight equal to 1,000 kilograms or 2,205 pounds avoirdupois. In Canada, the metric ton is the most widely used measure of oil quantity by weight. There are approximately 7 to 9 barrels (245 to 315 Imperial gallons) of oil per metric ton, depending on the specific gravity of the crude oil or petroleum product. 

KILOGRAM (kg). A unit of mass and is based upon a cylinder of platinum-iridium alloy kept by the International Bureau of Weights and Measures at Paris. A duplicate in the custody of the National Bureau of Standards at Washington is the mass standard for the United States. The kilogram is the only base unit still defined by an artifact. (A kilogram equals (1) 1,000 grams (2) 2.205 pounds (3) 9,842 x 10-4 long tons or (4) 1 102 x 10-3 short tons. 

How does the United States standardize measurements of mass and weight ... 

We shall first review the common units of weight and volume in the metric system and then describe methods of expressing results. The gram (g) is the basic unit of mass and is the unit employed most often in macro analyses. For small samples or trace constituents, chemists use smaller units. The milligram (mg) is 10 g, the microgram (/xg) is 10 g, and the nanogram (ng) is 10 g. The basic unit of volume is the liter (L). The milliliter (mL) is 10" L and is used conunonly in volumetric analysis. The microliter (/xL) is 10 L (10 mL), and the nanoliter (nL) is 10 L (1Q- mL). (Prefixes for even smaller quantities include pico for 10 and femto for 10" .)... 

Picking at random a unit of mass and determining the expected weight of the molecule of which it is a part will give the weight-average molecular weight. This is, in fact, another application of Eq. (5.89)]. Thus,... 

The mole is a unit of mass and is expressed as the molecular weight of substance in grams (g). A molar solution will contain one mole solute per liter of solution. A molal solution contains one mole solute per 1000 g of solvent. 

The expression (2-4) provides the definition of a partial molal quantity if the gram-molecular weight (mole) is the unit of mass and of a partial specific quantity if the gram is the unit of mass. In the future, the symbol i will denote a partial molal quantity defined by the relation... 

Cartesians, mass-weighted Cartesian displacements, mass-scaled Cartesians, and mass-scaled Jacobis. In mass-weighted coordinates, mass is unity and unitless, and the coordinates have units of length times square root of mass in mass-scaled coordinates, the reduced mass for all coordinates is a constant p (with units of mass), and the coordinates have units of length. We almost always use mass-scaled coordinates the main exception is in the subsection on curvihnear internal coordinates, where much of the analysis involving internal coordinates is done in terms of unsealed coordinates. 

Mass. The unit of mass is the kilogram and is the mass of a particular cylinder of Pt—Ir alloy which is preserved in France by the International Bureau of Weights and Measures. 

The English system of units is complicated compared to the metric system. In the English system, the units of mass are pounds-mass (Ibm) and the units of weight are pounds-force (Ibf). By definition, a weight (i.e., force) of one Ibf equals the force produced by one Ibm under the acceleration of gravity. Therefore, the constant, g, which has the same numerical value as g (32.17) and units of Ibm-ft/lbf-sec, is used in the definition of weight ... 

For scientific work the fundamental standard of mass is the international prototype kilogram, which is a mass of platinum-iridium alloy made in 1887 and deposited in the International Bureau of Weights and Measures near Paris. Authentic copies of the standard are kept by the appropriate responsible authorities in the various countries of the world these copies are employed for the comparison of secondary standards, which are used in the calibration of weights for scientific work. The unit of mass that is almost universally employed in laboratory work, however, is the gram, which may be defined as the one-thousandth part of the mass of the international prototype kilogram. 

SI (le Systeme International d UniUs) units are used in many countries to express clinical laboratory and serum drug concentration data. Instead of employing units of mass (such as micrograms), the SI system uses moles (mol) to represent the amount of a substance. A molar solution contains 1 mol (the molecular weight of the substance in grams) of the solute in 1 L of solution. The following formula is used to convert units of mass to moles (mcg/mL to pmol/L or, by substitution of terms, mg/mL to mmol/L or ng/mL to nmol/L). 

The standard unit of length in the SI system is the meter. One meter equals about 3.048 ft. The standard unit of volume is the liter. One liter is a little over a quart. Soda has been sold in 2-liter bottles for many years in the United States, one of the few areas that the SI system has penetrated our daily lives. The standard unit of mass is the gram. Technically, mass and weight are not the same, however, this distinction tends to be ignored often in practice. One thousand grams is about 2.2 pounds. 

If the masses (in c) are expressed in g and kg, respectively, in the last two situations, then the units of the ordinate reduce to cm2 s 2 and m2 s"2, respectively, which do not bear much resemblance to molecular weight units. Note, however, that the acceleration of gravity in appropriate units can be factored out of these latter quantities to leave units of cm and m, respectively, for reduced osmotic pressure. ... 

Because mass is independent of location, it is customary in science to measure matter by its mass rather than its weight. Conceptual Chemistry adheres to this convention by presenting matter in units of mass, such as kilograms, grams, and milligrams. Such weight units as pounds and tons (1 ton = 2000 pounds) are occasionally provided as a reference because of their familiarity. 

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